Two-wire transmitter

Abstract

A two-wire transmitter is provided. The two-wire transmitter is connected to an external circuit via two transmission lines, uses the external circuit as a power source, and outputs a current signal to the external circuit. The two-wire transmitter includes a measurement data processing circuit connected to a sensor that outputs an electric signal based on measurement data, outputs a first signal based on the measurement data; a current output circuit that determines the current signal based on the first signal; and a shunt adjustment circuit that determines a circuit voltage of the two-wire transmitter based on the first signal.

Description

[0001] This application is a divisional application of the invention patent application with application number 201911327149.X, application date December 20, 2019, entitled "Dual-line Transmitter". Technical Field

[0002] This disclosure relates to a two-wire transmitter. Background Technology

[0003] Previously, two-wire transmitters that output a specified current signal to an external circuit based on an electrical signal acquired from a sensor are known (for example, see Patent Document 1).

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2012-99088 Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] The two-wire transmitter disclosed in Patent Document 1 includes a shunt regulator circuit. The shunt regulator circuit controls the circuit voltage applied to the two-wire transmitter according to changes in the current signal output to the external circuit, aiming to stably control the circuit voltage.

[0009] This disclosure is made in view of the above-mentioned problems, and its purpose is to provide a two-wire transmitter that can improve the stability of the operation.

[0010] Methods for solving problems

[0011] Several embodiments of a two-wire transmitter are implemented, connected to an external circuit via two transmission lines, using the external circuit as a power source, and outputting a current signal to the external circuit. The two-wire transmitter includes: a measurement data processing circuit connected to a sensor that outputs an electrical signal based on the measurement data, and outputting a first signal based on the measurement data; a current output circuit that determines the current signal based on the first signal; and a shunt adjustment circuit that determines the circuit voltage of the two-wire transmitter based on the first signal. In this way, both the current output circuit and the shunt adjustment circuit operate based on the first signal, thereby reducing the time deviation of the input signals to each circuit. As a result, the stability of operation is improved.

[0012] In one embodiment of the two-wire transmitter, the current output circuit, based on the first signal, reduces the current flowing as the current signal, while the shunt adjustment circuit, based on the first signal, increases the circuit voltage. This ensures sufficient power supply to the two-wire transmitter, resulting in improved operational stability.

[0013] In one embodiment of the two-wire transmitter, the measurement data processing circuit includes: a signal processing circuit that processes the electrical signal and outputs a control signal; and a first signal generation circuit that outputs the first signal based on the control signal. In this way, the current output circuit can output a current signal based on the measurement data. As a result, the stability of the operation is improved.

[0014] In one embodiment of the two-wire transmitter, the control signal includes a PWM signal controlled in a PWM manner, and the first signal generation circuit includes an LPF that converts the PWM signal into the first signal and outputs it. In this way, the current output circuit can output a current signal based on measured data. As a result, the stability of operation is improved.

[0015] In one embodiment of the two-wire transmitter, the control signal includes a digital signal, and the first signal generation circuit includes a DA converter to convert the digital signal into an analog signal and output it as the first signal. This reduces the number of components. As a result, the component mounting area is reduced or the cost is lowered, and the stability of operation is improved.

[0016] In one embodiment of the two-wire transmitter, the measurement data processing circuit includes an abnormal state detection circuit. This allows the current signal output by the signal output circuit to be burned out in the event of an abnormality in the signal processing circuit.

[0017] Invention Effects

[0018] According to this disclosure, a two-wire transmitter is provided that can improve the stability of motion. Attached Figure Description

[0019] Figure 1 This is a circuit diagram showing a comparative example of a two-wire transmitter.

[0020] Figure 2 This is a block diagram illustrating an example configuration of a two-wire transmitter according to one embodiment.

[0021] Figure 3 This is a circuit diagram showing an example of the configuration of a measurement data processing circuit.

[0022] Figure 4 This is a circuit diagram showing an example of the configuration of a current output circuit.

[0023] Figure 5 This is a circuit diagram showing an example of the configuration of a shunt adjustment circuit.

[0024] Figure 6 This is a circuit diagram illustrating an example of the configuration of a two-wire transmitter according to another embodiment.

[0025] Figure 7 This is a circuit diagram showing an example of the configuration of a two-wire transmitter equipped with a DA converter.

[0026] Figure 8 This is a circuit diagram showing an example of the configuration of a two-wire transmitter equipped with an abnormal state detection circuit.

[0027] Explanation of reference numerals in the attached figures

[0028] 10 External circuit; 50 Sensor; 100 Two-wire transmitter; 102 Measurement data processing circuit; 103 Switching circuit; 104 Signal processing circuit; 105 First signal generation circuit; 106 Current output circuit; 108 Branch adjustment circuit; 110 Reference voltage output section; 113 Comparison circuit; 121, 122 DA converter. Detailed Implementation

[0029] like Figure 1 As shown, the comparative example two-wire transmitter 900 is connected to the external circuit 10 via two transmission lines L1 and L2. The two-wire transmitter 900 operates using power supplied by the external circuit 10. That is, the two-wire transmitter 900 operates using the external circuit 10 as its power source. The external circuit 10 has a voltage source Eb and a resistor R1. The voltage source Eb and the resistor R1 are connected in series with the transmission lines L1 and L2. The two-wire transmitter 900 draws a current, denoted by Iou, from the transmission line L1 via diode D1, and a current, denoted by Iout, flows into the transmission line L2 via resistor R3, thereby outputting a current signal Iout to the external circuit 10.

[0030] In the comparative example, the two-wire transmitter 900 is connected to the sensor 50. The two-wire transmitter 900 acquires measurement data from the sensor 50. The sensor 50 operates via power supplied from the two-wire transmitter 900. The sensor 50 measures physical quantities such as pressure or temperature, converts them into a measurement signal S1, and outputs it to the two-wire transmitter 900.

[0031] The comparative example of the two-wire transmitter 900 includes a measurement data processing circuit 102, a current output circuit 106, and a shunt adjustment circuit 108.

[0032] The measurement data processing circuit 102 includes a signal processing circuit 104. The signal processing circuit 104 acquires a measurement signal S1 from the sensor 50 and performs predetermined processing such as linear correction on the measurement signal S1. The signal processing circuit 104 modulates the measured signal S1, which has undergone predetermined processing, using pulse width modulation (PWM), and outputs the modulated signal as a switching control signal to the switch SW1. The signal modulated using PWM is also called a PWM (Pulse Width Modulation) signal. The PWM signal output to the switch SW1 is also called a current signal PWM signal. The switch SW1 has two fixed contacts and one movable contact. The switch SW1 is connected to a first reference voltage source PR1 at one fixed contact, which outputs a voltage represented by VR1. The switch SW1 is connected to a second reference voltage source PR2 at the other fixed contact, which outputs a voltage represented by VR2. The switch SW1 is connected to a signal line L3 at the movable contact. The movable contact of switch SW1 switches between two fixed contacts according to the level change of the input current signal using a PWM signal. Therefore, the measurement data processing circuit 102 outputs the signal of voltage variation between VR1 and VR2 as signal S2 to signal line L3.

[0033] The current output circuit 106 determines the current signal value output to the external circuit 10 based on the signal S2 input via signal line L3. The current signal Iout is represented by a current of 4mA to 20mA. The current output circuit 106 includes an LPF (Low Pass Filter) 3, a buffer amplifier Q1, an error amplifier Q2, and transistors Q3 and Q4. Resistor R8 is connected in parallel with the emitter and collector of transistor Q4. The two-wire transmitter 900 can be started by power supplied from the external circuit 10 via resistor R8. LPF3, with resistor R2 and capacitor C1, smooths the signal S2. Buffer amplifier Q1 buffers the smoothed signal S2 and outputs it as signal Va'. The voltage on transmission line L2 is represented by Vb. Error amplifier Q2 detects the error in the voltage obtained by dividing the potential difference between signal Va' and voltage Vb using resistors R4 and R5, and the voltage obtained by dividing the voltage VR1 output from the first reference voltage source PR1 using resistors R6 and R7. It controls the current flowing into transistors Q3 and Q4 to ensure these voltages are consistent. The current introduced into transistor Q4 from external circuit 10 becomes the current signal Iout corresponding to the measurement signal S1 output by sensor 50. Typically, the current signal Iout is determined such that when the measurement result of sensor 50 is 0% of the set span, the current signal Iout is 4mA, and when the measurement result of sensor 50 is 100% of the set span, the current signal Iout is 20mA. The smaller the measurement signal S1 output by sensor 50, the smaller the current signal Iout.

[0034] The measurement data processing circuit 102 also includes a reference voltage output unit 110. The signal processing circuit 104 outputs an electrical signal corresponding to the measurement signal S1 acquired from the sensor 50 to the reference voltage output unit 110.

[0035] The comparative example two-wire transmitter 900 also includes a reference voltage processing circuit 112. The reference voltage output unit 110 outputs a reference voltage to the reference voltage processing circuit 112 in correspondence with the electrical signal input from the signal processing circuit 104. The reference voltage output unit 110 uses a PWM signal as the reference voltage signal representing the reference voltage. The PWM signal representing the reference voltage is called the reference voltage PWM signal. The smaller the electrical signal input from the signal processing circuit 104, that is, the smaller the measurement signal S1 output from the sensor 50, the higher the duty cycle of the reference voltage output unit 110 outputs the reference voltage PWM signal. The larger the electrical signal input from the signal processing circuit 104, that is, the larger the measurement signal S1 output from the sensor 50, the lower the duty cycle of the reference voltage output unit 110 outputs the reference voltage PWM signal. Here, when the measurement signal S1 output from the sensor 50 is small, the current signal Iout is, for example, a small value such as 4mA.

[0036] The reference voltage processing circuit 112 includes LPF4, error amplifier Q5, resistors R10 and R11. LPF4 includes resistor R9 and capacitor C2. The reference voltage processing circuit 112 uses LPF4 to smooth the reference voltage with a PWM signal, and uses error amplifier Q5, resistors R10 and R11 to perform negative feedback amplification, and outputs the output Vref to the shunt adjustment circuit 108.

[0037] The shunt adjustment circuit 108 includes an error amplifier Q6, a transistor Q7, and resistors R13 and R14. The error amplifier Q6 detects the error between the output Vref of the error amplifier Q5 and the voltage obtained by dividing the circuit voltage V1 applied to the circuit using resistors R13 and R14. Together with transistor Q7, it controls the circuit voltage V1 to make them consistent. D The circuit voltage V1 corresponds to the voltage applied between the ground wire and the power supply line. The ground wire is the wiring connected to the ground point denoted as COM. The voltage of the ground wire is also called the ground voltage or the COM voltage. The power supply line is the wiring connected to the transmission line L1 via diode D1 and transistor Q4 or via diode D1 and resistor R8.

[0038] Based on the above operation, when the current flowing into the two-wire transmitter 900 from the external circuit 10 is small due to the small current signal Iout, the circuit voltage V1 becomes high. This increases the power that can be consumed inside the two-wire transmitter 900. When using a DC-DC converter with the circuit voltage V1 as the input voltage, the current usable on the output side (secondary side) of the DC-DC converter increases.

[0039] The comparative example of the two-wire transmitter 900 also includes a comparator circuit 113. The comparator circuit 113 includes a comparator Q8. A voltage smoothed by a PWM signal using LPF4 is input to the inverting input terminal of the comparator Q8. A voltage obtained by dividing the circuit voltage V1 using resistors R13 and R14 is input to the non-inverting input terminal of the comparator Q8. The comparator Q8 compares these voltages. If the voltage input to the non-inverting input terminal decreases, that is, if the circuit voltage V1 decreases, the comparator Q8 inverts the voltage output to the signal processing circuit 104, thus alerting the signal processing circuit 104 to an anomaly. Upon detecting the inversion of the voltage input from the comparator Q8, the signal processing circuit 104 performs processing such as saving the current value of the measurement signal S1.

[0040] As explained above, in the comparative example of the two-wire transmitter 900, the smaller the measurement signal S1 input from the sensor 50, the higher the duty cycle of the reference voltage output unit 110 outputs a reference voltage PWM signal. Thus, the smaller the measurement signal S1 input from the sensor 50, the higher the circuit voltage V1 controlled by the shunt regulating circuit 108 becomes. In other words, when the current signal Iout corresponding to the current supplied from the external circuit 10 is small, the circuit voltage V1 becomes high. As a result, the power that can be consumed within the circuit of the two-wire transmitter 900 increases.

[0041] In the comparative example of the two-wire transmitter 900, the transient response characteristics of LPF3, which outputs a signal to the current output circuit 106, and LPF4, which outputs a signal to the shunt adjustment circuit 108, are sometimes different from each other. When the transient response characteristics of LPF3 and LPF4 are different from each other, the current output circuit 106 or the shunt adjustment circuit 108 may operate differently than intended.

[0042] For example, when the transient response of LPF3 is faster than that of LPF4, the change in the current signal generated by the current output circuit 106 is faster than the change in the circuit voltage V1 generated by the shunt adjustment circuit 108.

[0043] When the current output circuit 106 increases the current signal, the voltage drop across resistors R1 and R3 increases due to the increased current signal. This increased voltage drop across the resistors reduces the emitter voltage of transistor Q4. On the other hand, since the transient response of LPF4 is slower than that of LPF3, the circuit voltage V1 is kept high for control. Because the circuit voltage V1 is kept high, the collector-emitter voltage of transistor Q4 decreases. This decrease in collector-emitter voltage causes transistor Q4 to saturate. With transistor Q4 saturated, it is difficult to increase the collector current of transistor Q4. When the current output circuit 106 increases the collector current of transistor Q4, thus increasing the current signal, the output voltage of error amplifier Q2 needs to increase, which in turn increases the collector current of transistor Q3 and the base current of transistor Q4. However, due to the upper limit of the output voltage of error amplifier Q2, the collector current of transistor Q3 may not be able to increase sufficiently. As a result, the magnitude of the current signal that should be output based on the measured data may not be guaranteed.

[0044] When the current output circuit 106 reduces the current signal, the circuit voltage V1 increases more slowly than the current signal decreases. In other words, when the current signal decreases, the circuit voltage V1 may remain low. In this case, the power supplied to each circuit of the two-wire transmitter 900 decreases. As a result, the two-wire transmitter 900 may enter a stopped (reset) state.

[0045] For example, if the transient response of LPF3 is slower than that of LPF4, the change in the current signal generated by the current output circuit 106 is slower than the change in the circuit voltage V1 generated by the shunt regulating circuit 108. When the current output circuit 106 wants to increase the current signal, the circuit voltage V1 may decrease first. In this case, the power supplied to each circuit of the two-wire transmitter 900 is reduced. As a result, the two-wire transmitter 900 may enter a stopped (reset) state. Conversely, if the current output circuit 106 wants to decrease the current signal, the circuit voltage V1 may increase first. In this case, transistor Q4 may saturate. As a result, the collector current of transistor Q4 cannot be increased sufficiently, and the magnitude of the current signal that should be output based on the measured data may not be guaranteed.

[0046] The comparative example two-wire transmitter 900 may operate differently from its intended operation during startup. During startup, the circuit voltage V1 applied to the two-wire transmitter 900 rises over time. If the error amplifier Q5 does not operate during the rise of circuit voltage V1, the output of error amplifier Q5 becomes high impedance. In this case, the output of error amplifier Q5 is pulled down to the COM voltage through resistors R10 and R11. In this state, the COM voltage is input to the non-inverting input terminal of error amplifier Q6, included in the shunt adjustment circuit 108. The voltage at the inverting input terminal of error amplifier Q6 is higher than the voltage applied to resistor R14, and therefore higher than the voltage at the non-inverting input terminal. In this state, if error amplifier Q6 operates first, error amplifier Q6 outputs the COM voltage. The COM voltage output by error amplifier Q6 is used to input the COM voltage to the gate of transistor Q7. The gate and drain of transistor Q7 become the same COM voltage, thereby turning transistor Q7 ON. When transistor Q7 is ON, the current flowing into the two-wire transmitter 900 flows into transistor Q7, making it difficult for current to flow into other circuits. As a result of this difficulty in current flowing into circuits other than transistor Q7, the two-wire transmitter 900 sometimes fails to start.

[0047] Therefore, this disclosure describes a two-wire transmitter that can operate as intended even in the presence of circuit parameter errors, such as differences in transient response characteristics between the two LPFs.

[0048] (Implementation of this disclosure)

[0049] like Figure 2 As shown, one embodiment of the two-wire transmitter 100 includes a measurement data processing circuit 102, a current output circuit 106, and a shunt adjustment circuit 108. The two-wire transmitter 100 may also include a comparison circuit 113. The measurement data processing circuit 102 may include a signal processing circuit 104 and a first signal generation circuit 105.

[0050] exist Figure 2 In the diagram, solid lines represent electrical wiring used to receive power from external circuit 10. External circuit 10 has a voltage source Eb and a resistor R1. Voltage source Eb and resistor R1 are connected in series to transmission lines L1 and L2. Two-wire transmitter 100 operates using power supplied by external circuit 10. That is, two-wire transmitter 100 operates using external circuit 10 as its power source. Current output circuit 106 draws a current corresponding to current signal Iout from external circuit 10. Current output circuit 106 generates a current signal Iout output to external circuit 10 by controlling the magnitude of the drawn current. Current signal Iout is represented by a current of 4mA to 20mA. Current signal Iout can be represented by different current values. External circuit 10 can obtain information from two-wire transmitter 100 based on current signal Iout. Branch adjustment circuit 108 determines the circuit voltage V1 applied to each component of two-wire transmitter 100. The signal processing circuit 104, the first signal generation circuit 105, and the comparison circuit 113 can operate with the circuit voltage V1 determined by the shunt adjustment circuit 108, or with a voltage obtained by stepping down the circuit voltage V1 using a step-down power supply IC (Integrated Circuit) or the like. The power supplied to each component of the two-wire transmitter 100 is determined based on the circuit voltage V1.

[0051] Sensor 50 measures physical quantities such as pressure or temperature, converts the measured data into an electrical signal, and outputs it to two-wire transmitter 100. Sensor 50 can operate using power from two-wire transmitter 100. Sensor 50 can also operate using power from other power sources instead of two-wire transmitter 100.

[0052] exist Figure 2In the diagram, dashed lines represent the signal flow between the constituent parts. Signal processing circuit 104 acquires an electrical signal based on measurement data from sensor 50. This electrical signal based on measurement data is also called the measurement signal. The measurement signal acquired by signal processing circuit 104 from sensor 50 is denoted as S1. Signal processing circuit 104 outputs a signal based on the measurement signal to first signal generation circuit 105. This signal based on the measurement signal is denoted as S2. First signal generation circuit 105 generates a first signal Va based on the signal acquired from signal processing circuit 104. That is, measurement data processing circuit 102 generates the first signal Va based on the measurement data. Measurement data processing circuit 102 outputs the first signal Va to current output circuit 106 and shunt adjustment circuit 108. Current output circuit 106 determines the current signal Iout based on the input first signal Va. Typically, the current signal Iout is determined such that when the measurement result of sensor 50 is 0% of the set span, the current signal Iout is 4mA, and when the measurement result of sensor 50 is 100% of the set span, the current signal Iout is 20mA. The smaller the measurement signal output by sensor 50, the smaller the current signal Iout becomes. The shunt adjustment circuit 108 determines the circuit voltage V1 based on the input first signal Va. The circuit voltage V1 corresponds to the voltage applied between the ground wire and the power supply line. The ground wire is the wiring connected to the ground point denoted as COM. The voltage of the ground wire is also called the ground voltage or COM voltage. The ground wire is the wiring connected to the transmission line L2. The power supply line is the wiring connected to the transmission line L1 via the current output circuit 106. The external circuit 10 supplies power to the two-wire transmitter 100 via the power supply line. The shunt adjustment circuit 108 outputs a signal based on the circuit voltage V1 to the comparator circuit 113. The comparator circuit 113 generates a signal indicating whether the circuit voltage V1 is abnormal based on the signal obtained from the shunt adjustment circuit 108, and outputs it to the signal processing circuit 104.

[0053] Reference Figure 3 This indicates the operation of the measurement data processing circuit 102.

[0054] The two-wire transmitter 100 can be connected to the sensor 50 via the measurement data processing circuit 102, in Figure 3 The transducer 100 is connected to sensor 50. The transducer 100 acquires measurement data from sensor 50. Sensor 50 can operate using power from the transducer 100. Sensor 50 can also operate using power from another power source, without power from the transducer 100.

[0055] The measurement data processing circuit 102 includes a signal processing circuit 104, a switching circuit 103, and a first signal generation circuit 105.

[0056] The signal processing circuit 104 can be constructed from a general-purpose integrated circuit such as a CPU (Central Processing Unit). The signal processing circuit 104 can perform various functions by executing a predetermined program. The signal processing circuit 104 may have a storage unit. The two-wire transmitter 100 may be a separate component from the signal processing circuit 104 and also have a storage unit. The storage unit can store various information used during the operation of the signal processing circuit 104 or programs used to implement the functions of the signal processing circuit 104. The storage unit can function as the working memory of the signal processing circuit 104. The storage unit may be constructed from, for example, a semiconductor memory.

[0057] The signal processing circuit 104 acquires the measurement signal S1 from the sensor 50. The signal processing circuit 104 can communicate with the sensor 50 based on standards such as RS485.

[0058] The signal processing circuit 104 converts the measured signal S1 into a PWM signal and outputs it to the switching circuit 103. The PWM signal output to the switching circuit 103 is also called a current signal PWM signal. Before converting the measured signal S1 into a PWM signal, the signal processing circuit 104 may perform predetermined processing such as linear correction on the measured signal S1. Prescribed processing may include, for example, linear correction. The signal processing circuit 104 is not limited to modulating the measured signal S1 by pulse width modulation; for example, it may modulate the measured signal S1 by various modulation methods such as pulse density modulation or pulse amplitude modulation.

[0059] The switching circuit 103 includes a switch SW1, a first reference voltage source PR1, and a second reference voltage source PR2. The first reference voltage source PR1 and the second reference voltage source PR2 output voltages represented by VR1 and VR2, respectively. The switch SW1 has two fixed contacts and one movable contact. Each fixed contact is connected to the first reference voltage source PR1 and the second reference voltage source PR2. The movable contact is connected to a signal line L3 that outputs a signal to the first signal generation circuit 105.

[0060] The movable contact of switch SW1 contacts any fixed contact based on the level of the PWM signal input from signal processing circuit 104, thereby inputting either voltage VR1 or VR2 to signal line L3. The PWM signal includes two level signals, referred to as High and Low, respectively. The High level is also called the H level. The Low level is also called the L level. The PWM signal represents a value from 0 to 100% based on the duty cycle representing the time ratio of each level within a specified period. For example, a PWM signal that maintains the H level for half the time and the L level for the remaining half of the time within a specified period represents 50%. A PWM signal that maintains the L level for the entire specified period represents 0%. A PWM signal that maintains the H level for the entire specified period represents 100%. In this embodiment, the L level is the ground voltage. The voltage of the L level is also called the L voltage. The H level is a specified voltage. The specified voltage can be appropriately set. The voltage of the H level is also called the H voltage. The movable contact of switch SW1 can, for example, contact a fixed contact connected to the first reference voltage source PR1 when the PWM signal level is L, outputting a voltage represented by VR1 to signal line L3. The movable contact of switch SW1 can, for example, contact a fixed contact connected to the second reference voltage source PR2 when the PWM signal level is H, outputting a voltage represented by VR2 to signal line L3. Switching circuit 103 controls the target terminal of the movable contact, thereby outputting a signal S2 to signal line L3 including either VR1 or VR2. Signal S2 is also called a control signal.

[0061] The first signal generation circuit 105 includes an LPF1, which includes a resistor R2 and a capacitor C1. The first signal generation circuit 105 may also include a buffer amplifier Q1. The buffer amplifier Q1 is an operational amplifier. The first signal generation circuit 105 uses the LPF1 to smooth the signal S2, converting it into a DC signal. The first signal generation circuit 105 uses the buffer amplifier Q1 to buffer the DC signal converted by the LPF1, outputting it as the first signal Va. The first signal generation circuit 105 outputs the first signal Va to the current output circuit 106 via signal line L4. The first signal generation circuit 105 outputs the first signal Va to the shunt adjustment circuit 108 via signal line L5. The first signal generation circuit 105 is not limited to a structure that separates the LPF1 and the buffer amplifier Q1; it can also be configured as an active filter where the LPF1 and the buffer amplifier Q1 are integrated. In the first signal generation circuit 105, the LPF1 can be replaced by an active filter.

[0062] Reference Figure 4 This explains the operation of the current output circuit 106.

[0063] like Figure 4As shown, the current output circuit 106 is connected to the external circuit 10 via transmission lines L1 and L2. That is, the two-wire transmitter 100 of one embodiment can be connected to the external circuit 10. The external circuit 10 has a voltage source Eb and a resistor R1. The voltage source Eb and the resistor R1 are connected in series to the transmission lines L1 and L2. The current output circuit 106 draws a current, denoted by Iout, from the transmission line L1 via diode D1, and the current, denoted by Iout, flows into the transmission line L2 via resistor R3, thereby outputting a current signal to the external circuit 10. The current output circuit 106 obtains a first signal Va from the measurement data processing circuit 102. The current output circuit 106 outputs a current signal Iout to the external circuit 10 based on the first signal Va. That is, the two-wire transmitter 100 determines the current signal Iout to be output to the external circuit 10 based on the first signal Va. The external circuit 10 can obtain information based on the measurement data from the sensor 50 from the two-wire transmitter 100 based on the current signal Iout.

[0064] The current output circuit 106 includes an error amplifier Q2. The error amplifier Q2 is an operational amplifier. The non-inverting terminal of the error amplifier Q2 is connected via resistor R4 to the signal line L4, which receives the first signal Va from the measurement data processing circuit 102. The signal line L4 is connected to the ground line via resistors R4, R5, and R3. The voltage on the transmission line L2 is denoted by Vb. The voltage obtained by dividing the potential difference between the first signal Va and the voltage Vb using resistors R4 and R5 is input to the non-inverting terminal of the error amplifier Q2. The voltage Vb corresponds to the voltage drop across resistor R3 caused by the current flowing into resistor R3. The current flowing into resistor R3 corresponds to the current signal Iout. The voltage obtained by dividing the potential difference between the first signal Va and the voltage Vb using resistors R4 and R5 is input to the non-inverting terminal of the error amplifier Q2, thereby feeding back a signal related to the current signal Iout to the non-inverting terminal of the error amplifier Q2. The inverting terminal of error amplifier Q2 is connected to the first reference voltage source PR1 via resistor R6, and to the ground line via resistor R7. The voltage obtained by dividing the voltage VR1 output from the first reference voltage source PR1 using resistors R6 and R7 is input to the inverting terminal of error amplifier Q2.

[0065] The current output circuit 106 includes transistors Q3 and Q4. Transistor Q3 is an npn transistor. Transistor Q4 is a pnp transistor. Transistor Q4 is connected to the transmission line L1 connecting the external circuit 10 and the two-wire transmitter 100 via diode D1 on the emitter side, and to the power supply line on the collector side. Resistor R8 is connected in parallel with the emitter and collector of transistor Q4. The two-wire transmitter 100 can be started by power supplied from the external circuit 10 via resistor R8. The base of transistor Q4 is connected to the collector of transistor Q3. The emitter of transistor Q3 is connected to the ground line via diode D2. The emitter of transistor Q3 can be connected to the ground line without the diode, or via a resistor or other component. The output of error amplifier Q2 is connected to the base of transistor Q3. That is, the output of error amplifier Q2 is input to the base of transistor Q3.

[0066] Error amplifier Q2 detects the difference between the voltage at the input non-inverting terminal and the voltage at the input inverting terminal, amplifies this difference, and outputs it. Error amplifier Q2 controls the current flowing into transistors Q3 and Q4 to make the voltage at the input non-inverting terminal match the voltage at the input inverting terminal. Specifically, the base current and collector current of transistor Q3 are determined based on the output of error amplifier Q2. In transistor Q4, the emitter current is determined by the base current. The base current of transistor Q4 matches the collector current of transistor Q3, which is determined based on the output of error amplifier Q2. Thus, the emitter current of transistor Q4 is determined based on the output of error amplifier Q2. The emitter current of transistor Q4 corresponds to the current signal, denoted by Iout, introduced from external circuit 10. The non-inverting terminal of error amplifier Q2 is connected to ground through resistor R3. That is, the output of error amplifier Q2 is fed back to the non-inverting terminal of error amplifier Q2 through resistor R3.

[0067] The parameters of each circuit element of the current output circuit 106 can be appropriately set so that the measurement range of the sensor 50's measurement data corresponds to the magnitude range of the current signal. When the current signal is a standard signal for measurement equipment, the parameters can be set so that the magnitude range of the current signal corresponding to the measurement range of the measurement data is between 4mA and 20mA. Regarding the measurement signal of the sensor 50, it can be represented as a 0% value corresponding to the lower limit of the sensor 50's measurement range, and as a 100% value corresponding to the upper limit of the measurement range. In other words, the measurement data of the sensor 50 can be represented by values ​​from 0% to 100%. When the measurement signal represents 0% of the value, the magnitude of the current signal can be set to the lower limit of 4mA, and when the measurement signal represents 100% of the value, the magnitude of the current signal can be set to the upper limit of 20mA.

[0068] For example, the magnitude of the current signal can be set based on the conditions represented by the following items (a) to (f).

[0069] (a) R6 >> R7

[0070] (b) R4=R5

[0071] (c) R5 >> R3

[0072] (d) VR1 = 0.4V

[0073] (e) VR2 = 2.0V

[0074] (f) When the level of the input PWM signal is L, switch SW1 outputs VR1 to signal line L3, and when the level of the input PWM signal is H, it outputs VR2 to signal line L3.

[0075] Under the conditions described in items (a) to (f) above, the current output circuit 106 operates as follows: Based on item (a), the voltage at the inverting terminal of error amplifier Q2 is the COM voltage. Error amplifier Q2 controls the current of transistors Q3 and Q4 to make the voltage at the non-inverting terminal the COM voltage. At this time, based on item (b), Vb = -Va holds true, and based on item (c), Iout = Va / R3 holds true. Here, the resistance value of resistor R3 is 100Ω. Based on items (d) to (f), when the PWM signal level is constant at L level, the first signal Va output by the first signal generation circuit 105 is 0.4V. In this case, the current signal Iout is the lower limit of 4mA. When the PWM signal level is constant at H level, the first signal Va is 2.0V. In this case, the current signal Iout is the upper limit of 20mA. Thus, the lower and upper limits of the current signal can be set.

[0076] Reference Figure 5 This explains the operation of the shunt adjustment circuit 108.

[0077] The shunt adjustment circuit 108 acquires a first signal Va from the measurement data processing circuit 102. Based on the first signal Va, the shunt adjustment circuit 108 controls the circuit voltage V1 applied to the two-wire transmitter 100. The circuit voltage V1 corresponds to the voltage applied between the ground wire and the power supply line. Figure 5 In this configuration, the grounding wire is the wiring connected to the grounding point, denoted as COM. The voltage of the grounding wire is also called the ground voltage or COM voltage. The power supply wire is the wiring denoted as V1.

[0078] like Figure 5As shown, the shunt adjustment circuit 108 includes an error amplifier Q6. The error amplifier Q6 is an operational amplifier. The inverting terminal of the error amplifier Q6 is connected via resistor R17 to signal line L5, which receives the first signal Va from the measurement data processing circuit 102. Node N1, located between resistor R17 and the inverting terminal of the error amplifier Q6, is connected to ground via resistor R14. Node N1 is connected to the power supply line via resistors R13 and R18. The voltage at node N1 is the voltage obtained by dividing the first signal Va using resistors R17 and R14. That is, the voltage obtained by dividing the first signal Va using resistors R17 and R14 is input to the inverting terminal of the error amplifier Q6. The non-inverting terminal of the error amplifier Q6 is connected to a second reference voltage source PR2. The voltage VR2 output by the second reference voltage source PR2 is input to the non-inverting terminal of the error amplifier Q6.

[0079] The shunt adjustment circuit 108 includes transistor Q7. Transistor Q7 is a p-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor). Transistor Q7 is not limited to a p-channel MOSFET; it can also be a pnp transistor. By swapping the input to the inverting input terminal and the input to the non-inverting input terminal of error amplifier Q6, transistor Q7 can be replaced with an n-channel MOSFET or an npn transistor. Transistor Q7 is connected to the power supply line at its source and to ground at its drain. The output of error amplifier Q6 is connected to the gate of transistor Q7. That is, the output of error amplifier Q6 is input to the gate of transistor Q7.

[0080] Error amplifier Q6 detects the difference between the voltage at the input non-inverting terminal and the voltage at the input inverting terminal, amplifies this difference, and outputs it. Error amplifier Q6 controls the current Ic flowing into transistor Q7 to make the voltage at the input non-inverting terminal match the voltage at the input inverting terminal. Specifically, shunt adjustment circuit 108 controls circuit voltage V1 to make the voltage at the inverting terminal of error amplifier Q6 become VR2. That is, the voltage at node N1 is controlled to be VR2. When the voltage at node N1 is VR2, the current Id flowing into resistor R14 is calculated using Id = VR2 / R14 and is a constant value.

[0081] Based on the voltage difference between the first signal Va and node N1, the current Ia flowing into resistor R17 is calculated as Ia = (Va - VR2) / R17.

[0082] Based on the voltage difference between the circuit voltage V1 and the node N1, the current Ib flowing into resistors R18 and R13 is calculated using Ib = (V1 - VR2) / (R13 + R18).

[0083] The current Id corresponds to the sum of currents Ia and Ib. If current Id is assumed to be a constant value, then the larger current Ia is, the smaller Ib is. If calculated separately using formulas for currents Ia and Ib, then the larger the first signal Va is, the smaller the circuit voltage V1 is.

[0084] Since Id=Ia+Ib holds true, the circuit voltage V1 is calculated using the following formula (1). V1=[1+(R13+R18) / R17+(R13+R18) / R14]×VR2-(R13+R18) / R17×Va (1)

[0085] If the specific values ​​shown in items (g) to (i) are applied as resistance values, the circuit voltage V1 is 12V when the current signal Iout is 4mA, and the circuit voltage V1 is 6V when the current signal Iout is 20mA.

[0086] (g) R14 = 150kΩ

[0087] (h) R17 = 80kΩ

[0088] (i) R18 + R13 = 300kΩ

[0089] The first term on the right-hand side of equation (1) indicates that the circuit voltage V1 is determined based on VR2. The second term on the right-hand side of equation (1) indicates that the circuit voltage V1 is determined based on Va. When the two-wire transmitter 100 is started, the output of the buffer amplifier Q1 of the first signal generation circuit 105 can become high impedance. When the output of the buffer amplifier Q1 is high impedance, the first signal Va output by the buffer amplifier Q1 may be non-constant. However, even when Va is non-constant, the circuit voltage V1 can still be determined based on the first term on the right-hand side of equation (1) above. As a result, the state of non-constant circuit voltage V1 at startup is avoided.

[0090] By determining the circuit voltage V1, the output of the error amplifier Q6 in the shunt regulating circuit 108 is prevented from being fixed at the COM voltage. Even when the output of the error amplifier Q6 is fixed at the COM voltage, the transistor Q7 is turned ON. When the transistor Q7 is turned ON, all or most of the current supplied to the two-wire transmitter 100 can flow into the transistor Q7. If this occurs, the current supplied to other circuits of the two-wire transmitter 100 will be insufficient, hindering the startup of the two-wire transmitter 100. By preventing the output of the error amplifier Q6 from being fixed at the COM voltage, the excessive current flowing into the transistor Q7 during startup is avoided. As a result, the startup of other circuits of the two-wire transmitter 100 is prevented from being hindered.

[0091] As explained above, in the two-wire transmitter 100 of this embodiment, both the current output circuit 106 and the shunt adjustment circuit 108 operate based on the first signal Va generated by the first signal generation circuit 105. According to the two-wire transmitter 100 of this embodiment, the current output circuit 106 and the shunt adjustment circuit 108 operate based on the universal first signal Va. This avoids the aforementioned problems caused by the difference in transient response between LPF3 and LPF4. As a result, operational stability is improved.

[0092] Furthermore, according to the two-wire transmitter 100 of this embodiment, when the two-wire transmitter 100 is started, the circuit voltage V1 is determined based on the voltage VR2 output by the second reference voltage source PR2. This avoids a state where the circuit voltage V1 is not constant during startup. As a result, the circuit of the two-wire transmitter 100 can start up more reliably.

[0093] like Figure 3 As shown, the two-wire transmitter 100 may also include a reference voltage output section 110, an LPF2 including resistor R9 and capacitor C2, and a comparator circuit 113. The comparator circuit 113 includes a comparator Q8. The non-inverting input terminal of the comparator Q8 is connected to the shunt adjustment circuit 108. The inverting input terminal of the comparator Q8 is connected to the LPF2 including resistor R9 and capacitor C2.

[0094] The reference voltage output unit 110 outputs a PWM signal based on the measurement signal S1 from the sensor 50. The reference voltage output unit 110 controls the duty cycle of the PWM signal based on the measurement signal S1. The smaller the measurement signal S1, the higher the duty cycle of the PWM signal. LPF2, including resistor R9 and capacitor C2, outputs a smoothed DC signal from the PWM signal output by the reference voltage output unit 110 to the inverting input terminal of comparator Q8. In other words, the DC signal based on the measurement signal S1 is input to the inverting input terminal of comparator Q8.

[0095] Comparator circuit 113 obtains a signal related to circuit voltage V1 from shunt adjustment circuit 108. This signal is input to the non-inverting input terminal of comparator Q8. The signal related to circuit voltage V1 corresponds to the voltage obtained by dividing circuit voltage V1 using the sum of the resistance values ​​of resistors R18, R13, and R14 (see reference). Figure 5 ).

[0096] Comparator Q8 compares a signal related to the circuit voltage V1 with a DC signal based on the measured signal S1 to determine whether the circuit voltage V1 is above a specified value or has decreased to below a specified value. Comparator Q8 outputs a signal related to the determination result to signal processing circuit 104. Comparator Q8 can output a signal at level H when the circuit voltage V1 is above the specified value, and a signal at level L when the circuit voltage V1 decreases to below the specified value. Signal processing circuit 104 can confirm whether the circuit voltage V1 is above the specified value based on the level of the signal output by comparator Q8. If the circuit voltage V1 decreases to below the specified value, signal processing circuit 104 can determine that the two-wire transmitter 100 is in an abnormal state. If the two-wire transmitter 100 is determined to be in an abnormal state, signal processing circuit 104 can perform processing corresponding to the abnormal state. Processing corresponding to the abnormal state includes, for example, saving the value of the measured signal S1 from sensor 50.

[0097] like Figure 6 As shown, another embodiment of the two-wire transmitter 100 can reduce several circuit elements. The measurement data processing circuit 102 in... Figure 3 This includes the first reference voltage source PR1 and the switch SW1. In contrast, in Figure 6 The first reference voltage source PR1 and switch SW1 are not included. Figure 6 Examples of its composition do not include those related to Figure 3 The structure corresponding to the switch circuit 103 shown is illustrated. Figure 6 In this circuit, the signal processing circuit 104 outputs the PWM signal as signal S2 directly to the signal line L3 without passing through the structure corresponding to the switching circuit 103. The voltage H of signal S2, which is used as the PWM signal, can be appropriately set. The voltage H of signal S2 can be set based on the voltage VR2 output by the second reference voltage source PR2. The voltage H of signal S2 can be set to a value larger than VR2, equal to VR2, or smaller than VR2.

[0098] The input signal S2 from signal line L3 is smoothed by LPF1, which includes resistor R2 and capacitor C1, buffered in buffer amplifier Q1, and transformed into the first signal Va. The voltage of the first signal Va is determined based on the voltage H of signal S2 and the duty cycle. For example, when the duty cycle of signal S2 is 50%, the voltage of the first signal Va is half the voltage H of signal S2. Assume that the voltage H of signal S2 is 2.0V. Under this assumption, if the first signal Va is controlled between, for example, 0.4V and 2.0V, the duty cycle of signal S2 is controlled between 20% and 100%.

[0099] exist Figure 3In the embodiment shown, the voltage of the first signal Va, corresponding to the case where the measured signal of sensor 50 is 100%, is the same as VR2. The smaller the value of the measured signal, the smaller Va is. That is, when the measured signal is any value between 0% and 100%, Va is below VR2.

[0100] On the other hand, Figure 6 In another embodiment shown, the H voltage of signal S2 can be set to a voltage different from VR2. When the H voltage of signal S2 is set to a voltage different from VR2, the voltage of the first signal Va, which corresponds to the case where the measured signal of sensor 50 is 100%, is different from VR2.

[0101] In the shunt adjustment circuit 108, current flows into resistor R17 based on the potential difference between VR2 and Va. When Va is lower than VR2, the current in resistor R17 flows from the error amplifier Q6 side towards the buffer amplifier Q1 side. This direction from the error amplifier Q6 side towards the buffer amplifier Q1 side is also called the first direction. When Va is higher than VR2, the current in resistor R17 flows from the buffer amplifier Q1 side towards the error amplifier Q6 side. This direction from the buffer amplifier Q1 side towards the error amplifier Q6 side is also called the second direction. In other words, the direction of the current in resistor R17 changes depending on whether Va is lower or higher than VR2.

[0102] When Va and VR2 are set to the same value corresponding to a specified value where the measured signal is above 0% and below 100%, the relationship between Va and VR2 changes according to the value of the measured signal. When the measured signal is the specified value, Va and VR2 become the same value. In this case, no current flows through resistor R17. When the measured signal is less than the specified value, Va is lower than VR2. In this case, the current in resistor R17 flows in the first direction. When the measured signal is greater than the specified value, Va is higher than VR2. In this case, the current in resistor R17 flows in the second direction.

[0103] When Va and VR2 are set to the same value corresponding to a measured signal of 100%, the direction of the current flowing in resistor R17 is only the first direction. In this case, it can be said that the specified value is set to 100%.

[0104] The greater the difference between the measured signal and the specified value, the greater the current in resistor R17, and the greater the power dissipation in resistor R17. Here, it is assumed that the probability distribution of the measured signal value is the same between 0% and 100%. Under this assumption, when the specified value is set to 50%, the power dissipation in resistor R17 is minimized. On the other hand, when the specified value is set to 100%, the power dissipation in resistor R17 becomes maximum. In other words, setting the specified value to be greater than 0% and less than 100% reduces the power dissipation in resistor R17 compared to setting the specified value to 100%.

[0105] The inverting input terminal of the error amplifier Q2 in the current output circuit 106 is at Figure 3 It is connected to the first reference voltage source PR1 via resistors R6 and R7. In contrast, Figure 6 It is connected to the ground wire. Additionally, the current output circuit 106 is in... Figure 6 Resistors R6 and R7 are not included.

[0106] exist Figure 6 In the current output circuit 106, the structure in which the inverting input terminal of the error amplifier Q2 is connected to the ground wire corresponds to the following: Figure 3 The voltage VR1 output from the first reference voltage source PR1 is set to 0V. That is to say, Figure 6 The current output circuit 106 and the current output circuit for Figure 3 Similarly, the action described can determine the current signal based on the first signal Va.

[0107] exist Figure 6 The illustrated two-wire transmitter 100 can be powered by a ratio Figure 1 The illustrated two-wire transmitter 100 has fewer components. As a result, it is possible to reduce the component mounting area or lower the cost.

[0108] like Figure 7 As shown, one embodiment of the two-wire transmitter 100 may include a DA converter 121. In Figure 3 In the two-wire transmitter 100, the switching circuit 103 and the first signal generation circuit 105 are included. Figure 7 The signal processing circuit 104 is replaced by a DA converter 121. The DA converter 121 converts the signal based on the measured signal output from the signal processing circuit 104 into a first signal Va, replacing the functions of the switching circuit 103 and the first signal generation circuit 105. In this case, the signal processing circuit 104 outputs a control signal as a signal based on the measured signal. The control signal includes a digital signal indicating that the measured signal is any value between 0% and 100%. The DA converter 121 converts the digital signal into an analog signal and outputs it as the first signal Va.

[0109] The two-wire transmitter 100 may include a DA converter 122. Figure 3 The two-wire transmitter 100 includes a reference voltage output section 110 and an LPF2 comprising a resistor R9 and a capacitor C2. Figure 7 The reference voltage output from the signal processing circuit 104 is replaced by a DA converter 122. The DA converter 122 converts the reference voltage output from the signal processing circuit 104 into a DC signal, thereby replacing the functions of the reference voltage output unit 110 and LPF2.

[0110] The DA converter 121 or DA converter 122 can be configured as a different structure from the signal processing circuit 104, or it can be included in the signal processing circuit 104. A portion of the circuit elements of the two-wire transmitter 100 are replaced by the DA converter 121 or DA converter 122, thereby reducing the number of components. As a result, it is possible to reduce the component mounting area or lower the cost, and improve the stability of operation.

[0111] If the signal processing circuit 104 becomes abnormal due to loss of control, the first signal Va becomes unstable. In this case, efforts are made to burn out the current signal output by the current output circuit 106. Burnout refers to the processing of a current signal flowing as a current signal of 3.6mA or less or 21.6mA or more.

[0112] For example, according to Figure 8 The two-wire transmitter 100 shown in one embodiment is capable of depletion. Figure 8 Detailed descriptions of the structure of the current output circuit 106 and the shunt adjustment circuit 108 are omitted in the text.

[0113] Figure 8 The illustrated two-wire transmitter 100 includes switches SW2, SW3, SW4, and SW5, a counter 114, an OR gate 0G, and an inverter INV. These structures are also referred to as an abnormal state detection circuit. The abnormal state detection circuit is not limited to... Figure 8 The method illustrated can be implemented in various other ways.

[0114] Switch SW4 has three fixed contacts. The first fixed contact of switch SW4 is connected to the positive electrode of the first reference voltage source PR1 for output voltage VR1. The second fixed contact of switch SW4 is connected to the positive electrode of the second reference voltage source PR2 for output voltage VR2. The third fixed contact of switch SW4 is connected to the positive electrode of the third reference voltage source PR3 for output voltage VR3. The movable contact of switch SW4 is connected to signal line L3. This configuration of switch SW4 allows it to selectively output any one of voltages VR1, VR2, and VR3 to current output circuit 106 according to the operating state of signal processing circuit 104.

[0115] Counter 114 is a free-run counter that detects abnormalities in signal processing circuit 104. Counter 114 outputs an error signal ERR at a specified level corresponding to the state of signal processing circuit 104. Counter 114 is reset to zero at the edge of the reset signal CLR input from signal processing circuit 104. Under normal operation of signal processing circuit 104, the error signal ERR is reset to zero and becomes level L. If signal processing circuit 104 malfunctions due to loss of control of the internal CPU, the error signal ERR is not reset but overflows, becoming level H.

[0116] The error signal ERR is input as a switching signal to switches SW2 and SW3, and also to one input terminal of OR gate OG. The output signal V3 of comparator Q8 is input to the other input terminal of OR gate OG via inverter INV. The output signal iV3 of inverter INV is also input to switch SW4 as a switching signal. The 'i' in iV3 indicates an inverted signal. The output signal of OR gate OG is input to switch SW5 as the voltage switching signal VSEL.

[0117] Switch SW2 has two fixed contacts that selectively output signals indicating whether the signal processing circuit 104 is in a normal or abnormal state. A PWM signal is input from the signal processing circuit 104 to one fixed contact of switch SW2. The output signal of switch SW3 is input to the other fixed contact of switch SW2. The output signal from the movable contact of switch SW2 is input as a switching signal to switch SW4.

[0118] The movable contact of switch SW2 operates based on whether the signal processing circuit 104 is in a normal or abnormal state. In the normal state (where the error signal ERR indicates a low level), the movable contact of switch SW2 selects the fixed contact of the input current signal (PWM signal). In the abnormal state (where the error signal ERR indicates a high level), the movable contact of switch SW2 selects the fixed contact of the input abnormal direction indication signal DIR from switch SW3.

[0119] Switch SW3 has two fixed contacts that selectively output current exceeding either the upper or lower limit, indicating an abnormal state of signal processing circuit 104. A circuit voltage V1 is input to one fixed contact of switch SW3. The other fixed contact of switch SW3 is connected to ground. The output signal from the movable contact of switch SW3 is input to the other fixed contact of switch SW2 as an abnormal direction indication signal DIR.

[0120] When the signal processing circuit 104 is in an abnormal state, the movable contact of switch SW3 selects either fixed contact to output a signal indicating whether the abnormality is caused by exceeding either the upper or lower limit as a first signal Va to the current output circuit 106. For example, the movable contact of switch SW3 can select a fixed contact that is the input circuit voltage V1, outputting the circuit voltage V1 as an abnormal direction indication signal DIR, so that the current output by the current output circuit 106 is 21.6mA or more when the upper limit is exceeded. Alternatively, the movable contact of switch SW3 can select a fixed contact that is the input ground voltage, outputting the ground voltage as an abnormal direction indication signal DIR, so that the current output by the current output circuit 106 is 3.6mA or less when the lower limit is exceeded. Figure 8 In the input signal ERR, which is used as the switching signal for switch SW3, other signals can also be input.

[0121] Switch SW5 has two fixed contacts that select the voltage input to comparator circuit 113. A reference voltage PWM signal is input to one fixed contact of switch SW5. The connection point of resistors R15 and R16, which are connected in series, is connected to the other fixed contact of switch SW5. The output signal from the movable contact of switch SW5 is input to one end of resistor R9, which forms LPF2. Circuit voltage V1 is input to one end of resistor R15, which is connected in series, and the other end of resistor R16 is connected to a general-purpose potential point.

[0122] The movable contact of switch SW5 selects the fixed contact to be connected based on the voltage switching signal VSEL output by the OR gate OG. When the output signal V3 of comparator Q8 is at level L or when the signal processing circuit 104 becomes abnormal, the voltage switching signal VSEL is at level H. When the voltage switching signal VSEL is at level H, the movable contact of switch SW5 selects the fixed contact whose voltage is obtained by dividing the circuit voltage V1 using the series circuit of resistors R15 and R16. When the output signal V3 of comparator Q8 becomes at level H and the signal processing circuit 104 is normal, the voltage switching signal VSEL becomes at level L. When the voltage switching signal VSEL is at level L, the movable contact of switch SW5 selects the fixed contact whose reference voltage is input using a PWM signal.

[0123] Figure 8 The two-wire transmitter 100 shown in one embodiment includes an abnormal state detection circuit, which enables the current signal output by the current output circuit 106 to be depleted according to the abnormal state in the event of an abnormality in the signal processing circuit 104.

[0124] The embodiments of this disclosure have been described based on the accompanying drawings and examples. However, it should be noted that those skilled in the art can easily make various modifications or alterations based on this disclosure. Therefore, it should be noted that such modifications or alterations are included within the scope of this disclosure. For example, the functions included in each component or step can be reconfigured in a logically consistent manner, and multiple components or steps can be combined into one or separated.

Claims

1. A two-wire transmitter, connected to an external circuit via two transmission lines, using the external circuit as a power source, and outputting a current signal to the external circuit, comprising: The measurement data processing circuit is connected to a sensor that outputs an electrical signal based on the measurement data, and has an output terminal configured to output a first signal based on the measurement data. A current output circuit determines the current signal based on the first signal; and The shunt adjustment circuit determines the circuit voltage of the two-wire transmitter based on the first signal. The measurement data processing circuit includes: a signal processing circuit, which processes the electrical signal and outputs a control signal as a digital signal; and the first signal generation circuit, which outputs the first signal based on the control signal. The first signal generation circuit includes a DA converter as a circuit element, which directly accepts the control signal as a digital signal and converts it into the first signal as an analog signal for output. The first signal is output from the output terminal and is not changed after being output from the output terminal. While maintaining the state of the signal generated by the measurement data processing circuit, it is bibranched toward each of the current output circuit and the shunt adjustment circuit so that it is input to both the current output circuit and the shunt adjustment circuit as the same signal as the signal generated by the measurement data processing circuit.

2. The two-wire transmitter according to claim 1, wherein, The current output circuit determines the current signal based on the difference between the first signal and the ground voltage, or a first reference voltage that is higher than the ground voltage. The shunt adjustment circuit determines the circuit voltage of the two-wire transmitter based on the difference between the first signal and the second reference voltage output from the reference voltage source.

3. The two-wire transmitter according to claim 1, wherein, The current output circuit has a first error amplifier that determines the current signal based on the output from the first error amplifier when the first signal is input to the non-inverting terminal of the first error amplifier and a ground voltage or a first reference voltage higher than the ground voltage is input to the inverting terminal of the first error amplifier. The shunt adjustment circuit has a second error amplifier that determines the circuit voltage of the two-wire transmitter based on the output of the second error amplifier when the first signal is input to the inverting terminal of the second error amplifier and the second reference voltage output from the reference voltage source is input to the non-inverting terminal of the second error amplifier.

4. The two-wire transmitter according to claim 1, wherein, The current output circuit makes the current flowing as the current signal smaller based on the first signal, and the branch circuit makes the circuit voltage larger based on the first signal.

5. The two-wire transmitter according to claim 2, wherein... The current output circuit makes the current flowing as the current signal smaller based on the first signal, and the branch circuit makes the circuit voltage larger based on the first signal.

6. The two-wire transmitter according to claim 3, wherein, The current output circuit makes the current flowing as the current signal smaller based on the first signal, and the branch circuit makes the circuit voltage larger based on the first signal.

7. The two-wire transmitter according to any one of claims 1 to 6, wherein, The measurement data processing circuit includes an abnormal state detection circuit.

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