JP2019186626A - Amplifier and offset adjustment method for the amplifier - Google Patents

Abstract

To enable adjustment of offset voltage even when an output voltage of an amplifier unit is out of an input range of an ADC(Analog/Digital Converter).SOLUTION: An amplifier includes: a saturation determination unit 1401 for determining whether an output of ADC 12 is saturated or not by reading a voltage value of a differential signal after conversion by the ADC12; a coarse adjustment unit 1402 performing rough adjustment for adjusting voltage of an offset adjustment signal output by a DAC(Digital/Analog Converter) 13 so that voltage of a differential signal after amplification by an amplifier unit 15 is settled within an input range of the ADC 12, when the saturation determination unit 1401 determines that the output is saturated; and a fine adjustment unit 1403 performing fine adjustment for adjusting the voltage of the offset adjustment signal output by the DAC 13 so that an offset voltage included in the differential signal input to the amplifier unit 15 is settled within a target threshold value, when the saturation determination unit 1401 determines that the output is not saturated or when the coarse adjustment unit 1402 completes the coarse adjustment.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an amplifier for adjusting an offset voltage included in an input differential signal and an offset adjustment method for the amplifier.

  As shown in FIG. 7, the instrumentation amplifier (amplifier) is connected to a sensor circuit (such as a torque sensor or a pressure sensor) such as a Wheatstone bridge circuit, and amplifies a differential signal input from the sensor circuit ( For example, see Patent Document 1). Since the instrumentation amplifier removes the common mode component and amplifies only the signal component, it is suitable for connection to a sensor circuit such as a Wheatstone bridge circuit that generates a common mode in principle. This instrumentation amplifier includes an amplifier section and a single-ended amplifier.

On the other hand, the torque sensor is used for force control of the smart robot, but is affected by the stress generated during the assembly of the torque sensor or the smart robot. As a result, the torque detected by the torque sensor becomes larger than the actual torque, that is, the offset becomes larger.
On the other hand, when the offset voltage (offset component) included in the differential signal input from the sensor circuit is adjusted by an instrumentation amplifier, generally, an offset adjustment circuit is provided in a single-ended amplifier as shown in FIG. A way to connect is taken. However, in this case, since the amplifier component amplifies the offset component as well as the signal component, there is a problem that the gain of the instrumentation amplifier cannot be made very large.

  In order to solve this problem, the applicant proposed in Japanese Patent Application No. 2017-220926 an instrumentation amplifier including an offset adjustment circuit that is connected to an amplifier unit and adjusts an offset voltage by an input offset adjustment signal. Yes. With this offset adjustment circuit, an instrumentation amplifier having an offset adjustment function and capable of obtaining a large gain can be realized. In this instrumentation amplifier, an ADC (analog / digital converter) is used instead of the single-ended amplifier.

JP 2017-130743 A

In the instrumentation amplifier having the above-described offset adjustment circuit, when the offset adjustment signal is generated using a DAC (digital analog converter), the offset adjustment signal is set based on the offset voltage calculated from the output of the ADC.
On the other hand, if the offset voltage input to the amplifier unit is large or the gain of the amplifier unit is large and the output voltage of the amplifier unit is outside the ADC input range, the ADC output is saturated. For example, in a torque sensor used in a smart robot, the fluctuation of the offset voltage may be increased due to the influence of stress generated during assembly, and the output of the ADC may be saturated. In this state, the instrumentation amplifier cannot accurately calculate the offset voltage from the output of the ADC, and cannot generate an appropriate offset adjustment signal.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an instrumentation amplifier capable of adjusting an offset voltage even when the output voltage of the amplifier section is outside the input range of the ADC. It is said.

  An amplifier according to the present invention includes an amplifier unit that amplifies an input differential signal, an analog-to-digital converter that converts the differential signal amplified by the amplifier unit from an analog signal to a digital signal, and offset adjustment that is a differential signal A digital analog converter that outputs a signal; an offset adjustment circuit that is connected to the amplifier unit and adjusts an offset voltage included in a differential signal input to the amplifier unit by an offset adjustment signal output from the digital analog converter; Reads the voltage value of the differential signal after conversion by the analog-to-digital converter, determines whether the output of the analog-to-digital converter is saturated, and determines that the output is saturated by the saturation determination unit The differential signal voltage after amplification by the amplifier A coarse adjustment unit that performs coarse adjustment to adjust the voltage of the offset adjustment signal output by the digital-analog converter so as to be within the input range of the inverter, and a case where the saturation determination unit determines that the output is not saturated or When the coarse adjustment by the coarse adjustment unit is completed, the voltage of the offset adjustment signal output by the digital analog converter is adjusted so that the offset voltage included in the differential signal input to the amplifier unit is within the target threshold. And a fine adjustment unit for performing fine adjustment.

  According to the present invention, since it is configured as described above, the offset voltage can be adjusted even when the output voltage of the amplifier section is outside the input range of the ADC.

It is a figure which shows the structural example of the instrumentation amplifier which concerns on Embodiment 1 of this invention. It is a figure which shows the structural example of the adjustment part in Embodiment 1 of this invention. It is a flowchart which shows the operation example by the adjustment part in Embodiment 1 of this invention. It is a flowchart which shows the operation example by the saturation determination part in Embodiment 1 of this invention. It is a flowchart which shows the operation example by the rough adjustment part in Embodiment 1 of this invention. It is a flowchart which shows the operation example by the fine adjustment part in Embodiment 1 of this invention. It is a figure which shows the structure of the conventional instrumentation amplifier. It is a figure which shows the structure at the time of adding the offset adjustment function to the conventional instrumentation amplifier.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
Embodiment 1 FIG.
FIG. 1 is a diagram showing a configuration example of an instrumentation amplifier (amplifier) 1 according to Embodiment 1 of the present invention. FIG. 1 also shows a sensor circuit 2 to which an instrumentation amplifier 1 is connected. The sensor circuit 2 is a torque sensor, a pressure sensor, or the like, and is a circuit that outputs a differential signal, such as a Wheatstone bridge circuit.
The instrumentation amplifier 1 is connected to the sensor circuit 2 and amplifies the signal component by removing the common mode component from the differential signal input from the sensor circuit 2. The instrumentation amplifier 1 also has a function of adjusting an offset voltage (offset component) included in the differential signal. As shown in FIG. 1, the instrumentation amplifier 1 includes an amplifier unit 11 with an offset adjustment function, an ADC (analog / digital converter) 12, a DAC (digital / analog converter) 13, and an adjustment unit 14. The amplifier unit 11 with an offset adjustment function includes an amplifier unit 15 and an offset adjustment circuit 16.

  The amplifier unit 15 amplifies the differential signal input from the sensor circuit 2. The amplifier unit 15 is configured by a differential amplifier or the like. Note that the voltage of one of the two signals constituting the differential signal input to the amplifier unit 15 is Vi1, and the voltage of the other signal is Vi2.

  The ADC 12 converts the differential signal input from the amplifier unit 15 from an analog signal to a digital signal. Note that a voltage of a signal output from one of the pair of output terminals of the ADC 12 is Vo1, and a voltage of a signal output from the other of the pair of output terminals of the ADC 12 is Vo2.

  The DAC 13 outputs an offset adjustment signal. The offset adjustment signal is a differential signal that adjusts an offset voltage included in the differential signal input to the amplifier unit 15. Note that the voltage of one of the two signals constituting the offset adjustment signal is Vt1, and the voltage of the other signal is Vt2.

  The offset adjustment circuit 16 is connected to the amplifier unit 15 and adjusts an offset voltage included in a differential signal input from the sensor circuit 2 to the amplifier unit 15 by an offset adjustment signal output from the DAC 13.

  The adjustment unit 14 adjusts the offset adjustment signal based on the differential signal converted by the ADC 12. As illustrated in FIG. 2, the adjustment unit 14 includes a saturation determination unit 1401, a coarse adjustment unit 1402, and a fine adjustment unit 1403. The adjustment unit 14 is realized by a CPU (Central Processing Unit) that executes a program stored in a memory or the like.

  The saturation determination unit 1401 reads the conversion value of the ADC 12 (the voltage value of the differential signal after conversion by the ADC 12) and determines whether the output of the ADC 12 is saturated. At this time, the saturation determination unit 1401 determines that the output of the ADC 12 is saturated when the conversion value of the ADC 12 is the maximum value or the minimum value that the conversion value can take, and otherwise the ADC 12 Is determined not to be saturated. Note that the determination processing by the saturation determination unit 1401 may be performed only once, for example, when the instrumentation amplifier 1 is activated for the first time, or may be periodically performed.

  The coarse adjustment unit 1402 determines the offset adjustment signal so that the voltage of the differential signal input to the ADC 12 falls within the input range of the ADC 12 when the saturation determination unit 1401 determines that the output of the ADC 12 is saturated. Make coarse adjustments to adjust the voltage. At this time, the coarse adjustment unit 1402 adds a fixed voltage to one of the two signals constituting the offset adjustment signal and subtracts the fixed voltage from the other until the output of the ADC 12 is not saturated. .

  The fine adjustment unit 1403 is included in the differential signal input to the amplifier unit 15 when the saturation determination unit 1401 determines that the output of the ADC 12 is not saturated or when the coarse adjustment by the coarse adjustment unit 1402 is completed. Fine adjustment is performed to adjust the voltage of the offset adjustment signal so that the offset voltage to be within the target threshold value. At this time, the fine adjustment unit 1403 adds a voltage 3/2 times the offset voltage to one of the two signals constituting the offset adjustment signal until the offset voltage falls within the target threshold, A voltage that is 3/2 times the offset voltage is subtracted.

Next, offset adjustment by the adjustment unit 14 of the instrumentation amplifier 1 according to Embodiment 1 will be described with reference to FIGS.
In the instrumentation amplifier 1, the offset adjustment circuit 16 can adjust the offset voltage included in the differential signal by injecting an appropriate offset adjustment signal to the differential signal input to the amplifier unit 15. The offset voltage can be calculated from the output of the ADC 12, and the instrumentation amplifier 1 adjusts the offset adjustment signal so that the relationship of the following expression (1) is satisfied. In Expression (1), Vin is an offset voltage.
Vt1−Vt2 = 3 × Vin (1)

As shown in Expression (1), the instrumentation amplifier 1 sets (Vt1−Vt2), which is the voltage of the offset adjustment signal, to be three times the offset voltage, so that the differential signal input to the amplifier unit 15 The offset voltage, which is an offset component, can be amplified without increasing the signal component, and the gain can be increased. In addition, the instrumentation amplifier 1 does not divide the offset adjustment signal voltage evenly by setting the voltage of the offset voltage to be equally divided into three times the offset voltage as in the following equations (2) and (3). The gain can be further increased with respect to the case.
Vt1 = (3/2) × Vin (2)
Vt2 = − (3/2) × Vin (3)

On the other hand, when the offset voltage input to the amplifier unit 15 is large or the gain of the amplifier unit 15 is large, and the output voltage of the amplifier unit 15 is outside the input range of the ADC 12, the output of the ADC 12 is saturated. For example, in a torque sensor used in a smart robot, the offset voltage may vary greatly due to the influence of stress generated during assembly, and the output of the ADC 12 may be saturated. In this state, the instrumentation amplifier 1 cannot accurately calculate the offset voltage from the output of the ADC 12 and cannot inject an appropriate offset adjustment signal for the differential signal input to the amplifier unit 15.
Therefore, in the instrumentation amplifier 1 according to the first embodiment, if the output of the ADC 12 is saturated, and if it is determined that the output is saturated, first, the voltage of the differential signal input to the ADC 12 is set. Adjust the voltage (rough adjustment) so that it is within the input range. Thereby, the instrumentation amplifier 1 can set the voltage of the differential signal input to the ADC 12 within the input range, and can calculate the offset voltage from the output of the ADC 12. After that, the instrumentation amplifier 1 performs voltage adjustment (fine adjustment) so that the calculated offset voltage is within the target threshold according to the equations (1) to (3).

  In the operation example by the adjustment unit 14 of the instrumentation amplifier 1, as shown in FIG. 3, the saturation determination unit 1401 first determines whether or not the output of the ADC 12 is saturated (step ST301).

  If the saturation determination unit 1401 determines in step ST301 that the output of the ADC 12 is saturated, the coarse adjustment unit 1402 performs coarse adjustment of the offset adjustment signal (step ST302). That is, the coarse adjustment unit 1402 adjusts the voltage of the offset adjustment signal so that the voltage of the differential signal input to the ADC 12 falls within the input range of the ADC 12.

  On the other hand, when the saturation determination unit 1401 determines in step ST301 that the output of the ADC 12 is not saturated, or when the coarse adjustment by the coarse adjustment unit 1402 is completed in step ST302, the fine adjustment unit 1403 outputs the offset adjustment signal. Is finely adjusted (step ST303). That is, the fine adjustment unit 1403 adjusts the voltage of the offset adjustment signal so that the offset voltage included in the differential signal input to the amplifier unit 15 falls within the target threshold value.

Next, an operation example by the saturation determination unit 1401 in step ST301 will be described with reference to FIG.
In the operation example by the saturation determination unit 1401, as shown in FIG. 4, the saturation determination unit 1401 first reads the converted value of the ADC 12 (step ST401). Note that the value of the differential signal after conversion by the ADC 12 slightly varies due to the influence of circuit noise and the like. Therefore, it is desirable that the saturation determination unit 1401 uses a voltage value obtained by averaging the voltage values of a plurality of differential signals after conversion by the ADC 12 as the conversion value of the ADC 12.

Next, saturation determination section 1401 determines whether or not the read conversion value of ADC 12 is the maximum value or the minimum value (step ST402).
In step ST402, when the saturation determination unit 1401 determines that the converted value of the ADC 12 is the maximum value or the minimum value, the saturation determination unit 1401 determines that the output of the ADC 12 is saturated (step ST403).
On the other hand, in step ST402, when the saturation determination unit 1401 determines that the converted value of the ADC 12 is not the maximum value or the minimum value, the saturation determination unit 1401 determines that the output of the ADC 12 is not saturated (step ST404).

Next, an operation example by the coarse adjustment unit 1402 in step ST302 will be described with reference to FIG.
In the operation example by the coarse adjustment unit 1402, as shown in FIG. 5, the coarse adjustment unit 1402 first initializes the number of adjustments of the offset adjustment signal to 1 (step ST501).

Next, coarse adjustment section 1402 adjusts the offset adjustment signal using a fixed voltage (step ST502). At this time, as shown in the following equations (4) and (5), the coarse adjustment unit 1402 adds a fixed voltage to one of the two signals constituting the offset adjustment signal, and subtracts the fixed voltage from the other. To do. In equations (4) and (5), Vx is a fixed voltage. Vx is assigned a positive value when the converted value of the ADC 12 is the maximum value, and is assigned a negative value when the converted value of the ADC 12 is the minimum value.
Vt1 = Vt1 + Vx (4)
Vt2 = Vt2-Vx (5)

  Thereafter, the DAC 13 outputs the offset adjustment signal adjusted by the coarse adjustment unit 1402, and the offset adjustment circuit 16 injects this offset adjustment signal into the differential signal input to the amplifier unit 15.

  Next, coarse adjustment section 1402 reads the converted value of ADC 12 (step ST503). The coarse adjustment unit 1402 preferably uses a voltage value obtained by averaging the voltage values of a plurality of differential signals after conversion by the ADC 12 as the conversion value of the ADC 12 for the same purpose as described above.

  Next, coarse adjustment section 1402 determines whether or not the read conversion value of ADC 12 is the maximum value or the minimum value (step ST504).

  In step ST504, when the coarse adjustment unit 1402 determines that the converted value of the ADC 12 is the maximum value or the minimum value, the coarse adjustment unit 1402 determines that the output of the ADC 12 is saturated, and the number of adjustments of the offset adjustment signal is N. It is determined whether it is the first time (step ST505).

  In this step ST505, when it is determined that the number of adjustments is not the Nth, the coarse adjustment unit 1402 increments the number of adjustments (step ST506). Thereafter, the sequence returns to step ST502, and the adjustment of the offset adjustment signal is repeated.

  On the other hand, in step ST505, if coarse adjustment section 1402 determines that the number of adjustments is the Nth, it determines that there is an error (step ST507).

  On the other hand, in Step ST504, when the coarse adjustment unit 1402 determines that the converted value of the ADC 12 is not the maximum value or the minimum value, it determines that the output of the ADC 12 is not saturated, and the sequence ends.

Next, an operation example by the fine adjustment unit 1403 in step ST303 will be described with reference to FIG.
In the operation example by the fine adjustment unit 1403, as shown in FIG. 6, the fine adjustment unit 1403 first initializes the number of adjustments of the offset adjustment signal to 1 (step ST601).

Next, fine adjustment section 1403 reads the conversion value of ADC 12 and calculates an offset voltage from the conversion value (step ST602). Note that the fine adjustment unit 1403 preferably uses a voltage value obtained by averaging the voltage values of a plurality of differential signals after conversion by the ADC 12 as the conversion value of the ADC 12 for the same purpose as described above.
Further, the offset voltage is obtained from the following equation (6). Vzero in Equation (6) is the LSB (quantization unit: Last Significant Bit) of the ADC 12, and is a value obtained by dividing the reference voltage of the ADC 12 by the number of bits. Further, Gain in Expression (6) is the amplification factor of the amplifier unit 15 and is obtained from the configuration of the amplifier unit 15. The Vzero and Gain are obtained in advance.
Vin = conversion value of ADC12 × Vzero / Gain (6)

Next, fine adjustment section 1403 adjusts the offset adjustment signal using the offset voltage (step ST603). At this time, the fine adjustment unit 1403 adds 3/2 times the offset voltage to one of the two signals constituting the offset adjustment signal as shown in the following equations (7) and (8), and from the other Subtract 3/2 times the offset voltage.
Vt1 = Vt1 + (3/2) × Vin (7)
Vt2 = Vt2− (3/2) × Vin (8)

  Thereafter, the DAC 13 outputs the offset adjustment signal adjusted by the fine adjustment unit 1403, and the offset adjustment circuit 16 injects this offset adjustment signal into the differential signal input to the amplifier unit 15. As a result, the offset voltage included in the differential signal input to the amplifier unit 15 can be reduced.

  Next, fine adjustment section 1403 reads the conversion value of ADC 12 and calculates an offset voltage from the conversion value (step ST604). The process in step ST604 is the same as the process in step ST602.

  Next, fine adjustment section 1403 determines whether or not the calculated offset voltage is within the target threshold value (step ST605). Vth in FIG. 6 is a target threshold value.

  If it is determined in step ST605 that the offset voltage is not within the target threshold value, fine adjustment section 1403 determines whether the number of adjustments of the offset adjustment signal is the Mth (step ST606).

  If the fine adjustment unit 1403 determines in step ST606 that the number of adjustments is not the Mth, the fine adjustment unit 1403 increments the number of adjustments (step ST607). Thereafter, the sequence returns to step ST603, and the adjustment of the offset adjustment signal is repeated.

On the other hand, in step ST606, fine adjustment section 1403 determines that there is an error when it is determined that the number of adjustments is the Mth (step ST608).
Note that in step ST608, the fine adjustment unit 1403 may determine that the offset voltage at this time is the minimum value without determining the error, and end the sequence.

  On the other hand, in step ST605, when fine adjustment section 1403 determines that the offset voltage is within the target threshold, the number of times that the offset voltage is repeatedly determined to be within the target threshold (the number of determinations within the threshold) is X times. It is determined whether or not there is (step ST609). That is, since the value of the offset voltage varies somewhat due to the influence of circuit noise or the like, the fine adjustment unit 1403 confirms that the offset voltage has surely converged within the target threshold value.

  In step ST609, when fine adjustment section 1403 determines that the number of in-threshold determinations is not X, the sequence returns to step ST604 and repeats the calculation of the offset voltage.

  On the other hand, in step ST609, fine adjustment section 1403 ends the sequence when determining that the number of determinations within the threshold is X.

  As described above, in the instrumentation amplifier 1 according to the first embodiment, the coarse adjustment for adjusting the voltage of the differential signal input to the ADC 12 to be within the input range of the ADC 12 and the offset input to the amplifier unit 15. Two-stage adjustment is performed, with fine adjustment to adjust the voltage to be within the target threshold. As a result, the instrumentation amplifier 1 can adjust the offset voltage even when the output voltage of the amplifier unit 15 is outside the input range of the ADC 12, and it is not necessary to adjust the output voltage of the amplifier unit 15 in advance. The instrumentation amplifier 1 can adjust the offset voltage even when the offset voltage dynamically changes.

As described above, according to the first embodiment, the instrumentation amplifier 1 includes the amplifier unit 15 that amplifies the input differential signal and the differential signal amplified by the amplifier unit 15 from the analog signal to the digital signal. Included in the differential signal input to the amplifier unit 15 by the offset adjustment signal output from the DAC 13 and connected to the amplifier unit 15 and the DAC 13 that outputs an offset adjustment signal that is a differential signal. An offset adjustment circuit 16 that adjusts the offset voltage, a voltage value of the differential signal after conversion by the ADC 12, a saturation determination unit 1401 that determines whether or not the output of the ADC 12 is saturated, and an output that is output by the saturation determination unit 1401 Is determined to be saturated, the voltage of the differential signal amplified by the amplifier unit 15 falls within the input range of the ADC 12. As described above, the coarse adjustment unit 1402 that performs coarse adjustment for adjusting the voltage of the offset adjustment signal output from the DAC 13 and the case where the saturation determination unit 1401 determines that the output is not saturated or the coarse adjustment by the coarse adjustment unit 1402 is performed. When the adjustment is completed, a fine adjustment unit 1403 performs fine adjustment to adjust the voltage of the offset adjustment signal output from the DAC 13 so that the offset voltage included in the differential signal input to the amplifier unit 15 is within the target threshold. And with. As a result, the instrumentation amplifier 1 according to Embodiment 1 can adjust the offset voltage even when the output voltage of the amplifier unit 15 is outside the input range of the ADC 12.
The instrumentation amplifier 1 is effective for adjusting an offset voltage in a sensor having a sensor circuit 2 such as a pressure gauge, a strain gauge, or a hot wire anemometer.

  In the present invention, any constituent element of the embodiment can be modified or any constituent element of the embodiment can be omitted within the scope of the invention.

1 Instrumentation amplifier
2 Sensor circuit 11 Amplifier unit with offset adjustment function 12 ADC
13 DAC
14 Adjustment unit 15 Amplifier unit 16 Offset adjustment circuit 1401 Saturation determination unit 1402 Coarse adjustment unit 1403 Fine adjustment unit

Claims (4)

An amplifier for amplifying the input differential signal;
An analog-to-digital converter that converts the differential signal amplified by the amplifier unit from an analog signal to a digital signal;
A digital-to-analog converter that outputs an offset adjustment signal that is a differential signal;
An offset adjustment circuit that is connected to the amplifier unit and adjusts an offset voltage included in a differential signal input to the amplifier unit by an offset adjustment signal output by the digital-analog converter;
Read the voltage value of the differential signal after conversion by the analog-digital converter, and determine whether the output of the analog-digital converter is saturated,
When the output is determined to be saturated by the saturation determination unit, the output from the digital / analog converter is such that the voltage of the differential signal amplified by the amplifier unit is within the input range of the analog / digital converter. A coarse adjustment unit for performing coarse adjustment to adjust the voltage of the offset adjustment signal to be
When the saturation determination unit determines that the output is not saturated or when the coarse adjustment by the coarse adjustment unit is completed, the offset voltage included in the differential signal input to the amplifier unit is within the target threshold value. And a fine adjustment unit that performs fine adjustment to adjust the voltage of the offset adjustment signal output from the digital-analog converter so as to be within the range. The coarse adjustment unit adds a fixed voltage to one of the two signals constituting the offset adjustment signal output by the digital-analog converter until the output of the analog-digital converter is not saturated. The amplifier according to claim 1, wherein the fixed voltage is subtracted from the other. The fine adjustment unit is one of two signals constituting the offset adjustment signal output by the digital-analog converter until the offset voltage included in the differential signal input to the amplifier unit falls within a target threshold value. The amplifier according to claim 1, wherein a voltage that is 3/2 times the offset voltage is added to the voltage, and a voltage that is 3/2 times the offset voltage is subtracted from the other voltage. An amplifier unit that amplifies the input differential signal, an analog-to-digital converter that converts the differential signal amplified by the amplifier unit from an analog signal to a digital signal, and a digital analog that outputs an offset adjustment signal that is a differential signal An amplifier comprising: a converter; and an offset adjustment circuit that is connected to the amplifier unit and adjusts an offset voltage included in a differential signal input to the amplifier unit by an offset adjustment signal output from the digital-analog converter. An offset adjustment method,
A saturation determination unit reads the voltage value of the differential signal after amplification by the analog-digital converter, and determines whether the output of the analog-digital converter is saturated;
When the coarse adjustment unit determines that the output is saturated by the saturation determination unit, the voltage of the differential signal after amplification by the amplifier unit is within the input range of the analog-digital converter. Performing coarse adjustment to adjust the voltage of the offset adjustment signal output by the digital-analog converter;
When the fine adjustment unit determines that the output is not saturated by the saturation determination unit or when the coarse adjustment by the coarse adjustment unit is completed, the offset voltage included in the differential signal input to the amplifier unit Performing a fine adjustment to adjust the voltage of the offset adjustment signal output from the digital-to-analog converter so that the value falls within a target threshold value.

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