Ion concentration measuring device

Abstract

A pH sensor 1 is provided to a measurement target 101 including measurement ions 102 and non-measurement ions 103, and acquires the concentration of the measurement ions 102. The pH sensor 1 is provided with a measurement target electrode 14 which is provided to the measurement target 101 and which controls the potential of the measurement target, a measurement target power source 16 which applies a voltage to the measurement target electrode 14, a measurement ISFET 2 including a measurement ion sensitive film 7, a measuring film electrode 17 which is provided to the measurement ion sensitive film 7 and which controls the potential of the measurement ion sensitive film 7, a measuring film power source 18 which applies a voltage to the measuring film electrode 17, a capacitor 71 which is connected to the measurement target electrode 14 and to the measuring film electrode 17 and which generates a potential difference between the measurement target electrode 14 and the measuring film electrode 17, and a power supply control unit 19 that controls the magnitude of the voltage outputted from the measurement target power source 16 and controls the magnitude of the voltage outputted from the measuring film power source 18.

Description

Technical Field

[0001] The present invention relates to an ion concentration measuring device.

Background Art

[0002] The hydrogen ion exponent (hereinafter referred to as "pH") is an important physical quantity in the agricultural field and the water quality inspection field. As means for measuring pH, means using litmus test paper, means using a glass electrode, and means using an ion selective field effect transistor (Ion Sensitive Field Effect Transistor: ISFET) are known. For example, Non-Patent Document 1 discloses a technique related to ISFET.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] For example, in the agricultural field, it is known that there are suitable pH values of soil for each crop. Therefore, in the agricultural field, a technique for continuously measuring the pH of an object in which fine particles such as soil are mixed over a long period of time is desired. The ISFET disclosed in Patent Document 1 outputs a voltage corresponding to the ion concentration of the measurement object. However, it is known that the output voltage of the ISFET fluctuates even when the ion concentration does not change over time. The fluctuation of the output voltage of the ISFET not caused by the change in the ion concentration is called drift.

[0005] Drift occurs due to fluctuations in the gate voltage of the ISFET. These gate voltage fluctuations are caused by ions that are not the target of measurement penetrating (invading) the ion-sensitive film. Therefore, the inventors of this application have come up with a mechanism to suppress the intrusion of ions that are not the target of measurement by controlling the potential difference between the target of measurement and the ISFET (Patent Document 2).

[0006] To sustain long-term measurements, it is desirable to drive the device with less power. The device described in Patent Document 2 can effectively suppress drift. However, suppressing drift requires controlling the potential difference between the measurement target and the ISFET, thus necessitating a continuous power supply. Therefore, to achieve long-term measurements, it is desirable to achieve both drift suppression and a reduction in power consumption.

[0007] The present invention provides an ion concentration measuring device that can obtain good output over a long period of time. [Means for solving the problem]

[0008] An ion concentration measuring device, one embodiment of the present invention, is installed on a target to be measured, which includes both a target ion and a non-target ion, to obtain the concentration of the target ion. The ion concentration measuring device comprises a target electrode placed on the target to control the potential of the target, a power supply for the target to supply a voltage to the target electrode, a measuring sensor unit including a target ion sensitive membrane that generates a voltage corresponding to the concentration of the target ion, a measuring membrane electrode placed on the target ion sensitive membrane to control the potential of the target ion sensitive membrane, a measuring membrane power supply that supplies a voltage to the measuring membrane electrode, a potential difference generating unit connected to the target electrode and the measuring membrane electrode to generate a potential difference between the target electrode and the measuring membrane electrode, and a power control unit that controls the magnitude of the voltage output from the power supply and the magnitude of the voltage output from the measuring membrane power supply.

[0009] An ion concentration measuring device generates a potential difference between the measurement target electrode placed on the object to be measured and the measurement film electrode placed on the measurement ion-sensitive film of the measurement sensor. The electric field based on this potential difference suppresses the penetration of non-measured ions into the measurement ion-sensitive film. Furthermore, the potential difference between the measurement target electrode and the measurement film electrode can also be generated by a measurement target power supply and a measurement film power supply. The potential difference between the measurement target electrode and the measurement film electrode can also be generated by a potential difference generation unit. An ion concentration measuring device does not necessarily need to use a measurement target power supply and a measurement film power supply to generate a potential difference between the measurement target electrode and the measurement film electrode. As a result, the ion concentration measuring device can reduce the energy required to suppress drift caused by the penetration of measured ions. Therefore, the ion concentration measuring device can obtain a good output with suppressed drift over a long period of time.

[0010] The potential difference generating section of the ion concentration measuring device described above may be a capacitor. This configuration makes it possible to reduce the amount of energy consumed to suppress drift.

[0011] The power control unit of the ion concentration measuring device described above may switch between a first operation, which generates a first potential difference between the electrode to be measured and the measuring film electrode using the power supply for the device to be measured and the power supply for the measuring film, and a second operation, which generates a second potential difference between the electrode to be measured and the measuring film electrode using a potential difference generation unit. According to the second operation, a potential difference for suppressing drift can be generated without using the power supply for the device to be measured and the power supply for the measuring film. Therefore, the energy consumption by the power supply for the device to be measured and the power supply for the measuring film can be reduced.

[0012] The power control unit of the ion concentration measuring device described above may, when in the first operation, allow the output of a voltage corresponding to the concentration of the measured ion from the measurement sensor unit. When in the second operation, the power control unit may prohibit the output of a voltage corresponding to the concentration of the measured ion from the measurement sensor unit. According to these operations, when measurement is performed and the output of a voltage corresponding to the concentration of the measured ion is allowed, a first potential difference can be generated using the power supply for the device being measured and the power supply for the measurement membrane. When measurement is not performed and the output of a voltage corresponding to the concentration of the measured ion is prohibited, a second potential difference can be generated using the potential difference generation unit.

[0013] The power control unit of the ion concentration measuring device described above may, when in the first operation, perform in parallel a measurement operation that allows the measurement sensor to output a voltage corresponding to the concentration of the measured ions, and a charging operation that charges the potential difference generating unit. This operation allows the measurement operation and the charging operation to be performed simultaneously.

[0014] In the ion concentration measuring device described above, the second potential difference may be equal to the first potential difference. This setting simplifies voltage control.

[0015] In the first operation, the power control unit of the ion concentration measuring device described above may perform one of the following operations before the other: a measurement operation that allows the measurement sensor to output a voltage corresponding to the concentration of the measured ions, and a charging operation that charges the potential difference generating unit. This operation allows the timing of the measurement operation and the charging operation to be staggered.

[0016] In the ion concentration measuring device described above, the second potential difference may be different from the first potential difference. With this setting, the first potential difference can be set to a value suitable for measurement, and the second potential difference can be set to a value suitable for suppressing drift over a long period of time.

[0017] The measurement sensor unit of the ion concentration measuring device described above may be an ion-selective field-effect transistor having a substrate, an insulating film provided on the substrate, and a measurement ion-sensitive film provided on the insulating film. With this configuration, an output voltage corresponding to the ion concentration can be obtained.

[0018] In the ion concentration measuring device described above, during the first operation, a voltage may be applied between the source and drain of the ion-selective field-effect transistor. During the second operation, the drain and source of the ion-selective field-effect transistor may be connected to a first reference potential section. According to the first operation, an output voltage corresponding to the ion concentration can be obtained. According to the second operation, the influence of external noise can be suppressed during periods when measurements are not being taken.

[0019] In the ion concentration measuring device described above, when the second operation is performed, the measuring membrane electrode may be connected to the second reference potential section. This operation also helps to suppress the influence of external noise during periods when measurements are not being taken.

[0020] In the ion concentration measuring device described above, the potential of the first reference potential section may be the same as the potential of the second reference potential section. A simple circuit configuration can suppress the influence of external noise. [Effects of the Invention]

[0021] According to the present invention, an ion concentration measuring device is provided that can obtain good output over a long period of time. [Brief explanation of the drawing]

[0022] [Figure 1] Figure 1 shows the configuration of the ion concentration measuring device according to the first embodiment. [Figure 2] Figure 2 is a magnified view of the ion-sensitive membrane shown in Figure 1. [Figure 3] Figure 3 is a time chart showing the operation of the ion concentration measuring device of the first embodiment. [Figure 4] Figure 4 is a circuit diagram of the ion concentration measuring device shown in Figure 1. [Figure 5] Figure 5 shows the circuit configuration when the ion concentration measuring device shown in Figure 3 performs measurement and charging operations. [Figure 6] Figure 6 shows the circuit configuration of the ion concentration measuring device shown in Figure 3 when it performs a storage operation. [Figure 7] Figure 7 is a flowchart showing the operation of the ion concentration measuring device according to the first embodiment. [Figure 8] Figure 8 is a time chart showing the operation of the ion concentration measuring device of the second embodiment. [Figure 9] Figure 9 is a circuit diagram of the ion concentration measuring device according to the second embodiment. [Figure 10] Figure 10 shows the circuit configuration when the ion concentration measuring device shown in Figure 9 is performing a measurement operation. [Figure 11] Figure 11 shows the circuit configuration when the ion concentration measuring device shown in Figure 9 is performing a charging operation. [Figure 12] Figure 12 shows the circuit configuration of the ion concentration measuring device shown in Figure 9 when it performs a storage operation. [Figure 13] Figure 13 is a flowchart showing the operation of the ion concentration measuring device according to the second embodiment. [Figure 14] Figure 14 shows the configuration of a modified ion concentration measuring device. [Modes for carrying out the invention]

[0023] Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the attached drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant explanations are omitted.

[0024] [First Embodiment] The ion concentration measuring device shown in Figure 1 is placed on the measurement target 101, which is, for example, soil. In the following description, the ion concentration measuring device will be referred to as "pH sensor 1". pH sensor 1 obtains the concentration of hydrogen ions, which are the measurement ions 102. In the following description, the concentration of hydrogen ions will be referred to as "pH". The measurement target 101 includes measurement ions 102 and non-measurement ions 103. As mentioned above, hydrogen ions are an example of measurement ions 102. Hydroxide ions are an example of non-measurement ions 103. pH sensor 1 is equipped with an ion-selective field-effect transistor (ISFET). The gate of the MOSFET of pH sensor 1 equipped with an ISFET is provided with a material that specifically adsorbs measurement ions 102 as a sensitive film. pH sensor 1 obtains the voltage change that occurs due to the difference in the adsorption density of measurement ions 102. With this configuration, pH sensor 1 can be placed directly on the soil. With this configuration, it is also possible to miniaturize and improve the accuracy of pH sensor 1.

[0025] The pH sensor 1 has, as its main components, a measuring ISFET 2 (measuring sensor unit) and a power supply unit 3.

[0026] The measurement ISFET 2 comprises a substrate 4, an insulating film 6, a measurement ion-sensitive film 7 (ion-trapping film), and a protective film 8. The substrate 4 is made of, for example, n-type silicon. A source 9 and a drain 11 are provided on a part of the substrate 4. The source 9 is p-type. The drain 11 is also p-type. The potential of the source 9 is the same as the potential of the substrate 4. The drain 11 is electrically connected to the substrate 4. The substrate power supply 12 provides the drain voltage to the measurement ISFET 2. The drain voltage is the potential difference between the source 9 and the drain 11.

[0027] An insulating film 6 is provided on the main surface of the substrate 4. The main surface of the substrate 4 includes the main surface of the source 9, the main surface of the drain 11, and the main surface of the channel 13. The insulating film 6 is made of, for example, silicon oxide (SiO2). The insulating film 6 is covered with a measurement ion sensitive film 7 and a protective film 8. The measurement ion sensitive film 7 and the protective film 8 are provided on the main surface of the insulating film 6. The measurement ion sensitive film 7 is provided on the channel 13 via the insulating film 6. The measurement ion sensitive film 7 functions as a gate in the FET. The measurement ion sensitive film 7 is in direct contact with the object to be measured 101. The measurement ion sensitive film 7 selectively captures measurement ions 102 contained in the object to be measured 101. The measurement ions 102 are hydrogen ions. Therefore, the material of the measurement ion sensitive film 7 has OH groups on its surface that promote specific adsorption of hydrogen ions. For example, Si3N4 or Ta2O5 may be used for the measurement ion sensitive film 7. A protective film 8 is provided on the main surface of the insulating film 6 that is not covered by the measurement ion-sensitive film 7. For example, a silicon oxide film 8a (SiOx) and a silicon nitride film 8b (SiNx) may be used as the protective film 8.

[0028] The power supply unit 3 includes a measurement target electrode 14, a measurement target power supply 16, a measurement film electrode 17, a measurement film power supply 18, and a power supply control unit 19. The power supply unit 3 further includes components such as a capacitor 71, which will be described in detail later.

[0029] The positive electrode of the power supply 16 under measurement is electrically connected to the electrode 14 under measurement. The negative electrode of the power supply 16 under measurement is electrically connected to the reference potential section 99. The voltage reference of the power supply 16 under measurement is the reference potential. The substrate 4 and source 9 are also connected to the reference potential section 99. Therefore, the voltage reference of the power supply 16 under measurement is the potential of source 9 or the potential of substrate 4. In the following explanation, the voltage reference of the power supply 16 under measurement will be assumed to be the potential of source 9.

[0030] The electrode 14 and power supply 16 to be measured maintain a constant difference between the potential of source 9 and the potential of the object being measured 101. In other words, the electrode 14 and power supply 16 to be measured maintain a constant potential of the object being measured 101 with respect to the potential of source 9. In the following explanation, the difference between the potential of source 9 and the potential of the object being measured 101 is referred to as the "measured voltage (V)". T ) is called the measured voltage (V T The voltage (V) may also be called the reference voltage. The electrode 14 to be measured is placed on the object to be measured 101. For example, if the object to be measured 101 is soil, the electrode 14 to be measured is placed on the soil. The electrode 14 to be measured is placed on the object to be measured 101 and the voltage (V) T Apply the voltage to be measured (V T The measured voltage (V) is output from the power supply 16 connected to the electrode 14 to be measured. The power supply 16 controls the measured voltage (V) according to the signal φ16 output from the power control unit 19. T ) controls the measured voltage (V T ) is variable.

[0031] The positive electrode of the measurement film power supply 18 is electrically connected to the measurement film electrode 17. The negative electrode of the measurement film power supply 18 is electrically connected to the reference potential unit 99. Therefore, the reference voltage of the measurement film power supply 18 is the reference potential. The substrate 4 and source 9 are also connected to the reference potential unit 99. Therefore, the reference voltage of the measurement film power supply 18 is the potential of source 9 or the potential of substrate 4. The reference voltage of the measurement film power supply 18 is the potential of the power supply being measured 16. The reference voltages of substrate 4, source 9, power supply being measured 16, and measurement film power supply 18 are all the same. Therefore, the reference voltages of substrate 4, source 9, power supply being measured 16, and measurement film power supply 18 are all common to each other. The measurement film power supply 18 controls the output voltage according to the signal φ18 output from the power supply control unit 19.

[0032] The measurement membrane electrode 17 and the measurement membrane power supply 18 control the voltage of the measurement ion-sensitive membrane 7. The pH sensor 1 actively controls the voltage of the measurement ion-sensitive membrane 7. The voltage of the measurement ion-sensitive membrane 7 is the difference between the potential of the measurement target 101 and the potential of the measurement ion-sensitive membrane 7. In the following description, the difference between the potential of the measurement target 101 and the potential of the measurement ion-sensitive membrane 7 is referred to as "membrane voltage (V F )". The potential of the measurement ion-sensitive membrane 7 is the sum of the potential applied from the measurement membrane electrode 17 and the potential generated by the capture of measurement ions 102 by the measurement ion-sensitive membrane 7. In the following description, the potential applied from the measurement membrane electrode 17 is referred to as "membrane control voltage (V C )". The reference of the membrane control voltage (Vc) is the potential of the source 9. The measurement membrane electrode 17 and the measurement membrane power supply 18 control the membrane control voltage (V C ).

[0033] The measurement membrane electrode 17 is disposed inside the measurement ion-sensitive membrane 7. The measurement membrane electrode 17 is embedded in the measurement ion-sensitive membrane 7. The measurement membrane electrode 17 has a striped shape. In other words, the measurement membrane electrode 17 has a stripe shape. The measurement membrane electrode 17 includes a plurality of electrode ridges 17a. The plurality of electrode ridges 17a are electrically connected to each other. The potentials of the plurality of electrode ridges 17a are the same as each other. The electrode ridge 17a contacts the main surface of the insulating film 6. The electrode ridge 17a extends in a predetermined direction. This predetermined direction is orthogonal to the direction of the drain current 200 flowing from the source 9 to the drain 11. The electrode ridges 17a are spaced apart from each other along the direction of the drain current 200. A part of the measurement ion-sensitive membrane 7 and the electrode ridges 17a are alternately arranged along the direction of the drain current 200. For example, the interval between adjacent electrode ridges 17a is 600 nm. The tip surface of the electrode ridge 17a is covered by the measurement ion-sensitive membrane 7. The tip surface of the electrode ridge 17a is not exposed to the measurement target 101. The tip surface of the electrode ridge 17a does not directly contact the measurement target 101.

[0034] The following explains that the suppression of drift is achieved by the above configuration. The output of the pH sensor 1 is the gate voltage (V GBased on ). Gate voltage (V G The gate voltage (V) is given by the following equation (1). G ) is the voltage to be measured (V T ) and film voltage (V F It is the sum of ).

number

[0035] Film voltage (V F The pH-dependent voltage (V) is based on the potential generated when hydrogen ions are trapped in the ion-sensitive membrane 7. PH ) defines pH-dependent voltage (V PH The reference for ) can be, for example, the potential of source 9. Then, equation (1) can be expressed as equation (2).

number

[0036] As described above, when hydroxide ions penetrate the measurement ion-sensitive membrane 7, the potential of the measurement ion-sensitive membrane 7 changes according to the number of hydroxide ions. Based on this potential caused by the penetrated hydroxide ions, the "drift voltage (V)" is calculated. D The term "drift" refers to the change in the output voltage of the measured ISFET2 as a result of charge accumulation on the measured ion-sensitive membrane 7 due to the penetration of non-measured ions 103 into the measured ion-sensitive membrane 7 over time. D The reference for the drift voltage (V) can be, for example, the potential of source 9. D Considering this, the gate voltage (V G ) is shown as equation (3).

number

[0037] Drift voltage (V DThe drift voltage (V) depends on the number of hydroxide ions that enter. As the number of entering hydroxide ions increases over time, the drift voltage (V) D ) becomes larger. As a result, the measured voltage (V T ) and pH-dependent voltage (V PH Even if the drift voltage (V) remains constant, D Due to fluctuations in ), the gate voltage (V G The drift voltage (V) changes. In other words, the output of pH sensor 1A changes. D The change in the output of pH sensor 1A caused by this is called "drift".

[0038] In response to the above problem, the inventors diligently considered the matter and came up with the idea that if the cause of drift is the intrusion of non-measured ions 103, then the drift can be suppressed by suppressing the intrusion of non-measured ions 103. pH sensor 1 measures the membrane voltage (V) of the ion-sensitive membrane 7. F By actively controlling ), the intrusion of non-measured ions 103 is suppressed.

[0039] As shown in Figure 2, the pH sensor 1 has a measuring membrane electrode 17 embedded in the measuring ion-sensitive membrane 7. The measuring membrane electrode 17 is controlled by a membrane control voltage (V C The measuring film electrode 17 is closer to the object to be measured 101 than the drain electrode. Therefore, the film voltage (V F ) is the membrane control voltage (V C ) and pH-dependent voltage (V PH ) and the sum (V F =V C +V PH ) can be defined as: film control voltage (V C ) is the gate voltage (V G This is a bias voltage that actively sets the reference for ). The measured ISFET2 has a gate voltage (V G ) is the threshold voltage (V TH It operates when the film control voltage (V) is greater than or equal to . C ) is the threshold voltage (V TH The film control voltage (V) is set to ) or higher. C The voltage fluctuation relative to ) is the pH-dependent voltage (V PH ) is the pH-dependent voltage (VPH ) corresponds to the captured measured ion 102.

[0040] In accordance with this regulation, the membrane control voltage (V C The film control voltage (V) is set to a value that satisfies the following equation (4). In other words, the film control voltage (V) C ) is the voltage to be measured (V T It is smaller than ). In other words, the film control voltage (V C ) is the film voltage (V F It is smaller than ). Also, if the measurement target 101 is used as a reference, the measurement ion-sensitive film 7 is negatively charged. For example, the measurement target voltage (V T When the ) is 500mV, the film control voltage (V C The voltage is 0mV.

number

[0041] The measurement ion-sensitive membrane 7, in which the measurement membrane electrode 17 is embedded, includes a sensitive region S1 and an electrode region S2. The sensitive region S1 is formed between the electrode ridges 17a. The sensitive region S1 does not include the electrode ridges 17a in the thickness direction of the measurement ion-sensitive membrane 7. According to the sensitive region S1, a change in potential can be obtained when measurement ions 102 are trapped on the main surface of the sensitive region S1. This change in potential is the pH-induced voltage (V). PH This is a change in ).

[0042] The electrode region S2 includes the electrode ridge 17a. The electrode region S2 includes a portion of the measurement ion-sensitive film 7 and the electrode ridge 17a in the thickness direction of the measurement ion-sensitive film 7. A film control voltage (V) is applied to the electrode ridge 17a. C When a voltage is applied, an electric field, as shown by the dashed arrow in Figure 2, is generated inside the measurement ion-sensitive film 7. Specifically, the electric field is generated in the portion of the measurement ion-sensitive film 7 on the main surface of the electrode ridge 17a and in the sensitive region S1.

[0043] Diameter control voltage (V C When the above equation (4) is satisfied, the film control voltage (V CThe electric field caused by ) exerts a repulsive force on the negatively charged non-measured ions 103. This repulsive force inhibits the approach of non-measured ions 103 to the measured ion-sensitive film 7. In other words, non-measured ions 103 have difficulty approaching the measured ion-sensitive film 7. Consequently, the penetration of non-measured ions 103 into the measured ion-sensitive film 7 is suppressed. As a result, the drift voltage (V) included in equation (3) D Since the gate voltage (V) does not change, no drift occurs. Therefore, the gate voltage (V) does not change. G ) becomes stable.

[0044] In short, the voltage of the solution at pH sensor 1 is the voltage to be measured (V) supplied from the power supply 16. T Based on the measurement of the film voltage (V) of the ion-sensitive film 7. F ) is the pH-dependent voltage (V) corresponding to the captured measured ion 102. PH ) and the film control voltage (V) supplied from the measurement film power supply 18. C ) is based on the following. The power control unit 19 measures the voltage to be measured (V) output by the power supply to be measured 16. T ) and the film control voltage (V) output by the measurement film power supply 18 C It controls the relationship with the measured voltage (V). Specifically, the power control unit 19 controls the relationship with the measured voltage (V). T ) for the film control voltage (V C The measurement film power supply 18 is controlled so that the polarity of the ) becomes the same as the polarity of the non-measured ion 103. Measured voltage (V T ) and membrane control voltage (V C According to the relationship, non-measured ions 103 are not attracted to the measured ion-sensitive membrane 7. As a result, drift in the output of the measured ISFET2 caused by the entry of non-measured ions 103 into the measured ion-sensitive membrane 7 can be suppressed. Therefore, the pH sensor 1 can obtain a stable pH over a long period of time.

[0045] [Drift suppression, charging circuit, and storage circuit] The pH sensor 1 shown in Figure 1 performs the operation shown in Figure 3, for example. First, the pH sensor 1 performs an operation to measure the ion concentration (hereinafter referred to as "measurement operation S10"). The duration of measurement operation S10 is, for example, 2 seconds. Next, the pH sensor 1 performs an operation in which it does not measure the ion concentration (hereinafter referred to as "storage operation S30"). The duration of storage operation S30 is, for example, 900 seconds. The pH sensor 1 alternates between measurement operation S10 and storage operation S30.

[0046] The circuit of pH sensor 1 when performing the storage operation S30 is different from the circuit of pH sensor 1 when performing the measurement operation S10. Hereinafter, the circuit of pH sensor 1 when performing the measurement operation S10 will be referred to as the "drift suppression / charging circuit." The circuit of pH sensor 1 when performing the storage operation S30 will be referred to as the "storage circuit." The drift suppression / charging circuit and the storage circuit will be described in detail below.

[0047] As shown in Figure 4, the power supply unit 3 includes a capacitor 71 (potential difference generating unit), a measurement target switch 72, a measurement film switch 73, a ground switch 74, an output switch 75, a circuit board switch 76, and a DC current source 98.

[0048] When performing the preservation operation S30, the capacitor 71 preserves the relationship between the potential of the object to be measured 101 and the potential of the measurement ion-sensitive membrane 7. "Preservation" means that when performing the measurement operation S10, the relationship between the potential of the object to be measured 101 and the potential of the measurement ion-sensitive membrane 7 is maintained. More specifically, "preservation" means that when performing the preservation operation S30, the relationship in equation (4) is satisfied. The capacitor 71 generates a potential difference between the object to be measured electrode 14 and the measurement membrane electrode 17 to satisfy the relationship in equation (4).

[0049] The first end 71a of the capacitor 71 is connected to the electrode 14 to be measured. The first end 71a of the capacitor 71 in the pH sensor 1 of the first embodiment is directly connected to the electrode 14 to be measured. No switch is located between the first end 71a of the capacitor 71 and the electrode 14 to be measured. The second end 71b of the capacitor 71 is connected to the measuring membrane electrode 17. The second end 71b of the capacitor 71 is also directly connected to the measuring membrane electrode 17. No switch is located between the second end 71b of the capacitor 71 and the measuring membrane electrode 17.

[0050] The measurement target switch 72 is positioned between the measurement target electrode 14 and the measurement target power supply 16. The measurement target switch 72 is positioned between the first terminal 71a of the capacitor 71 and the measurement target power supply 16. The measurement target switch 72 switches between a drift suppression / charging circuit and a storage circuit according to the signal φ72 output by the power supply control unit 19. When the measurement target switch 72 is in drift suppression / charging circuit mode, terminal 72a is connected to terminal 72b. As a result, when the drift suppression / charging circuit is in drift suppression / charging circuit mode, the measurement target power supply 16 is connected to the measurement target electrode 14. Furthermore, the measurement target power supply 16 is connected to the capacitor 71. When the storage circuit is in storage circuit mode, terminal 72a of the measurement target switch 72 is disconnected from terminal 72b. As a result, the measurement target power supply 16 is disconnected from the measurement target electrode 14. The measurement target power supply 16 is also disconnected from the capacitor 71.

[0051] The measuring film switch 73 is positioned between the measuring film electrode 17 and the measuring film power supply 18. The measuring film switch 73 is positioned between the second terminal 71b of the capacitor 71 and the measuring film power supply 18. The measuring film switch 73 switches between the drift suppression / charging circuit and the storage circuit according to the signal φ73 output by the power supply control unit 19. When the drift suppression / charging circuit is active, the measuring film switch 73 connects terminal 73a to terminal 73b. As a result, when the drift suppression / charging circuit is active, the measuring film power supply 18 is connected to the measuring film electrode 17. The measuring film power supply 18 is also connected to the capacitor 71. When the storage circuit is active, the measuring film switch 73 connects terminal 73a to terminal 73c. ​​As a result, when the storage circuit is active, the measuring film power supply 18 is disconnected from the measuring film electrode 17. The measuring film power supply 18 is also disconnected from the capacitor 71. When the storage circuit is active, the measuring film electrode 17 is connected to the ground switch 74. The capacitor 71 is also connected to the ground switch 74.

[0052] The grounding switch 74 is located between the measuring ISFET 2 and the reference potential section 99. More specifically, the grounding switch 74 is located between the source 9 of the measuring ISFET 2 and the reference potential section 99. The output terminal 77 is electrically connected to the wiring L1 connecting the source 9 of the measuring ISFET 2 and the grounding switch 74. The output terminal 77 outputs a voltage θ1 corresponding to the ion concentration. A DC current source 98 is located between the grounding switch 74 and the reference potential section 99. This configuration is a so-called source follower circuit.

[0053] The grounding switch 74 switches between the drift suppression / charging circuit and the storage circuit according to the signal φ74 output by the power control unit 19. When the drift suppression / charging circuit is active, the grounding switch 74 connects terminal 74a to terminal 74b. As a result, the source 9 of the measurement ISFET 2 is connected to the reference potential section 99 via the DC current source 98. When the storage circuit is active, the grounding switch 74 connects terminal 74a to terminal 74c. As a result, the source 9 of the measurement ISFET 2 is connected to the measurement film switch 73. When the storage circuit is active, the grounding switch 74 also connects terminal 74a to terminal 74d. As a result, the source 9 of the measurement ISFET 2 is connected to the reference potential section 99 without going through the DC current source 98.

[0054] The output switch 75 is located between the measurement ISFET 2 and the output terminal 77. The output switch 75 is located between the board power supply 12 and the reference potential section 99. The output switch 75 switches between the drift suppression / charging circuit and the storage circuit according to the signal φ75 output by the power control unit 19. When the drift suppression / charging circuit is active, the output switch 75 connects terminal 75a to terminal 75b. As a result, the source 9 of the measurement ISFET 2 is connected to the output terminal 77. When the storage circuit is active, the output switch 75 connects terminal 95a to terminal 75d. As a result, the source 9 of the measurement ISFET 2 is connected to the reference potential section 99. When the storage circuit is active, the output switch 75 also connects terminal 75c to terminal 75d. As a result, the drain 11 of the measurement ISFET 2 is connected to the reference potential section 99 via the board switch 76.

[0055] The board switch 76 is located between the measurement ISFET 2 and the board power supply 12. The board switch 76 switches between the drift suppression / charging circuit and the storage circuit according to the signal φ76 output by the power control unit 19. When the drift suppression / charging circuit is active, the board switch 76 connects terminal 76a to terminal 75b. As a result, the positive terminal of the board power supply 12 is connected to the drain 11 of the measurement ISFET 2. When the storage circuit is active, the board switch 76 disconnects terminal 76a from terminal 75b. As a result, the positive terminal of the board power supply 12 is disconnected from the drain 11 of the measurement ISFET 2. Furthermore, the board switch 76 connects terminal 76a to terminal 76c. As a result, the drain 11 of the measurement ISFET 2 is connected to the reference potential unit 99 via the output switch 75.

[0056] When the drain 11 of the measurement ISFET2 is connected to the reference potential section 99 via the output switch 75, the source 9 of the measurement ISFET2 is also connected to the reference potential section 99 via the switch 74. The substrate 4 of the measurement ISFET2 is always connected to the reference potential section 99. When it is a storage circuit, the source 9, drain 11, and substrate 4 of the measurement ISFET2 are all connected to the reference potential section 99. When it is a storage circuit, the potential of the source 9, the potential of the drain 11, and the potential of the substrate 4 of the measurement ISFET2 are the same as each other.

[0057] In the above explanation, the measurement target switch 72, the measurement film switch 73, the ground switch 74, the output switch 75, and the board switch 76 were each explained individually. Next, the overall circuit configuration when it is a drift suppression / charging circuit and the overall circuit configuration when it is a storage circuit will be explained.

[0058] [Drift suppression and charging circuit] Figure 5 shows the overall circuit configuration when the circuit is configured as a drift suppression and charging circuit. In Figure 5, the dashed lines indicate connections that do not function when the circuit is configured as a drift suppression and charging circuit. The states of each switch when the circuit is configured as a drift suppression and charging circuit are as follows: The switch 72 to be measured: The power supply 16 to be measured is connected to the electrode 14 to be measured, and the power supply 16 to be measured is also connected to the capacitor 71. Measurement film switch 73: The measurement film power supply 18 is connected to the measurement film electrode 17 and also to the capacitor 71. Grounding switch 74: Connects the source 9 of the measuring ISFET 2 to the reference potential section 99. Output switch 75: Connect the source 9 of the measurement ISFET2 to the output terminal 77. Board switch 76: Connects the board power supply 12 to the drain 11 of the measurement ISFET2.

[0059] According to the state of the switch described above, a predetermined potential difference is generated between the electrode to be measured 14 and the measurement film electrode 17. As a result, the intrusion of non-measured ions 103 can be suppressed. According to the state of the switch described above, the capacitor 71 is charged according to the potential difference between the power supply to be measured 16 and the measurement film power supply 18. According to the state of the switch described above, a source-drain current 200 is generated according to the gate voltage of the measurement ISFET 2. As a result, an output voltage θ1 corresponding to the source-drain current 200 is provided to the output terminal 77.

[0060] [Save circuit] Figure 6 shows the overall circuit configuration when it is a storage circuit. In Figure 6, the dashed lines indicate connections that are not functioning when it is a storage circuit. The state of each switch when it is a storage circuit is as follows: The switch 72 to be measured disconnects the power supply 16 to be measured from the electrode 14 and also disconnects the power supply 16 to be measured from the capacitor 71. Measurement film switch 73: Disconnects the measurement film power supply 18 from the measurement film electrode 17 and also disconnects the measurement film power supply 18 from the capacitor 71. Connect the measurement film electrode 17 to the ground switch 74 and also connect the capacitor 71 to the ground switch 74. Grounding switch 74: Connects the measuring film switch 73 to the reference potential unit 99 and also connects the source 9 of the measuring ISFET 2 to the reference potential unit 99. Output switch 75: The source 9 of the measurement ISFET2 is connected to the reference potential section 99, and the board switch 76 is also connected to the reference potential section 99. Board switch 76: Disconnects the board power supply 12 from the drain 11 of the measurement ISFET2.

[0061] Depending on the state of the switch described above, a potential difference can be generated between the electrode to be measured 14 and the measuring film electrode 17 due to the charge present in the capacitor 71.

[0062] According to the switch state described above, the second terminal 71b of capacitor 71 is connected to the reference potential section 99. The measuring film electrode 17 is also connected to the reference potential section 99. The source 9 and drain 11 of measuring ISFET 2 are also connected to the reference potential section 99. In other words, the potentials of the measuring film electrode 17, the source 9 of measuring ISFET 2, and the drain 11 of measuring ISFET 2 can be made equal to each other.

[0063] According to the state of the switches described above, the source 9 and drain 11 of the measuring ISFET2 form a closed circuit. This closed circuit is connected to the reference potential section 99. Therefore, the potential of the closed circuit is fixed at the reference potential. Consequently, the influence of external noise can be suppressed.

[0064] [Operation] The operation of pH sensor 1 will be explained with reference to the flow chart in Figure 7.

[0065] First, pH sensor 1 performs a measurement operation (S10). The measurement operation (S10) includes switching the circuit (S11), setting the drift suppression voltage (S12), and obtaining a voltage corresponding to the ion concentration (S13).

[0066] Specifically, pH sensor 1 outputs signals φ72~φ77 from power control unit 19. As a result, pH sensor 1 switches to the drift suppression / charging circuit (see Figure 5) (S11).

[0067] Next, the pH sensor 1 performs an operation to set the drift suppression voltage (S12). The voltage of the power supply 16 for measurement and the voltage of the measurement membrane power supply 18 are determined according to the polarity of the non-measured ions 103. For example, assume that the non-measured ions 103 are hydroxide ions. Hydroxide ions have a negative charge. In this case, the voltages of the power supply 16 for measurement and the measurement membrane power supply 18 are set so that the potential of the measurement ion-sensitive membrane 7 is lower than the potential of the measurement target 101. The drift suppression voltage is determined by the voltage output from the power supply 16 for measurement and the voltage output from the measurement membrane power supply 18. If the non-measured ions 103 have a positive charge, the voltages of the power supply 16 for measurement and the measurement membrane power supply 18 are set so that the potential of the measurement ion-sensitive membrane 7 is higher than the potential of the measurement target 101.

[0068] For example, if the measured ion 102 is a hydrogen ion, the voltage of the power supply 16 to be measured may be set to 2000mV. For the pH sensor 1, for example, the voltage of the measuring membrane power supply 18 may be set to 1500mV. As a result, the potential difference between the electrode to be measured 14 and the measuring membrane electrode 17 is 500mV. The potential of the measuring membrane electrode 17 is 500mV lower than the potential of the electrode to be measured 14.

[0069] The larger the absolute value of the potential difference between the electrode to be measured 14 and the measurement membrane electrode 17, the greater the effect of suppressing the intrusion of non-measured ions 103. On the other hand, the voltage applied to the measurement membrane electrode 17 (membrane voltage (V) T )) is the gate voltage (V) that affects the output voltage of pH sensor 1, as shown in equation (1). G This relates to the measurement accuracy of pH sensor 1. From the standpoint of accuracy, it is desirable to be able to calculate the membrane potential accurately. The membrane potential is obtained by dividing the output voltage (θ1) by the amplification factor. In order to calculate the membrane potential accurately, it is good to increase the amplification factor of the measurement ISFET 2. The amplification factor is the gate voltage (V G ) has an effect. Specifically, the gate voltage (V G The higher the gate voltage (V), the higher the amplification factor of pH sensor 1. G To increase the voltage of the measuring film electrode 17 (film voltage (V) TSet the voltage to a higher value. For example, the voltage of the measuring film electrode 17 may be set to 1500mV.

[0070] This voltage setting allows for the suppression of noise's influence on the output voltage (θ1). As a result, measurement accuracy can be improved.

[0071] Next, an operation (S13) is performed to obtain a voltage corresponding to the ion concentration. This operation S13 includes a drift suppression operation (S13a), a charging operation (S13b), and a measurement voltage acquisition operation (S13c) (see Figure 3).

[0072] The pH sensor 1 outputs a predetermined voltage from the measurement target power supply 16 by outputting a signal φ16 from the power control unit 19. Furthermore, the pH sensor 1 outputs a predetermined voltage from the measurement membrane power supply 18 by outputting a signal φ18 from the power control unit 19. The output of these voltages initiates the drift suppression operation (S13a) and the charging operation (S13b). In other words, in the pH sensor 1 of the first embodiment, the timing at which the charging operation (S13b) is initiated always coincides with the timing at which the drift suppression operation (S13a) is initiated. In this specification, "charging" refers to the state in which the capacitor 71 is receiving voltage from the measurement target power supply 16 and the measurement membrane power supply 18. In other words, "charging" does not depend on whether or not the accumulation of charge corresponding to the capacitance of the capacitor 71 has been completed.

[0073] When voltage output is started from both the measurement target power supply 16 and the measurement film power supply 18, the effect of suppressing the intrusion of non-measurement ions 103 is achieved. Furthermore, charging of the capacitor 71 is also started. When a potential difference is generated between the measurement target electrode 14 and the measurement film electrode 17, a potential difference is also generated between the first end 71a and the second end 71b of the capacitor 71. The potential difference generated in the capacitor 71 is equal to the potential difference between the measurement target electrode 14 and the measurement film electrode 17. As a result, charge accumulation occurs in proportion to the potential difference. In other words, the capacitor 71 is charged.

[0074] The pH sensor 1 outputs a predetermined voltage from the substrate power supply 12 when the power control unit 19 outputs a signal φ12. This voltage output initiates the measurement voltage acquisition operation (S13c). For example, the output voltage of the substrate power supply 12 may be 1500mV. A state is formed where a source-drain voltage is applied between the source 9 and drain 11 of the measurement ISFET 2. In this state, an output voltage θ1 corresponding to the gate voltage of the measurement ISFET 2 is output to the output terminal 77. The pH sensor 1 continues to output the voltage from the substrate power supply 12 for a predetermined measurement time. The duration may be, for example, 2 seconds. After the duration has elapsed, the substrate power supply 12, the power supply to be measured 16, and the power supply to the measurement film 18 stop outputting voltages.

[0075] As previously mentioned, the timing at which the charging operation (S13b) begins always coincides with the timing at which the drift suppression operation (S13a) begins. The timing at which these operations begin may coincide with the timing at which the voltage acquisition operation (S13c) begins, or they may be different.

[0076] For example, as shown in Figure 3, the drift suppression operation (S13a), the charging operation (S13b), and the voltage acquisition operation (S13c) may be started simultaneously. The voltage output may also be started simultaneously from the substrate power supply 12, the power supply to be measured 16, and the power supply to the measurement film 18. With this operation, the drift suppression operation (S13a), the charging operation (S13b), and the voltage acquisition operation (S13c) overlap throughout the entire measurement period.

[0077] The voltage acquisition operation (S13c) may be started after the drift suppression operation (S13a) and the charging operation (S13b) have been started. Alternatively, the voltage output from the substrate power supply 12 may be started after the voltage output from the power supply 16 to be measured and the power supply 18 to the measurement film have been started. This operation results in a period during which the drift suppression operation (S13a) and the charging operation (S13b) overlap. Subsequently, there is a period during which the drift suppression operation (S13a), the charging operation (S13b), and the voltage acquisition operation (S13c) overlap.

[0078] Next, the pH sensor 1 starts the storage operation (S30). Specifically, first, the pH sensor 1 outputs signals φ72~φ77 from the power control unit 19. As a result, it switches from the drift suppression / charging circuit (see Figure 5) to the storage circuit (see Figure 6) (S31). Switching from the drift suppression / charging circuit to the storage circuit switches the element that generates a potential difference between the electrode to be measured 14 and the measuring membrane electrode 17. Specifically, the element that generates a potential difference between the electrode to be measured 14 and the measuring membrane electrode 17 switches from the measurement target power supply 16 and the measuring membrane power supply 18 to the capacitor 71. The capacitor 71 generates the same potential difference between the electrode to be measured 14 and the measuring membrane electrode 17 as during measurement. Specifically, the capacitor 71 generates a potential difference in which the potential of the measuring membrane electrode 17 is 500mV lower than the potential of the electrode to be measured 14.

[0079] When the circuit is in storage mode, no external energy supply is required to maintain the potential difference between the electrode 14 being measured and the film electrode 17 being measured. This is because, when the circuit is in storage mode, the electrode 14 being measured and the film electrode 17 being measured are electrically insulated from each other. Therefore, the charge stored in the capacitor 71 cannot, in principle, move between the electrode 14 being measured and the film electrode 17 being measured. As a result, discharge from the capacitor 71 does not occur in principle. Consequently, the state of the charge stored in the capacitor 71 is maintained. As a result, the potential difference between the electrode 14 being measured and the film electrode 17 being measured can be maintained without requiring an external energy supply.

[0080] When the circuit is in storage mode, a potential difference exists between the electrode 14 being measured and the measuring membrane electrode 17. However, the electrode 14 being measured and the measuring membrane electrode 17 are electrically insulated from each other. Therefore, no current flows between the electrode 14 being measured and the measuring membrane electrode 17. The source 9 and drain 11 of the measuring ISFET 2 are short-circuited by the ground switch 74 and the output switch 75. The closed circuit formed by the short circuit is connected to the reference potential section 99. Therefore, when the circuit is in storage mode, no current continues to flow through these closed circuits. As a result, the lifespan of the pH sensor 1 can be extended.

[0081] pH sensor 1 maintains the configuration of the storage circuit for a predetermined storage time (S32). The storage time may be, for example, 900 seconds. After the storage time has elapsed, pH sensor 1 switches from the storage circuit to the drift suppression / charging circuit (S11).

[0082] [Effects and Effects] The pH sensor 1, which is an ion concentration measuring device, is installed on a measurement target 101 that includes measurement ions 102 and non-measurement ions 103. The pH sensor 1 obtains the concentration of measurement ions 102. The pH sensor 1 is located on the measurement target 101 and includes a measurement target electrode 14 that controls the potential of the measurement target, a measurement target power supply 16 that supplies voltage to the measurement target electrode 14, a measurement ISFET 2 including a measurement ion sensitive membrane 7 that generates a voltage corresponding to the concentration of measurement ions 102, a measurement membrane electrode 17 located on the measurement ion sensitive membrane 7 that controls the potential of the measurement ion sensitive membrane 7, a measurement membrane power supply 18 that supplies voltage to the measurement membrane electrode 17, a capacitor 71 connected to the measurement target electrode 14 and the measurement membrane electrode 17 that creates a potential difference between the measurement target electrode 14 and the measurement membrane electrode 17, and a power control unit 19 that controls the magnitude of the voltage output from the measurement target power supply 16 and the magnitude of the voltage output from the measurement membrane power supply 18.

[0083] The pH sensor 1 generates a potential difference between the measurement target electrode 14, which is placed on the object to be measured 101, and the measurement membrane electrode 17, which is placed on the measurement ion-sensitive membrane 7 of the measurement ISFET 2. The electric field based on the potential difference suppresses the penetration of non-measured ions 103 into the measurement ion-sensitive membrane 7. As a result, the voltage drift output from the measurement ISFET 2 is suppressed. The potential difference between the measurement target electrode 14 and the measurement membrane electrode 17 can be generated by the measurement target power supply 16 and the measurement membrane power supply 18. The potential difference between the measurement target electrode 14 and the measurement membrane electrode 17 can also be generated by the capacitor 71. It is not always necessary to use the measurement target power supply 16 and the measurement membrane power supply 18 for the potential difference between the measurement target electrode 14 and the measurement membrane electrode 17. Therefore, it is possible to reduce the energy required to suppress the output voltage drift. As a result, a good output with suppressed drift can be obtained over a long period of time.

[0084] For example, when no voltage is supplied to the measurement film power supply 18, the operation (threshold voltage) of the measurement ISFET2 is determined by the potential difference between the power supply 16 being measured and the substrate 4. When the measurement ISFET2 is in the ON state, current flows between the drain 11 and the source 9. Therefore, the voltage of channel 13 is the midpoint between the voltage of drain 11 and the voltage of source 9. At this time, the voltage of the substrate power supply 12 is greater than the voltage of the power supply 16 being measured (threshold voltage < voltage of power supply 16 being measured < voltage of substrate power supply 12). As a result, the measurement ISFET2 may not be able to sufficiently suppress the drift described later.

[0085] The measurement film power supply 18 makes it possible to satisfy both the conditions for suppressing output voltage drift (described later) and the conditions for operating the measurement ISFET 2. The condition for suppressing output voltage drift is that the voltage of the measurement film power supply 18 is smaller than the voltage of the power supply 16 being measured (voltage of measurement film power supply 18 < voltage of power supply 16 being measured). The condition for operating the measurement ISFET 2 is that the threshold voltage is smaller than the voltage of the measurement film power supply 18, AND the voltage of the measurement film power supply 18 is smaller than the voltage of the substrate power supply 12 (threshold voltage < voltage of measurement film power supply 18 < voltage of substrate power supply 12). Therefore, the measurement film power supply 18 eliminates non-uniformity in the measurement ion-sensitive film 7. As a result, drift suppression becomes easier. Therefore, drift can be sufficiently suppressed. Furthermore, the voltage of the substrate power supply 12, which is the voltage of the drain 11, can be set to a desired value.

[0086] The pH sensor 1 has a capacitor 71 as a potential difference generating unit. This configuration makes it possible to suppress energy consumption required to suppress drift.

[0087] The power control unit 19 switches between a first operation, which generates a first potential difference between the electrode to be measured 14 and the measurement film electrode 17 using the power supply 16 and the measurement film power supply 18, and a second operation, which generates a second potential difference between the electrode to be measured 14 and the measurement film electrode 17 using the capacitor 71. The second operation makes it possible to generate a potential difference to suppress drift without using the power supply 16 and the measurement film power supply 18. Therefore, energy consumption by the power supply 16 and the measurement film power supply 18 can be suppressed.

[0088] When the first operation is in progress, the power control unit 19 allows the output of a voltage corresponding to the concentration of the measured ion 102 from the measurement ISFET 2. When the second operation is in progress, the power control unit 19 prohibits the output of a voltage corresponding to the concentration of the measured ion 102 from the measurement ISFET 2. According to these operations, when measurement is performed and the output of a voltage corresponding to the concentration of the measured ion 102 is permitted, the power supply for the device being measured 16 and the power supply for the measuring film 18 can generate a first potential difference. When measurement is not performed and the output of a voltage corresponding to the concentration of the measured ion 102 is prohibited, the capacitor 71 can generate a second potential difference.

[0089] When the first operation is in progress, the power control unit 19 performs a measurement operation (S10) that allows the measurement ISFET2 to output a voltage corresponding to the concentration of the measured ion 102, and a charging operation (S30) that charges the capacitor 71, in parallel. This operation allows the measurement operation (S10) and the charging operation (S30) to be performed simultaneously.

[0090] The second potential difference is equal to the first potential difference. This setting simplifies voltage control.

[0091] The measurement ISFET2 is an ion-selective field-effect transistor having a substrate 4, an insulating film 6 provided on the substrate 4, and a measurement ion-sensitive film 7 provided on the insulating film 6. With this configuration, an output voltage corresponding to the ion concentration can be obtained.

[0092] In the first operation, a voltage is applied between the source 9 and drain 11 of the measurement ISFET2. In the second operation, the source 9 and drain 11 of the measurement ISFET2 are connected to the reference potential section 99. According to the first operation, an output voltage (θ1) corresponding to the ion concentration can be obtained. According to the second operation, the influence of external noise can be suppressed during periods when no measurements are being taken.

[0093] In the second operation, the measuring film electrode 17 may be connected to the reference potential unit 99. This operation also helps to suppress the influence of external noise during periods when no measurements are being taken.

[0094] The potential of the reference potential section 99 to which the source 9 and drain 11 of the measurement ISFET2 are connected is the same as the potential of the reference potential section 99 to which the measurement film electrode 17 is connected. This simple circuit configuration makes it possible to suppress the influence of external noise.

[0095] [Second Embodiment] The ion concentration measuring device of the second embodiment will now be described. Figure 8 is a timing chart showing the operation of the ion concentration measuring device of the second embodiment (pH sensor 1A: see Figure 9).

[0096] As described above, in the ion concentration measuring device (pH sensor 1) of the first embodiment, the charging operation (S13b) overlapped with the drift suppression operation (S13a). In such an operation, the charging voltage of the capacitor 71 matches the voltage of the drift suppression operation (S13a), which is performed in parallel with the measurement voltage acquisition operation (S13c).

[0097] In contrast, the pH sensor 1A of the second embodiment has a charging operation (S23b) that does not overlap with the measurement operation (S10A). In such an operation, the charging voltage of the capacitor 71 does not necessarily match the voltage of the drift suppression operation (S13a) which is performed in parallel with the measurement voltage acquisition operation (S13c). Therefore, it is possible to set the charging voltage of the capacitor 71 to a value different from the voltage of the drift suppression operation (S13a) which is performed in parallel with the measurement voltage acquisition operation (S13c). In other words, the characteristics of the potential difference generated using the power supply 16 to be measured and the power supply 18 to be measured can be made different from the characteristics of the potential difference generated using the capacitor 71.

[0098] Figure 9 is a circuit diagram of the pH sensor 1A of the second embodiment. As shown in Figure 9, the pH sensor 1A has a power supply unit 3A in which capacitor switches 78 and 79 are added to the configuration of the pH sensor 1.

[0099] A connection point P2 is provided in the wiring L2 connecting the electrode 14 to be measured and the switch 72 to be measured. The connection point P2 is connected to the first end 71a of the capacitor 71. The capacitor switch 78 is provided between the connection point P2 and the first end 71a of the capacitor 71. The capacitor switch 78 operates based on the signal φ78 from the power control unit 19.

[0100] A connection point P3 is provided in the wiring L3 connecting the measuring film electrode 17 and the measuring film switch 73. The connection point P3 is connected to the second terminal 71b of the capacitor 71. The capacitor switch 79 is provided between the connection point P3 and the second terminal 71b of the capacitor 71. The capacitor switch 79 operates based on the signal φ79 from the power supply control unit 19.

[0101] As in the first embodiment, if capacitor switches 78 and 79 are not provided, two connection configurations can be realized. The first connection configuration is a drift suppression / charging circuit (see Figure 5). In the first connection configuration, the electrode 14 to be measured and the capacitor 71 are connected to the power supply 16 to be measured. Furthermore, in the first connection configuration, the film electrode 17 and the capacitor 71 are connected to the film power supply 18. The second connection configuration is a storage circuit (see Figure 6). In the second connection configuration, the electrode 14 to be measured and the capacitor 71 are disconnected from the power supply 16 to be measured. In the second connection configuration, the film electrode 17 and the capacitor 71 are disconnected from the film power supply 18.

[0102] As in the second embodiment, when capacitor switches 78 and 79 are provided, a third connection configuration can be realized in addition to the first and second connection configurations. The third connection configuration is a drift suppression circuit. The drift suppression circuit performs a drift suppression function. However, the drift suppression circuit does not charge the capacitor 71.

[0103] As shown in Figure 10, the third connection configuration connects the electrode 14 to be measured to the power supply 16 and disconnects the capacitor 71 from the power supply 16. The third configuration connects the film electrode 17 to the film power supply 18 and disconnects the capacitor 71 from the film power supply 18.

[0104] [Drift suppression circuit] Figure 10 shows the overall circuit configuration when the circuit is configured as a drift suppression circuit. In Figure 10, the dashed lines indicate the parts that are not functioning when the circuit is configured as a drift suppression circuit. The states of each switch when the circuit is configured as a drift suppression circuit are as follows: The switch 72 to be measured: connect the power supply 16 to be measured to the electrode 14. Measurement film switch 73: Connects the measurement film power supply 18 to the measurement film electrode 17. Grounding switch 74: Connects the source 9 of the measuring ISFET 2 to the reference potential section 99. Output switch 75: Connect the source 9 of the measurement ISFET2 to the output terminal 77. Board switch 76: Connects the board power supply 12 to the drain 11 of the measurement ISFET2. Capacitor switch 78: Disconnects the first terminal 71a of capacitor 71 from connection point P2. Capacitor switch 79: Disconnects the second terminal 71b of capacitor 71 from connection point P3.

[0105] In the third connection configuration, when the measurement voltage acquisition operation (S13c) and the drift suppression operation (S13a) are performed in parallel, the power receiving operation (S23b) of the capacitor 71 is not performed. After the measurement voltage acquisition operation (S13c) is completed, the charging operation (S23b) of the capacitor 71 is performed by switching to the first connection configuration s.

[0106] For example, in the drift suppression operation (S13a) performed simultaneously with the measurement voltage acquisition operation (S13c), the voltage is set to generate a first potential difference. In the third connection configuration, no voltage is applied to the capacitor 71. Therefore, the capacitor 71 is not charged. When the charging operation (S23b) is performed together with the drift suppression operation (S23a) after the measurement voltage acquisition operation (S13c) is completed, it is possible to apply a voltage different from that of the drift suppression operation (S13a) to the capacitor 71. Therefore, the capacitor 71 can be charged based on a voltage different from that of the drift suppression operation (S13a).

[0107] Figure 11 shows the overall circuit configuration when it is the first connection configuration, the drift suppression and charging circuit. The state of each switch when it is the drift suppression and charging circuit is as follows: The switch 72 to be measured: The power supply 16 to be measured is connected to the electrode 14 to be measured, and the power supply 16 to be measured is also connected to the capacitor 71. Measurement film switch 73: The measurement film power supply 18 is connected to the measurement film electrode 17 and also to the capacitor 71. Grounding switch 74: Connects the source 9 of the measuring ISFET 2 to the reference potential section 99. Output switch 75: Connect the source 9 of the measurement ISFET2 to the output terminal 77. Board switch 76: Connects the board power supply 12 to the drain 11 of the measurement ISFET2. Capacitor switch 78: Connects the first terminal 71a of capacitor 71 to connection point P2. Capacitor switch 79: Connects the second terminal 71b of capacitor 71 to connection point P3.

[0108] Figure 12 shows the overall circuit configuration when it is a storage circuit, which is the second connection configuration. The state of each switch when it is a charging circuit is as follows: The switch 72 to be measured disconnects the power supply 16 to be measured from the electrode 14 and also disconnects the power supply 16 to be measured from the capacitor 71. Measurement film switch 73: Disconnects the measurement film power supply 18 from the measurement film electrode 17 and also disconnects the measurement film power supply 18 from the capacitor 71. Connect the measurement film electrode 17 to the ground switch 74 and also connect the capacitor 71 to the ground switch 74. Grounding switch 74: Connects the measuring film switch 73 to the reference potential unit 99 and also connects the source 9 of the measuring ISFET 2 to the reference potential unit 99. Output switch 75: The source 9 of the measurement ISFET2 is connected to the reference potential section 99, and the board switch 76 is also connected to the reference potential section 99. Board switch 76: Disconnects the board power supply 12 from the drain 11 of the measurement ISFET2. Capacitor switch 78: Connects the first terminal 71a of capacitor 71 to connection point P2. Capacitor switch 79: Connects the second terminal 71b of capacitor 71 to connection point P3.

[0109] [Operation] An example of the operation of the pH sensor 1A in the second embodiment will be explained with reference to the flowchart in Figure 13.

[0110] pH sensor 1 performs a measurement operation (S10A). The measurement operation (S10A) includes switching the circuit (S11A), setting the drift suppression voltage (S12), and obtaining a voltage corresponding to the ion concentration (S13A).

[0111] In the circuit switching operation (S11A), the circuit is switched to the drift suppression circuit shown in Figure 10. The operation to set the drift suppression voltage (S12) is the same as in the first embodiment, so a detailed explanation is omitted.

[0112] Next, an operation (S13A) is performed to obtain a voltage corresponding to the ion concentration. Operation S13A includes a drift suppression operation (S13a) and a measurement voltage acquisition operation (S13c) (see Figure 8). The operation (S13A) of the second embodiment does not include a charging operation. After a predetermined time has elapsed, the voltage output from the substrate power supply 12, the power supply for the device to be measured 16, and the power supply for the measurement film 18 is stopped.

[0113] Next, the charging operation (S20) is performed. Specifically, the pH sensor 1A outputs signals φ72 to φ79 from the power control unit 19. As a result, the system switches from the drift suppression circuit (see Figure 10) to the drift suppression and charging circuit (see Figure 11) (S21). Next, the charging voltage is set (S22). The charging voltage is determined by the voltage output from the power supply 16 to be measured and the voltage output from the measuring membrane power supply 18. Therefore, in operation (S20), the voltage output from the power supply 16 to be measured is set by signal φ16. In operation (S20), the voltage output from the measuring membrane power supply 18 is set by signal φ18.

[0114] Next, voltage output is started from the power supply 16 to be measured and from the measurement film power supply 18. As a result, charging of the capacitor 71 begins. In the drift suppression / charging circuit (Figure 11), the power supply 16 to be measured is connected to the electrode 14 to be measured. Furthermore, the measurement film power supply 18 is connected to the measurement film electrode 17. Therefore, a drift suppression function is also generated. After a predetermined time has elapsed, voltage output is stopped from the power supply 16 to be measured and the measurement film power supply 18.

[0115] Next, the pH sensor 1 starts the storage operation (S30). The storage operation (S30) is the same as in the first embodiment, so a detailed explanation is omitted.

[0116] [Effects and Effects] The pH sensor 1A of the second embodiment, like the pH sensor 1 of the first embodiment, can also provide a good output over a long period of time.

[0117] Furthermore, when the pH sensor 1A is in the first operation, the power control unit 19 performs the measurement operation (S10) before the charging operation (S20) of the measurement operation (S10), which allows the output of a voltage corresponding to the concentration of the measured ion 102 from the measurement ISFET 2, and the charging operation (S20), which charges the capacitor 71. This operation allows the timing of the measurement operation (S10) and the charging operation (S20) to be staggered.

[0118] [Differentiation] The present invention has been described in detail above based on its embodiments. However, the present invention is not limited to the above embodiments. The present invention can be modified in various ways without departing from its spirit.

[0119] For example, in the first embodiment described above, the unmeasured ions 103 have a negative charge. The unmeasured ions 103 may also have a positive charge. In this case, the power control unit 19 controls at least one of the power supply to be measured 16 and the measurement film power supply 18 so that the membrane control voltage (VC) is greater than the voltage to be measured (VT). With this configuration, drift caused by the unmeasured ions 103, which have a positive polarity, can be suppressed.

[0120] In the first embodiment, hydrogen ions were used as an example of the positively charged measurement ion 102. The measurement ion 102 may be any other positively charged ion (positive ion). For example, potassium ions can be used as an example of a positively charged measurement ion 102. If the ion to be measured has a positive charge, the potential of the measurement membrane electrode 17 should be lowered than the potential of the measurement target electrode 14. In the first embodiment, a positively charged measurement ion 102 was used as an example, but ions with a negative charge may also be used as the measurement target. Examples of negatively charged ions (negative ions) include hydroxide ions and chloride ions. If the ion to be measured has a negative charge, the potential of the measurement membrane electrode 17 should be higher than the potential of the measurement target electrode 14.

[0121] The application of ion concentration measuring devices is not limited to measuring soil pH. For example, ion concentration measuring devices may be applied to measuring pH in culture media. Ion concentration measuring devices may also be applied to measuring pH in concrete.

[0122] The application of ion concentration measuring devices is not limited to pH measurement. For example, they can be suitably applied to sensors that perform ion adsorption and desorption, such as potassium (K) ion sensors in soil.

[0123] In the above embodiment, a capacitor was used as an example of a potential difference generating unit. However, a different configuration may be used for the potential difference generating unit instead of a capacitor. For example, the potential difference generating unit may be powered by a power source separate from the power supply 16 to be measured and the power supply 18 to the measurement film. A power source that can be used as a potential difference generating unit may include, for example, a secondary battery, a rechargeable battery, or a battery power supply.

[0124] In the above embodiment, when it is a storage circuit, a configuration was shown in which the source 9 and drain 11 of the measurement ISFET2 and the substrate 4 are connected to a reference potential unit 99. In the above embodiment, when it is a storage circuit, the source 9 and drain 11 of the measurement ISFET2 and the substrate 4 are at a common potential (e.g., ground potential) provided by the reference potential unit 99.

[0125] As long as equation (4) is satisfied, the pH sensor 1 does not require the source 9 and the substrate 4 to be at the same potential. The potentials of the source 9 and the substrate 4 may be the same as shown in the above embodiment, or they may be different. Of these, it is more advantageous for the source 9 and the substrate 4 to be at the same potential as shown in the above embodiment than for them to be different. Furthermore, when the source 9 and the substrate 4 are at the same potential, the potential may be fixed or variable, as shown in the above embodiment.

[0126] In the above embodiment, the source 9 and the substrate 4 are set to the same reference potential. If the reference potential is the ground potential, then in the above embodiment, the polarity of the non-measured ions 103 that can be suppressed is determined to be negative. In other words, when the polarity of the non-measured ions 103 is negative, the circuit configuration shown in Figure 1 can be used. On the other hand, the circuit configuration shown in Figure 1 cannot handle cases where the polarity of the non-measured ions 103 is positive. For example, if a circuit configuration like the pH sensor 1B shown in Figure 14 is adopted, the polarity of the measured ions 102 can be measured not only when it is positive but also when it is negative.

[0127] The pH sensor 1B shown in Figure 14 has a circuit board 4 directly connected to the source 9 by wiring L4. With this connection configuration, the potential of the circuit board 4 is always the same as the potential of the source 9. Specifically, when it is a storage circuit, the potential of the circuit board 4 is the same as the potential of the source 9. Even when it is a drift suppression / charging circuit, the potential of the circuit board 4 is the same as the potential of the source 9. By appropriately setting the connections of the ground switch 74 and the output switch 75, the circuit board 4 and the source 9 can be connected to the reference potential unit 99. The circuit board 4 and the source 9 can also be disconnected from the reference potential unit 99.

[0128] As shown in Figure 14, the potential of the substrate 4 of the pH sensor 1B follows the potential of the source 9. When the drift suppression / charging circuit is in operation (during measurement), the potential of the source 9 is determined according to the source-drain voltage supplied by the substrate power supply 12. As a result, the potential of the substrate 4 is determined according to the potential of the source 9. When the storage circuit is in operation (non-measurement) and is disconnected from the reference potential section 99, the potential of the source 9 is determined according to the voltage supplied by the capacitor 71. As a result, the potential of the substrate 4 is determined according to the potential of the source 9. With this connection configuration, the potential of the substrate 4 and the potential of the source 9 are not fixed to a reference potential. Therefore, the potential can be set according to the polarity of the non-measured ions 103.

[0129] [Note] This disclosure includes the following components:

[0130] The ion concentration measuring device of the present disclosure is [1] "an ion concentration measuring device installed on a target for measurement including a target ion and a non-target ion to obtain the concentration of the target ion, comprising: a target electrode disposed on the target for measurement and controlling the potential of the target; a power supply for measurement that provides a voltage to the target electrode; a measurement sensor unit including a target ion sensitive membrane that generates a voltage corresponding to the concentration of the target ion; a membrane electrode disposed on the target ion sensitive membrane and controlling the potential of the target ion sensitive membrane; a membrane power supply that provides a voltage to the membrane electrode; a potential difference generating unit connected to the target electrode and the membrane electrode to generate a potential difference between the target electrode and the membrane electrode; and a power control unit that controls the magnitude of the voltage output from the power supply for measurement and controls the magnitude of the voltage output from the membrane power supply, wherein the ion concentration measuring device comprises."

[0131] The ion concentration measuring device of the present disclosure is [2] "the ion concentration measuring device according to [1] above, wherein the potential difference generating unit is a capacitor."

[0132] The ion concentration measuring apparatus of the present disclosure is [3] "the ion concentration measuring apparatus according to [1] or [2] above, wherein the power control unit switches between a first operation that generates a first potential difference between the electrode to be measured and the measuring membrane electrode using the power supply to be measured and the measuring membrane power supply, and a second operation that generates a second potential difference between the electrode to be measured and the measuring membrane electrode using the potential difference generating unit."

[0133] The ion concentration measuring device of the present disclosure is [4] "the ion concentration measuring device according to [3] above, wherein the power control unit permits the output of a voltage corresponding to the concentration of the measured ion from the measurement sensor unit when the first operation is in progress, and prohibits the output of a voltage corresponding to the concentration of the measured ion from the measurement sensor unit when the second operation is in progress."

[0134] The ion concentration measuring device of the present disclosure is [5] "the ion concentration measuring device according to [3] or [4] above, wherein the power control unit, when in the first operation, performs in parallel a measurement operation that permits the output of a voltage corresponding to the concentration of the measured ion from the measurement sensor unit and a charging operation that charges the potential difference generating unit."

[0135] The ion concentration measuring device of this disclosure is [6] "the ion concentration measuring device according to [5] above, wherein the second potential difference is equal to the first potential difference."

[0136] The ion concentration measuring device of the present disclosure is [7] "the ion concentration measuring device according to [3] or [4] above, wherein when the first operation is in progress, the power control unit performs one of the following operations before the other: a measurement operation which permits the output of a voltage corresponding to the concentration of the measured ion from the measurement sensor unit, and a charging operation which charges the potential difference generating unit."

[0137] The ion concentration measuring device of this disclosure is [8] "the ion concentration measuring device according to [7] above, wherein the second potential difference is different from the first potential difference."

[0138] The ion concentration measuring device of the present disclosure is [9] "the ion concentration measuring device according to any one of the above [3] to [8], wherein the measuring sensor unit is an ion-selective field-effect transistor having a substrate, an insulating film provided on the substrate, and the measuring ion-sensitive film provided on the insulating film."

[0139] The ion concentration measuring device of the present disclosure is

[10] "the ion concentration measuring device according to [9] above, wherein when the first operation is in progress, a voltage is applied between the source and drain of the ion-selective field-effect transistor, and when the second operation is in progress, the drain and source of the ion-selective field-effect transistor are connected to a first reference potential section."

[0140] The ion concentration measuring device of the present disclosure is

[11] "the ion concentration measuring device according to

[10] above, wherein when the second operation is in progress, the measuring membrane electrode is connected to a second reference potential unit."

[0141] The ion concentration measuring device of the present disclosure is

[12] "the ion concentration measuring device according to

[11] above, wherein the potential of the first reference potential section is the same as the potential of the second reference potential section." [Explanation of Symbols]

[0142] 1,1A…pH sensor, 2…Measurement ISFET, 3…Power supply unit, 4…Substrate, 6…Insulating film, 7…Measurement ion-sensitive film (ion-trapping film), 8…Protective film, 8a…Silicon oxide film, 8b…Silicon nitride film, 9…Source, 11…Drain, 12…Substrate power supply, 13…Channel, 14…Measurement target electrode, 16…Measurement target power supply, 17…Measurement film electrode, 18…Measurement film power supply, 19…Power supply control unit, 71…Capacitor, 72…Measurement target switch, 73…Measurement film switch, 74…Ground switch, 75…Output switch, 76…Substrate switch, 77…Output terminal, 78…Capacitor switch, 79…Capacitor switch, 98…DC current source, 99…Reference potential unit, 101…Measurement target, 102…Measurement ion, 103…Non-measurement ion, 200…Drain current, S10…Measurement operation, S20…Charging operation, S30…Storage operation.

Claims

1. An ion concentration measuring device installed on a target for measurement that includes both measured ions and non-measured ions, for obtaining the concentration of the measured ions, A measurement target electrode is placed on the measurement target and controls the potential of the measurement target, A power supply that can be connected to the electrode to be measured via a first switch and supplies voltage to the electrode to be measured, A measurement sensor unit including a measurement ion-sensitive membrane that generates a voltage corresponding to the concentration of the measurement ion, A measuring membrane electrode is placed on the measuring ion-sensitive membrane and controls the potential of the measuring ion-sensitive membrane, A measuring film power supply that can be connected to the measuring film electrode via a second switch and supplies voltage to the measuring film electrode, A potential difference generating unit is connected to the electrode to be measured and the measuring film electrode without going through the first switch and the second switch, and generates a potential difference between the electrode to be measured and the measuring film electrode, An ion concentration measuring device comprising: a power supply control unit that controls the magnitude of the voltage output from the power supply to be measured, and a power supply control unit that controls the magnitude of the voltage output from the power supply for the measuring film.

2. The ion concentration measuring device according to claim 1, wherein the potential difference generating unit is a capacitor.

3. The power supply control unit, A first operation is performed using the power supply for the object to be measured and the power supply for the measuring film to generate a first potential difference between the electrode to be measured and the measuring film electrode. The ion concentration measuring device according to claim 1, wherein the device switches between a second operation, which generates a second potential difference between the electrode to be measured and the measuring film electrode, using the potential difference generating unit.

4. The power supply control unit, When the first operation is performed, the measurement sensor unit is allowed to output a voltage corresponding to the concentration of the measured ion. The ion concentration measuring device according to claim 3, wherein when the second operation is performed, the output of a voltage corresponding to the concentration of the measured ion from the measuring sensor unit is prohibited.

5. The ion concentration measuring device according to claim 3, wherein the power control unit, when in the first operation, performs in parallel a measurement operation that permits the output of a voltage corresponding to the concentration of the measured ions from the measurement sensor unit and a charging operation that charges the potential difference generating unit.

6. The ion concentration measuring device according to claim 5, wherein the second potential difference is equal to the first potential difference.

7. The ion concentration measuring device according to claim 3, wherein, when the power control unit is in the first operation, it performs one of the following before the other: a measurement operation that permits the output of a voltage corresponding to the concentration of the measured ions from the measurement sensor unit, and a charging operation that charges the potential difference generating unit.

8. The ion concentration measuring device according to claim 7, wherein the second potential difference is different from the first potential difference.

9. The ion concentration measuring device according to claim 3, wherein the measurement sensor unit is an ion-selective field-effect transistor having a substrate, an insulating film provided on the substrate, and the measurement ion-sensitive film provided on the insulating film.

10. When the first operation is occurring, a voltage is applied between the source and drain of the ion-selective field-effect transistor. The ion concentration measuring device according to claim 9, wherein, when the second operation is performed, the drain and source of the ion-selective field-effect transistor are connected to the first reference potential section.

11. The ion concentration measuring device according to claim 10, wherein, when the second operation is performed, the measuring membrane electrode is connected to the second reference potential unit.

12. The ion concentration measuring device according to claim 11, wherein the potential of the first reference potential section is the same as the potential of the second reference potential section.

13. The power supply control unit, A first operation is performed in which the first switch is activated to connect the power supply to be measured to the electrode to be measured, and the second switch is activated to connect the power supply to the measuring film to the electrode to be measured, thereby generating a first potential difference between the power supply to be measured and the power supply to the measuring film and the electrode to be measured, The ion concentration measuring device according to claim 1, wherein the device switches between a second operation, which generates a second potential difference between the electrode to be measured and the measuring film electrode by performing at least one of the operations of turning off the first switch to disconnect the power supply to be measured from the electrode to be measured and turning off the second switch to disconnect the power supply to the measuring film electrode, and a second operation, which generates a second potential difference between the electrode to be measured and the measuring film electrode from the potential difference generating unit.

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