PH electrode internal resistance measuring circuit
By designing a pH electrode internal resistance measurement circuit, and using an excitation source circuit and an operational amplifier analog switch to measure the electrode internal resistance, the problem of pH electrode not being able to provide early warning was solved, and rapid and accurate electrode state detection and circuit miniaturization were achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- SHANGHAI XINCHENG TECHNOLOGY CO LTD
- Filing Date
- 2025-05-14
- Publication Date
- 2026-05-08
AI Technical Summary
Existing pH electrodes cannot provide early warning of wear and tear during use, leading to inconvenient maintenance. Most data acquisition circuits and transmitters on the market lack detection capabilities.
Design a pH electrode internal resistance measurement circuit. Apply a controllable excitation source through an excitation source circuit, combine the first and second measurement circuits to measure voltage, and the current measurement circuit to measure current. Accurate acquisition of voltage and current is achieved by using operational amplifiers and analog switches.
It enables rapid and accurate detection of pH electrode internal resistance. The circuit is simple and low-cost, which helps to miniaturize the circuit and avoids the use of transformers.
Smart Images
Figure CN224216781U_ABST
Abstract
Description
Technical Field
[0001] This utility model belongs to the field of pH electrode detection technology, and relates to a measurement circuit, particularly a pH electrode internal resistance measurement circuit. Background Technology
[0002] A pH electrode, also known as a pH probe or pH sensor, is the part of a pH meter that comes into contact with the substance being measured. It is a device that measures the pH value by measuring the electrode potential.
[0003] pH electrodes wear down during use and need to be replaced periodically. Currently, most pH electrode acquisition circuits and transmitters on the market do not have the function of detecting pH status, so they cannot provide early warnings. Maintenance personnel will only perform maintenance when the pH measurement value is abnormal, which is extremely inconvenient in field applications.
[0004] In view of this, there is an urgent need to design a new pH electrode detection method in order to overcome at least some of the above-mentioned defects in the existing pH electrode usage methods. Utility Model Content
[0005] This invention provides a pH electrode internal resistance measurement circuit, which can conveniently and quickly detect the pH electrode internal resistance. The circuit scheme is simple, low in cost, and accurate in measurement.
[0006] To solve the above-mentioned technical problems, according to one aspect of this utility model, the following technical solution is adopted:
[0007] A pH electrode internal resistance measurement circuit, comprising: an excitation source circuit, a first measurement circuit, a second measurement circuit, and a current measurement circuit;
[0008] The excitation source circuit is connected to the first end of the pH electrode and is used to apply a controllable excitation source when measuring the internal resistance of the pH electrode.
[0009] The first measuring circuit is connected to the first end of the pH electrode to measure the voltage at the pH+ end;
[0010] The second measuring circuit is connected to the second terminal of the pH electrode to measure the voltage at the pH-terminus;
[0011] The current measuring circuit is connected to the second end of the pH electrode to measure the current flowing through the pH electrode;
[0012] The first end of the pH electrode is a glass electrode, and the second end of the pH electrode is a reference electrode; or, the first end of the pH electrode is a reference electrode, and the second end of the pH electrode is a glass electrode.
[0013] As one embodiment of this utility model, the excitation source circuit includes a first digital-to-analog converter (DAC), a first five operational amplifier (U15), a first four analog switch (U14), a first five resistor (R15), a first six resistor (R16), a first seven resistor (R17), and a third three resistor (R33).
[0014] The non-inverting input terminal of the first five operational amplifier U15 is connected to the first digital-to-analog converter (DAC) through the third three-resistor R33; the inverting input terminal of the first five operational amplifier U15 is connected to the second terminal of the first five-resistor R15 and the first terminal of the first six-resistor R16, and the second terminal of the first six-resistor R16 is grounded.
[0015] The first end of the first five resistor R15 is connected to the second end of the first seven resistor R17 and the output end of the first five operational amplifier U15. The first end of the first seven resistor R17 is connected to the second end of the first four analog switch U14. The first end of the first four analog switch U14 is connected to the positive electrode of the pH electrode.
[0016] In one embodiment of this utility model, the first digital-to-analog converter (DAC) is used to output a set voltage, the first fifth operational amplifier (U15) amplifies the set voltage output by the first DAC in phase, and the first fourth analog switch (U14) controls the on / off state of the excitation source and the pH electrode.
[0017] As one embodiment of the present invention, the first measurement circuit includes a first sixth operational amplifier U16, a first seventh operational amplifier U17, a second digital-to-analog converter ADC2, a third fourth resistor R34, a third fifth resistor R35, a third sixth resistor R36, a third seventh resistor R37, and a second eighth capacitor C28.
[0018] The non-inverting input terminal of the first six operational amplifier U16 is connected to the positive terminal of the pH electrode, the inverting input terminal of the first six operational amplifier U16 is connected to the output terminal of the first six operational amplifier U16 and the first terminal of the third four resistor R34, and the second terminal of the third four resistor R34 is connected to the non-inverting input terminal of the first seven operational amplifier U17.
[0019] The inverting input terminal of the first seven operational amplifier U17 is connected to the second terminal of the third six resistor R36 and the first terminal of the third seven resistor R37, respectively. The first terminal of the third six resistor R36 is grounded. The second terminal of the third seven resistor R37 is connected to the output terminal of the first seven operational amplifier U17 and the first terminal of the third five resistor R35, respectively. The second terminal of the third five resistor R35 is connected to the first terminal of the second eight capacitor C28 and the second digital-to-analog converter ADC2, respectively. The second terminal of the second eight capacitor C28 is grounded.
[0020] In one embodiment of this utility model, the first sixth operational amplifier U16 is a high internal resistance operational amplifier that forms a voltage follower to buffer the voltage at the pH+ terminal. The first seventh operational amplifier U17 amplifies it in phase, making it easier for the second digital-to-analog converter ADC2 to accurately acquire it.
[0021] As one embodiment of this utility model, the second measurement circuit includes a first octet operational amplifier U18, a second hexet operational amplifier U26, a third digital-to-analog converter ADC3, a third octet resistor R38, a third nintet resistor R39, a fourth zero resistor R40, a fourth one resistor R41, and a third zero capacitor C30.
[0022] The non-inverting input terminal of the first eight-operation amplifier U18 is connected to the negative terminal of the pH electrode, the inverting input terminal of the first eight-operation amplifier U18 is connected to the output terminal of the first eight-operation amplifier U18 and the first terminal of the third eight-resistor R38, and the second terminal of the third eight-resistor R38 is connected to the non-inverting input terminal of the second six-operation amplifier U26.
[0023] The inverting input terminal of the second six operational amplifier U26 is connected to the second terminal of the fourth zero resistor R40 and the first terminal of the fourth one resistor R41, respectively; the first terminal of the fourth zero resistor R40 is grounded, and the second terminal of the fourth one resistor R41 is connected to the output terminal of the second six operational amplifier U26 and the first terminal of the third nine resistor R39, respectively.
[0024] The second terminal of the third nine-resistor R39 is connected to the first terminal of the third zero-capacitor C30 and the third digital-to-analog converter ADC3, respectively; the second terminal of the third zero-capacitor C30 is grounded.
[0025] In one embodiment of this utility model, the first eight operational amplifier U18 is a high internal resistance operational amplifier. The first eight operational amplifier U18 constitutes a voltage follower to buffer the voltage at the pH- end. Operational amplifier U26 amplifies it in phase, making it easier for the third digital-to-analog converter ADC3 to accurately acquire it.
[0026] As one embodiment of this utility model, the current measurement circuit includes a second seventh operational amplifier U27, a second eighth operational amplifier U28, a fourth digital-to-analog converter ADC4, a fourth second resistor R42, a fourth third resistor R43, a fourth fourth resistor R44, a fourth fifth resistor R45, a fourth seventh resistor R47, a third first capacitor C31, and a fourth fourth capacitor C44.
[0027] The non-inverting input terminal of the second op-amp U27 is grounded, and the inverting input terminal of the second op-amp U27 is connected to the negative terminal of the pH electrode, the first terminal of the fourth seven resistor R47, and the first terminal of the fourth four capacitor C44, respectively; the output terminal of the second op-amp U27 is connected to the second terminal of the fourth seven resistor R47, the second terminal of the fourth four capacitor C44, and the first terminal of the fourth two resistor R42, respectively.
[0028] The second terminal of the fourth resistor R42 is connected to the non-inverting input terminal of the second octa-op amplifier U28. The inverting input terminal of the second octa-op amplifier U28 is connected to the second terminal of the fourth resistor R44 and the first terminal of the fourth resistor R45, respectively. The first terminal of the fourth resistor R44 is grounded. The output terminal of the second octa-op amplifier U28 is connected to the second terminal of the fourth resistor R45 and the first terminal of the fourth resistor R43, respectively. The second terminal of the fourth resistor R43 is connected to the first terminal of the third capacitor C31 and the fourth digital-to-analog converter ADC4, respectively. The second terminal of the third capacitor C31 is grounded.
[0029] In one embodiment of this utility model, the second seventh operational amplifier U27 is a precision operational amplifier. The second seventh operational amplifier U27, resistor R47, and capacitor C64 constitute a transimpedance amplifier, which converts the current on the pH electrode into voltage. The second eighth operational amplifier U28 amplifies it in phase, making it easier for the fourth digital-to-analog converter ADC4 to accurately acquire it.
[0030] The beneficial effects of this invention are as follows: The pH electrode internal resistance measurement circuit proposed in this invention applies an excitation source to the pH electrode through an analog switch, simultaneously measuring changes in voltage and current. This allows for convenient and rapid detection of the pH electrode internal resistance. The circuit scheme is simple, low-cost, and provides accurate measurements. Applying the excitation source requires no transformer, using only operational amplifiers and analog switches, which contributes to circuit miniaturization. Attached Figure Description
[0031] Figure 1 This is a circuit diagram of the pH electrode internal resistance measurement circuit in one embodiment of the present invention. Detailed Implementation
[0032] The preferred embodiments of this utility model are described in detail below with reference to the accompanying drawings.
[0033] To further understand this utility model, preferred embodiments of this utility model are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of this utility model, and not for limiting the scope of the claims of this utility model.
[0034] The description in this section pertains to only a few typical embodiments, and this utility model is not limited to the scope of the embodiments described. Substitution of identical or similar prior art methods with some technical features in the embodiments is also within the scope of this utility model's description and protection.
[0035] The term "connection" in the specification includes both direct and indirect connections, such as connections made through active devices, passive devices, or electrical conduction media; it may also include connections made by other active or passive devices that are known to those skilled in the art and can achieve the same or similar functional purpose, such as connections made through circuits or components such as switches or follower circuits.
[0036] This invention discloses a pH electrode internal resistance measurement circuit. Figure 1 This is a circuit diagram of the pH electrode internal resistance measurement circuit in one embodiment of this utility model; please refer to... Figure 1 The pH electrode internal resistance measurement circuit includes: an excitation source circuit, a first measurement circuit, a second measurement circuit, and a current measurement circuit.
[0037] The excitation source circuit is connected to the first end of the pH electrode and is used to apply a controllable excitation source when measuring the internal resistance of the pH electrode; the first measurement circuit is connected to the first end of the pH electrode and is used to measure the voltage at the pH+ end; the second measurement circuit is connected to the second end of the pH electrode and is used to measure the voltage at the pH- end; the current measurement circuit is connected to the second end of the pH electrode and is used to measure the current flowing through the pH electrode.
[0038] The first end of the pH electrode is a glass electrode, and the second end of the pH electrode is a reference electrode; or, the first end of the pH electrode is a reference electrode, and the second end of the pH electrode is a glass electrode.
[0039] In one embodiment of this utility model, the excitation source circuit includes a first digital-to-analog converter (DAC), a first five-operation amplifier (U15), a first four-analog switch (U14), a first five-resistor (R15), a first six-resistor (R16), a first seven-resistor (R17), and a third three-resistor (R33). The non-inverting input terminal of the first five-operation amplifier (U15) is connected to the first DAC via the third three-resistor (R33); the inverting input terminal of the first five-operation amplifier (U15) is connected to the second terminal of the first five-resistor (R15) and the first terminal of the first six-resistor (R16), with the second terminal of the first six-resistor (R16) grounded. The first terminal of the first five-resistor (R15) is connected to the second terminal of the first seven-resistor (R17) and the output terminal of the first five-operation amplifier (U15), and the first terminal of the first seven-resistor (R17) is connected to the second terminal of the first four-analog switch (U14), with the first terminal of the first four-analog switch (U14) connected to the positive terminal of the pH electrode.
[0040] The first digital-to-analog converter (DAC) is used to output a set voltage. The first fifth operational amplifier (U15) amplifies the set voltage output by the first DAC in phase. The first fourth analog switch (U14) controls the on / off state of the excitation source and the pH electrode.
[0041] The first measurement circuit includes a first six-operation amplifier U16, a first seven-operation amplifier U17, a second digital-to-analog converter (ADC2), a third four-resistor R34, a third five-resistor R35, a third six-resistor R36, a third seven-resistor R37, and a second eight-capacitor C28. The non-inverting input of the first six-operation amplifier U16 is connected to the positive terminal of the pH electrode. The inverting input of the first six-operation amplifier U16 is connected to the output of the first six-operation amplifier U16 and the first terminal of the third four-resistor R34. The second terminal of the third four-resistor R34 is connected to the non-inverting input of the first seven-operation amplifier U17. The inverting input of the first seven-operation amplifier U17 is connected to the second terminal of the third six-resistor R36 and the first terminal of the third seven-resistor R37, with the first terminal of the third six-resistor R36 grounded. The second terminal of the third seven-resistor R37 is connected to the output of the first seven-operation amplifier U17 and the first terminal of the third five-resistor R35. The second terminal of the third five-resistor R35 is connected to the first terminal of the second eight-capacitor C28 and the second digital-to-analog converter (ADC2). The second terminal of the second eight-capacitor C28 is grounded. The first sixth operational amplifier U16 is a high internal resistance operational amplifier that forms a voltage follower to buffer the voltage at the pH+ terminal. The first seventh operational amplifier U17 amplifies it in phase, making it easier for the second digital-to-analog converter ADC2 to accurately acquire it.
[0042] In one embodiment of this utility model, the second measurement circuit includes a first octa-operated amplifier U18, a second hexa-operated amplifier U26, a third digital-to-analog converter ADC3, a third octa-resistor R38, a third ninta-resistor R39, a fourth zero-resistor R40, a fourth one-resistor R41, and a third zero-capacitor C30. The non-inverting input terminal of the first octa-operated amplifier U18 is connected to the negative terminal of the pH electrode. The inverting input terminal of the first octa-operated amplifier U18 is connected to the output terminal of the first octa-operated amplifier U18 and the first terminal of the third octa-resistor R38. The second terminal of the third octa-resistor R38 is connected to the non-inverting input terminal of the second hexa-operated amplifier U26. The inverting input terminal of the second hexa-operated amplifier U26 is connected to the second terminal of the fourth zero-resistor R40 and the first terminal of the fourth one-resistor R41. The first terminal of the fourth zero-resistor R40 is grounded, and the second terminal of the fourth one-resistor R41 is connected to the output terminal of the second hexa-operated amplifier U26 and the first terminal of the third ninta-resistor R39. The second terminal of the third nine-resistor R39 is connected to the first terminal of the third zero-capacitor C30 and the third digital-to-analog converter ADC3, respectively; the second terminal of the third zero-capacitor C30 is grounded. The first eight-operation amplifier U18 can be a high-impedance operation amplifier. The first eight-operation amplifier U18 forms a voltage follower to buffer the voltage at the pH- terminal. Operation amplifier U26 amplifies it in phase, making it easier for the third digital-to-analog converter ADC3 to accurately acquire it.
[0043] The current measurement circuit includes a second operational amplifier U27 (7th pin), a second operational amplifier U28 (8th pin), a fourth digital-to-analog converter (ADC4), a fourth resistor R42 (2nd pin), a fourth resistor R43 (3rd pin), a fourth resistor R44 (4th pin), a fourth resistor R45 (5th pin), a fourth resistor R47 (7th pin), a third capacitor C31 (1st pin), and a fourth capacitor C44 (4th pin). The non-inverting input of the second operational amplifier U27 is grounded, and its inverting input is connected to the negative terminal of the pH electrode, the first terminal of the fourth resistor R47, and the first terminal of the fourth capacitor C44. The output of the second operational amplifier U27 is connected to the second terminal of the fourth resistor R47, the second terminal of the fourth capacitor C44, and the first terminal of the fourth resistor R42 (2nd pin). The second terminal of the fourth resistor R42 is connected to the non-inverting input terminal of the second octa-op amplifier U28. The inverting input terminal of the second octa-op amplifier U28 is connected to the second terminal of the fourth resistor R44 and the first terminal of the fourth resistor R45, respectively. The first terminal of the fourth resistor R44 is grounded. The output terminal of the second octa-op amplifier U28 is connected to the second terminal of the fourth resistor R45 and the first terminal of the fourth resistor R43, respectively. The second terminal of the fourth resistor R43 is connected to the first terminal of the third capacitor C31 and the fourth digital-to-analog converter ADC4, respectively. The second terminal of the third capacitor C31 is grounded.
[0044] The second operational amplifier U27 can be a precision operational amplifier. The second operational amplifier U27, along with resistor R47 and capacitor C64, constitutes a transimpedance amplifier, which converts the current on the pH electrode into a voltage. The second operational amplifier U28 amplifies it in phase, making it easier for the fourth digital-to-analog converter ADC4 to accurately acquire it.
[0045] In one application scenario of this utility model, the measurement process is as follows:
[0046] Normal pH measurement: With analog switch U14 off, the excitation source is not applied to the pH electrode. At this time, the pH electrode voltage VpH = V_pH+ - V_pH-. The pH electrode voltage VpH can be obtained by acquiring these two voltages through the ADC. In particular, since both ends of pH are connected to the input terminals of the op-amp at this time, and the input impedance of the op-amp is measured in GΩ, the current I_pH flowing through pH can be considered to be 0.
[0047] pH internal resistance measurement: (1) Turn on the analog switch U14 and apply the excitation source to the pH electrode. At this time, the voltage across the pH electrode is determined by the excitation source and VpH. The voltage difference obtained by ADC1 and ADC2 is V_pH+'-V_pH-'. For ease of calculation, the value measured the second time is recorded as V2. (2) Measure the current I_pH flowing through the pH electrode at this time through ADC3. (3) Based on the pH electrode model, we can assume that the voltage across the pH electrode at this time is the sum of VpH and the voltage difference generated by the current flowing through the internal resistance, i.e., V2=VpH+I_pH*RpH. After simplification, we can get: RpH=(V2-VpH) / I_pH=[(V_pH+'-V_pH-')-(V_pH+-V_pH-)] / I_pH. (4) After the measurement is completed, turn off the analog switch U14 and continue to measure the pH value normally.
[0048] In summary, the pH electrode internal resistance measurement circuit proposed in this invention applies an excitation source to the pH electrode using an analog switch, simultaneously measuring changes in voltage and current. This allows for convenient and rapid calculation of the pH internal resistance. The circuit design is simple, low-cost, and provides accurate measurements. Applying the excitation source eliminates the need for a transformer, using only operational amplifiers and analog switches, which contributes to circuit miniaturization.
[0049] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.
[0050] The description and application of this utility model herein are illustrative and not intended to limit the scope of the utility model to the above embodiments. The effects or advantages involved in the embodiments may not be manifested in the embodiments due to various factors, and the description of effects or advantages is not intended to limit the embodiments. Variations and modifications of the embodiments disclosed herein are possible, and various substitutions and equivalents of the components in the embodiments are well known to those skilled in the art. It should be clear to those skilled in the art that this utility model can be implemented in other forms, structures, arrangements, proportions, and with other components, materials, and parts without departing from the spirit or essential characteristics of this utility model. Other variations and modifications can be made to the embodiments disclosed herein without departing from the scope and spirit of this utility model.
Claims
1. A pH electrode internal resistance measurement circuit, characterized in that, The pH electrode internal resistance measurement circuit includes: an excitation source circuit, a first measurement circuit, a second measurement circuit, and a current measurement circuit; The excitation source circuit is connected to the first end of the pH electrode and is used to apply a controllable excitation source when measuring the internal resistance of the pH electrode. The first measuring circuit is connected to the first end of the pH electrode to measure the voltage at the pH+ end; The second measuring circuit is connected to the second terminal of the pH electrode to measure the voltage at the pH-terminus; The current measuring circuit is connected to the second end of the pH electrode to measure the current flowing through the pH electrode; The first end of the pH electrode is a glass electrode, and the second end of the pH electrode is a reference electrode; or, the first end of the pH electrode is a reference electrode, and the second end of the pH electrode is a glass electrode.
2. The pH electrode internal resistance measuring circuit according to claim 1, characterized in that: The excitation source circuit includes a first digital-to-analog converter (DAC), a first fifth operational amplifier (U15), a first fourth analog switch (U14), a first fifth resistor (R15), a first sixth resistor (R16), a first seventh resistor (R17), and a third third resistor (R33). The non-inverting input terminal of the first five operational amplifier U15 is connected to the first digital-to-analog converter (DAC) through the third three-resistor R33; the inverting input terminal of the first five operational amplifier U15 is connected to the second terminal of the first five-resistor R15 and the first terminal of the first six-resistor R16, and the second terminal of the first six-resistor R16 is grounded. The first end of the first five resistor R15 is connected to the second end of the first seven resistor R17 and the output end of the first five operational amplifier U15. The first end of the first seven resistor R17 is connected to the second end of the first four analog switch U14. The first end of the first four analog switch U14 is connected to the positive electrode of the pH electrode.
3. The pH electrode internal resistance measuring circuit according to claim 2, characterized in that: The first digital-to-analog converter (DAC) is used to output a set voltage. The first fifth operational amplifier (U15) amplifies the set voltage output by the first DAC in phase. The first fourth analog switch (U14) controls the on / off state of the excitation source and the pH electrode.
4. The pH electrode internal resistance measuring circuit according to claim 1, characterized in that: The first measurement circuit includes a first sixth operational amplifier U16, a first seventh operational amplifier U17, a second digital-to-analog converter ADC2, a third fourth resistor R34, a third fifth resistor R35, a third sixth resistor R36, a third seventh resistor R37, and a second eighth capacitor C28. The non-inverting input terminal of the first six operational amplifier U16 is connected to the positive terminal of the pH electrode, the inverting input terminal of the first six operational amplifier U16 is connected to the output terminal of the first six operational amplifier U16 and the first terminal of the third four resistor R34, and the second terminal of the third four resistor R34 is connected to the non-inverting input terminal of the first seven operational amplifier U17. The inverting input terminal of the first seven operational amplifier U17 is connected to the second terminal of the third six resistor R36 and the first terminal of the third seven resistor R37, respectively. The first terminal of the third six resistor R36 is grounded. The second terminal of the third seven resistor R37 is connected to the output terminal of the first seven operational amplifier U17 and the first terminal of the third five resistor R35, respectively. The second terminal of the third five resistor R35 is connected to the first terminal of the second eight capacitor C28 and the second digital-to-analog converter ADC2, respectively. The second terminal of the second eight capacitor C28 is grounded.
5. The pH electrode internal resistance measuring circuit according to claim 4, characterized in that: The first sixth operational amplifier U16 is a high internal resistance operational amplifier that forms a voltage follower to buffer the voltage at the pH+ terminal. The first seventh operational amplifier U17 amplifies it in phase, making it easier for the second digital-to-analog converter ADC2 to accurately acquire it.
6. The pH electrode internal resistance measuring circuit according to claim 1, characterized in that: The second measurement circuit includes a first octet operational amplifier U18, a second hexet operational amplifier U26, a third digital-to-analog converter ADC3, a third octet resistor R38, a third nintet resistor R39, a fourth zero resistor R40, a fourth one resistor R41, and a third zero capacitor C30. The non-inverting input terminal of the first eight-operation amplifier U18 is connected to the negative terminal of the pH electrode, the inverting input terminal of the first eight-operation amplifier U18 is connected to the output terminal of the first eight-operation amplifier U18 and the first terminal of the third eight-resistor R38, and the second terminal of the third eight-resistor R38 is connected to the non-inverting input terminal of the second six-operation amplifier U26. The inverting input terminal of the second six operational amplifier U26 is connected to the second terminal of the fourth zero resistor R40 and the first terminal of the fourth one resistor R41, respectively; the first terminal of the fourth zero resistor R40 is grounded, and the second terminal of the fourth one resistor R41 is connected to the output terminal of the second six operational amplifier U26 and the first terminal of the third nine resistor R39, respectively. The second terminal of the third nine-resistor R39 is connected to the first terminal of the third zero-capacitor C30 and the third digital-to-analog converter ADC3, respectively; the second terminal of the third zero-capacitor C30 is grounded.
7. The pH electrode internal resistance measuring circuit according to claim 6, characterized in that: The first octet operational amplifier U18 is a high internal resistance operational amplifier. The first octet operational amplifier U18 forms a voltage follower to buffer the voltage at the pH- end. Operational amplifier U26 amplifies it in phase, making it easier for the third digital-to-analog converter ADC3 to accurately acquire it.
8. The pH electrode internal resistance measuring circuit according to claim 1, characterized in that: The current measurement circuit includes a second seventh operational amplifier U27, a second eighth operational amplifier U28, a fourth digital-to-analog converter ADC4, a fourth second resistor R42, a fourth third resistor R43, a fourth fourth resistor R44, a fourth fifth resistor R45, a fourth seventh resistor R47, a third first capacitor C31, and a fourth fourth capacitor C44. The non-inverting input terminal of the second op-amp U27 is grounded, and the inverting input terminal of the second op-amp U27 is connected to the negative terminal of the pH electrode, the first terminal of the fourth seven resistor R47, and the first terminal of the fourth four capacitor C44, respectively; the output terminal of the second op-amp U27 is connected to the second terminal of the fourth seven resistor R47, the second terminal of the fourth four capacitor C44, and the first terminal of the fourth two resistor R42, respectively. The second terminal of the fourth resistor R42 is connected to the non-inverting input terminal of the second octa-op amplifier U28. The inverting input terminal of the second octa-op amplifier U28 is connected to the second terminal of the fourth resistor R44 and the first terminal of the fourth resistor R45, respectively. The first terminal of the fourth resistor R44 is grounded. The output terminal of the second octa-op amplifier U28 is connected to the second terminal of the fourth resistor R45 and the first terminal of the fourth resistor R43, respectively. The second terminal of the fourth resistor R43 is connected to the first terminal of the third capacitor C31 and the fourth digital-to-analog converter ADC4, respectively. The second terminal of the third capacitor C31 is grounded.
9. The pH electrode internal resistance measuring circuit according to claim 8, characterized in that: The second operational amplifier U27 is a precision operational amplifier. The second operational amplifier U27, along with resistor R47 and capacitor C64, constitutes a transimpedance amplifier, which converts the current on the pH electrode into a voltage. The second operational amplifier U28 amplifies it in phase, making it easier for the fourth digital-to-analog converter ADC4 to accurately acquire it.