Circuit for measuring resistance

Abstract

The present application relates to a circuit for measuring resistance, comprising: a combined constant current source and constant voltage source for providing a constant supply current and a supply voltage, respectively, the supply current being less than or equal to 1 ampere; the resistor to be measured and the reference resistor are connected with each other in series and are electrically connected with the constant current source or the constant voltage source respectively; the load signal conditioner and the reference signal conditioner are electrically connected with the resistor to be measured and the reference resistor respectively; the two-way synchronous AD conversion module is electrically connected with the load signal conditioner and the reference signal conditioner respectively; the controller is electrically connected with the two-way synchronous AD conversion module; and the gear switcher is electrically connected with the controller and is used for switching between the constant current source and the constant voltage source. According to the circuit for measuring the resistance, the mode of combining the constant voltage source and the constant current source is adopted, and the resistance can be measured in a self-adaptive and high-precision mode in an ultra-wide range.

Description

Technical Field

[0001] The present application relates to the field of industrial measurement technology, and in particular, to a circuit for measuring resistance. Background Art

[0002] In the industrial field, especially in the aircraft manufacturing and assembly process, electrical connections between metals and metals, or metals and composite materials are often required. The quality of electrical connections directly affects the performance and safety of aircraft or other products. The resistance value of the connection is one of the important indicators for judging the quality of electrical connections. The resistance value of the electrical connection between metals is very small, usually in the milliohm level, while the resistance value of the electrical connection between metals and composite materials is often very large.

[0003] In order to measure the resistance value, various resistance measuring instruments are usually used. However, there are many problems with current resistance measuring instruments. For example, the resistance measuring instrument of Japan's Hioki Company has no high resistance measurement function and the reading is unstable; the resistance measuring instrument of France's CA Company has a large excitation current; the resistance measuring instrument of Shanghai Zhengyang requires an external power supply, etc.

[0004] At present, resistance measuring instruments on the market usually use voltage division measurement technology. The principle is to amplify the voltage value on the resistor to be measured through an amplifier, and then compare the amplified voltage value with the voltage value on a reference resistor with a known resistance value. The resistor to be measured and the reference resistor are passed through the same DC current, thereby obtaining the resistance value of the resistor to be measured. In addition, the four-wire method is also a commonly used resistance measurement technology. This technology uses two leads to measure the DC current on the resistor to be measured, and then uses the other two leads to measure the voltage across the resistor to be measured, thereby reducing the adverse effects of lead resistance and contact resistance on the test results.

[0005] However, the voltage divider measurement technology has low measurement accuracy and a narrow measurement range, especially the problem of large fluctuations in the measurement data of micro-ohm-level small resistors. It is difficult to achieve high-precision measurement of ultra-wide resistance range using the four-wire method. Utility Model Content

[0006] In this context, the object of the present application is to provide a circuit for measuring resistance, which has a wide measurement range and high accuracy, thereby at least partially solving the problems existing in the prior art.

[0007] According to one aspect of the present application, a circuit for measuring resistance is provided, comprising:

[0008] A combined constant current source and constant voltage source are used to provide a constant power supply current and power supply voltage, respectively, and the power supply current is less than or equal to 1 ampere;

[0009] The resistor to be measured and the reference resistor connected in series are respectively electrically connected to a constant current source or a constant voltage source;

[0010] The load signal conditioner and the reference signal conditioner are electrically connected to the resistor to be measured and the reference resistor respectively;

[0011] A dual-channel synchronous AD conversion module is electrically connected to the load signal conditioner and the reference signal conditioner respectively;

[0012] A controller is electrically connected to the dual-channel synchronous AD conversion module;

[0013] The gear switch is electrically connected to the controller and is used to switch between the constant current source and the constant voltage source. When the resistance to be measured is greater than 200 kΩ, it switches to the constant voltage source, otherwise it switches to the constant current source.

[0014] According to the circuit for measuring resistance of the present application, a mode of combining a constant voltage source and a constant current source is adopted, so that resistance can be measured adaptively and with high precision over an ultra-wide range.

[0015] In one embodiment of the circuit according to the present application, the constant current source has an adjustable micro resistor array composed of multiple resistors, and the temperature drift of the multiple resistors is 2-5ppm. Through the adjustable micro resistor array, it is possible to facilitate the subdivision switching of the constant current source, and its low temperature drift resistor ensures that the current provided by the constant current source is stable and is little affected by temperature, providing the necessary premise for high-precision measurement of resistance.

[0016] In one embodiment of the circuit according to the present application, the constant current source includes a small current constant current source module of less than 500μA and a larger current constant current source module of 500μA-1A, and the small current constant current source module and the larger current constant current source module respectively include a plurality of sub-constant current sources. In this way, a small current of less than 1A or even less than 500μA is provided, thereby improving the safety of the device where the resistor to be measured is located, extending its service life, and ensuring its performance.

[0017] In one embodiment of the circuit according to the present application, the constant current source has a current gear switch for switching to a corresponding sub-constant current source. In this way, the corresponding sub-constant current source can be selected according to the size of the resistance to be measured, thereby improving the applicability to specific practical applications.

[0018] In one embodiment of the circuit according to the present application, the constant current source adopts an enhanced HOWLAND design with a buffer and a filter, thereby reducing the output current ripple, improving the noise suppression capability, and providing a guarantee for high-precision resistance measurement.

[0019] In one embodiment of the circuit according to the present application, the adjustable micro resistor array has mutually matching resistor modules located at the front and rear ends thereof, that is, the adjustable micro resistor array achieves resistance matching, thereby making the output current more stable.

[0020] In one embodiment of the circuit according to the present application, the load signal conditioner and the reference signal conditioner are electrically connected to the dual-channel synchronous AD conversion module through their respective instrument amplifiers. The instrument amplifier can amplify the voltage signals on the resistor to be measured and the reference resistor, which is convenient for subsequent measurement and signal processing.

[0021] In one embodiment of the circuit according to the present application, the instrument amplifier is provided with a matching impedance, in this way, the instrument amplifier is ensured to work stably, the interference is reduced, and the noise is further suppressed.

[0022] In one embodiment of the circuit according to the present application, the current gear switch and the gear switch are adaptively switched according to the size of the resistance to be measured, thereby quickly and adaptively switching to the corresponding current source suitable for the resistance to be measured, so that the resistance to be measured can be measured more safely and accurately.

[0023] In one embodiment of the circuit according to the present application, the dual-channel synchronous AD conversion module operates using an isolated external clock signal, which can improve the clock accuracy and enable the AD conversion module to have better noise suppression performance.

[0024] In one embodiment of the circuit according to the present application, the dual-channel synchronous AD conversion module is electrically connected to the controller via a digital isolator. The digital isolator improves the anti-noise performance, ensures the stable transmission of the signal between the AD conversion module and the controller, and prevents external interference. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Further configurations and advantages of the circuit for measuring a resistance according to the present application are described in detail below with reference to the drawings using exemplary embodiments.

[0026] Figure 1 is a control logic diagram of a circuit for measuring resistance according to the present application;

[0027] Figure 2 is the functional block diagram of the circuit;

[0028] 3(a) and (b) are schematic diagrams showing the structures of the constant current generator and the constant voltage generator of the circuit respectively;

[0029] Figure 4 is a schematic diagram of the composition of the load signal conditioner and the reference signal conditioner of the circuit;

[0030] Figure 5 It is a schematic diagram of the structure of the synchronous AD conversion module of the circuit;

[0031] Figure 6 is the schematic diagram of the resistor voltage division measurement; and

[0032] Figure 7 This is the schematic diagram of the constant current source designed using enhanced HOWLAND. DETAILED DESCRIPTION

[0033] As mentioned above, in the prior art, resistance is usually measured by either the voltage division method or the four-wire method. Figure 6 As can be seen from the figure, the resistor to be measured is connected in series with the reference resistor, and the resistance value of the reference resistor is known. In the voltage division method, when the switch S is closed, the DC current flows through the resistor to be measured and the reference resistor, and the voltage on the two is measured respectively by the voltmeter V, and the measured voltage values ​​of the two are compared. According to the comparison result, according to the ratio of the voltage values ​​of the two, the resistance value of the resistor to be measured can be calculated. The voltage division method requires that the voltage provided by the DC power supply is constant. However, the voltage division method is only suitable for measuring large resistors, such as 200 kilo-ohms, more than 50 megohms, and even 200-300 megohms. It is not suitable for small resistors, and the voltage measurement of the reference resistor and the resistor to be measured is not synchronized, and the measurement accuracy is not high.

[0034] In order to measure small resistances, the four-wire method, also known as the Kelvin method, is often used in the prior art. In the four-wire method, four leads are connected to the resistor to be measured, two of which are used as current leads to introduce direct current into the resistor to be measured, and the other two leads independent of the current leads are used, for example, to measure the voltage at both ends of the resistor to be measured using a Kelvin fixture. In this way, the measured voltage and current are completely generated inside the resistor to be measured, and have nothing to do with the resistance of the connecting leads. Through this connection method, the direct current only flows through the resistor to be measured, and the voltage measurement only involves the two ends of the resistor to be measured, thereby eliminating the influence of the connecting lead resistance on the resistance measurement, which is beneficial for measuring small resistances. The four-wire method requires that the current provided by the DC power supply is constant. However, because the resistor to be measured is small, the four-wire method in the prior art usually uses a large current, such as more than 1 ampere, so that the voltage signal obtained is large enough, otherwise there will be problems such as a small voltage signal and a low signal-to-noise ratio. However, large currents can bring safety hazards and affect the performance and life of the device under test. In addition, the resistance measurement range of the four-wire method is small, such as up to 3 megohms.

[0035] In this context, the present application proposes a novel resistance measurement circuit, which adopts a mode of combining a DC constant voltage source and a constant current source, has a wide measurement range, high accuracy, and is safe and reliable. The working mode of the circuit for measuring resistance according to the present application will be described in detail below.

[0036] See also Figure 1, which shows a logic diagram of a circuit for measuring resistance according to the present application. The controller causes the gear switch electrically connected to it to operate according to the size of the resistance to be measured, thereby switching between a DC constant current source and a constant voltage source, and selecting a corresponding working mode. The controller is, for example, a CPU, an MCU, a PLC, etc., which has a corresponding control circuit to achieve switching of working modes during the measurement process. Specifically, when measuring a large resistance, a constant voltage mode is adopted, that is, the above-mentioned voltage division method, using a DC constant voltage source; and when measuring a small resistance, a constant current mode is adopted, using a DC constant current source, but combining the above-mentioned four-wire method with the voltage division method, and the specific method will be described in detail below. It should be pointed out that there is no clear dividing line between large resistance and small resistance here. In the present application, for example, resistances below 200kΩ are regarded as small resistances, and resistances above 200kΩ are large resistances. However, the value of 200kΩ is not fixed, but can be flexibly adjusted according to the actual application situation. The present application can even measure small resistances of 100-200 milliohms or even 0.1 micro-ohms. When the constant voltage mode is adopted, the control circuit selects and switches to the corresponding constant voltage gear, and when the constant current mode is adopted, the control circuit selects and switches to the corresponding constant current gear. Then a measuring circuit is used to measure the resistance to be measured, and the measured analog signal is fed back to the AD (Analog-Digital, analog-digital) conversion module for analog-to-digital conversion. After conversion, the digital measurement signal is sent to the controller, and the controller outputs the measurement result, such as displaying it on a display, broadcasting it in the form of sound, or outputting it in other ways. It can be seen from this that the circuit according to the present application combines the constant voltage mode with the constant current mode, which can measure both large resistance and small resistance, thereby achieving an ultra-wide range of resistance measurement.

[0037] From the above, we can see that Figure 2 , according to the circuit of the present application, a combined DC constant current source and a constant voltage source are included, which are used to provide a constant power supply current and a power supply voltage, respectively. It goes without saying that the circuit according to the present application also includes a resistor to be measured and a reference resistor connected in series with each other, which are electrically connected to the DC constant current source in constant current mode, or are electrically connected to the DC constant voltage source in constant voltage mode. The resistance of the reference resistor is known. In constant current mode, that is, when measuring a small resistor, contrary to the prior art, the power supply current provided by the constant current source in the present application is a small current less than or equal to 1 ampere, which flows through the resistor to be measured and the reference resistor connected in series, thereby fundamentally and significantly reducing the heat generated by the large current, thereby avoiding the resistance change and test data offset caused by temperature rise.

[0038] However, using a small current to measure a small resistance may cause some problems, that is, the measured voltage signal is small, while the external noise and interference are not reduced, resulting in a low signal-to-noise ratio. In order to solve these problems, the present application has taken a series of measures, including optimizing the constant current source to resist noise, improve the signal-to-noise ratio, and improve the measurement stability.

[0039] According to this application, see Figure 2 The load signal conditioner and the reference signal conditioner are electrically connected to the resistor to be measured and the reference resistor, respectively, to receive and amplify the voltage signals measured by the four-wire method from the resistor to be measured and the reference resistor, respectively, to improve the measurement accuracy and reduce noise. They also have the functions of anti-attenuation, pre-amplification, nonlinear compensation, etc. The structures of the load signal conditioner and the reference signal conditioner are the same. For their specific composition, see Figure 4 . By Figure 4 It can be seen that the load signal conditioner and the reference signal conditioner mainly include three components: a low bias current and low offset amplifier, a differential output buffer, and an anti-aliasing filter. The input signal is the voltage signal measured on the resistor to be measured and the reference resistor. A small amount of current will flow into the two input terminals of the actual amplifier, namely the so-called bias current, referred to as bias current, which will cause system errors. When there is a difference between the bias currents of the two input terminals of the amplifier, a current offset will be generated, which will cause amplifier noise. Selecting an amplifier with low bias current and low offset can effectively improve the measurement accuracy. Reduce noise. Since the current flowing through the resistor to be measured and the reference resistor in this application is relatively small, the voltage signal measured on the resistor to be measured and the reference resistor is also relatively small, so an amplifier is used to amplify it. The amplified signal passes through the differential output buffer. The basic principle of differential output is to compare the difference between the two input signals to extract useful information. The differential output uses two signal lines of opposite polarity to reduce the influence of external interference, so it has strong anti-noise and anti-interference properties. The output buffer is used to temporarily store the generated signal data so that it can be sent to the controller, so that the high-speed controller and the slow-speed peripherals can coordinate with each other, play a buffering role, and achieve synchronization of data transmission. The anti-aliasing filter is a low-pass filter specially used to suppress aliasing in the process of converting analog signals into digital signals. It is used to reduce the aliasing frequency components to a negligible level in the output level, thereby improving the signal fidelity. Aliasing is an error that occurs in the sampling process. When the frequency contained in the analog signal is higher than the frequency range required by the sampling theorem, these high-frequency information will be mistakenly represented as low-frequency information. The design goal of the anti-aliasing filter is to avoid the occurrence of aliasing by filtering out frequency components higher than the sampling frequency before the analog signal is input to the sampler.

[0040] In a preferred embodiment of the circuit according to the present application, the circuit has a dual-channel synchronous AD conversion module, see Figure 2, which is electrically connected to the load signal conditioner and the reference signal conditioner respectively. The structure of the dual-channel synchronous AD conversion module is as follows Figure 5 As shown. Figure 5 It can be seen that channels 1 and 2 respectively input voltage signals from the resistor to be measured and the reference resistor that have been processed by the above-mentioned load signal conditioner and reference signal conditioner. In the prior art, when the voltage divider method is used to measure the voltage on the resistor to be measured and the reference resistor, it is impossible to measure the voltage on the two resistors absolutely synchronously, which leads to instantaneous deviations and further affects the accuracy of the measurement results. The dual-channel synchronous AD conversion module of the circuit of the present application uses an internal timer as a clock, which can synchronize channels 1 and 2 within any set time window, thereby measuring the voltage on the resistor to be measured and the reference resistor absolutely synchronously. In addition, the dual-channel synchronous AD conversion module itself relies on an isolated external reference clock signal to work in order to improve noise suppression performance. Figure 5 As shown, the dual-channel synchronous AD conversion module adopts AD oversampling, which samples the analog signal at a sampling rate far higher than the Nyquist frequency, thereby improving the resolution and signal-to-noise ratio of the signal. The signal obtained after sampling is sent to the ∑Δ modulator, ∑ represents integration or summation, and Δ represents difference. The modulator is used to filter out quantization noise in the process of converting analog quantity to digital quantity, thereby obtaining accurate measurement results. The signal after such modulation is input to the subsequent 32-bit converter, where it is converted into a digital quantity represented by 32-bit binary digits, thereby significantly improving the measurement accuracy. In addition, the dual-channel synchronous AD conversion module also has a PGA (Programmable Gain Amplifier, programmable gain amplifier) ​​not shown inside, and its temperature drift is only 1 ppm, that is, one millionth, which greatly improves the stability of the measurement result. Then, the converted digital quantity is output to the controller electrically connected to the dual-channel synchronous AD conversion module as the voltage synchronization data on the resistor to be measured and the reference resistor.

[0041] In a preferred embodiment of the circuit according to the present application, after receiving the above-mentioned synchronization data, the controller performs a series of data processing and executes some functions, including calculating the resistance value of the resistor to be measured based on the ratio of the measured voltages on the resistor to be measured and the reference resistor, storing the measurement results in a storage unit, outputting the measurement results through an output unit such as a display, a speaker, etc., receiving commands input through an input unit, communicating with other peripherals through a communication unit, and so on.

[0042] In a preferred embodiment of the circuit according to the present application, see also Figure 2The dual-channel synchronous AD conversion module is electrically connected to the controller via a digital isolator. The digital isolator is, for example, a high-speed six-channel digital isolator, which can improve the anti-interference and anti-noise performance of resistance measurement, ensure stable signal transmission between the dual-channel synchronous AD conversion module and the controller, and prevent external factors from interfering with and damaging the circuit due to misoperation.

[0043] In a preferred embodiment of the circuit according to the present application, still refer to Figure 2 , the load signal conditioner and the reference signal conditioner are electrically connected to the dual-channel synchronous AD conversion module through their own instrument amplifiers. The instrument amplifier can be used to amplify the voltage signals conditioned by the load signal conditioner and the reference signal conditioner on the resistor to be measured and the reference resistor, so as to obtain a suitable amplification effect and further improve the signal-to-noise ratio. In addition, when the four-wire method is adopted, the instrument amplifier is convenient for measuring the voltage on the resistor to be measured and the reference resistor in actual operation, avoiding the mistest caused by the measurement loop being in a suspended state when the ejector pin test fixture is used. Preferably, the instrument amplifier is provided with a matching impedance, thereby improving the stability and anti-interference ability of the amplifier and further improving the signal-to-noise ratio. In addition, the power supply voltage of the instrument amplifier is calculated after analyzing the diamond diagram of the instrument amplifier, so that the instrument amplifier works in a relatively ideal state. The diamond diagram shows the relationship between the input common-mode voltage range and the output voltage of the instrument amplifier at a given gain. The method of using the diamond diagram is to put the rectangle formed by the input common-mode voltage range and the output voltage range of the instrument amplifier into the diamond diagram. If it does not exceed the diamond diagram range, the circuit can work normally. If it exceeds the diamond diagram range, the operation may be abnormal. Therefore, the diamond plot can be used to easily determine the effective operating voltage range of the instrumentation amplifier.

[0044] In a preferred embodiment of the circuit according to the present application, the constant current source adopts an enhanced HOWLAND design with a buffer and a filter, see Figure 7 . The circuit designed with HOWLAND is used to convert the input voltage into the output current. Compared with the current, the voltage is easier to control. In this way, the output current can be easily controlled by controlling the voltage at the input end, which is convenient for the switching of the constant current source. The constant current source designed with enhanced HOWLAND can work quickly and stably due to the buffer on the feedback loop, reducing the leakage current and having good linearity. The filter can reduce the ripple of the output current of the constant current source and improve the ability to suppress noise.

[0045] In a preferred embodiment of the circuit according to the present application, the constant current source includes a small current constant current source module of less than 500μA and a larger current constant current source module of 500μA-1A. Preferably, the small current constant current source module and the larger current constant current source module include a plurality of sub-constant current sources, respectively, see Figure 3 (a). It can be seen that in the present application, the constant current source can, for example, include a sub-constant current source of 1μA, 5μA, 50μA, 250μA, 500μA, 3mA, 100mA, and 500mA. However, the present application is not limited thereto, and the constant current source can also include sub-constant current sources of other sizes, such as sub-constant current sources of 70nA (nanoampere), 650mA, 800mA, etc. In this regard, it can be flexibly set as needed. Therefore, it is possible to switch to the corresponding constant current source module or sub-constant current source according to the size of the resistance to be measured, and in particular, very small resistances can be measured, thereby further improving the measurement accuracy and expanding the measurement range.

[0046] In a preferred embodiment of the circuit according to the present application, the constant current source has an adjustable micro resistor array composed of multiple resistors, and the temperature drift of the multiple resistors is very small, such as 2-5ppm, that is, two to five parts per million, and preferably a high-precision resistor, and the accuracy reaches 0.1-1 / 10000, for example. By adding an adjustable micro resistor array to the constant current source design, it is convenient to switch to the corresponding sub-constant current source adapted to the size of the resistance to be measured, and the functions of reasonable grading, reducing bias, reducing offset, and proportional linearity are achieved, and low noise, low power consumption, and fast step response can be achieved, and the subsequent circuit of the constant current source is easily divided into a reference end and a load end, so that the resistance measurement has higher accuracy and better stability. In addition, since the resistance temperature drift of the adjustable micro resistor array is very small, the measurement result is very stable and not easily affected by temperature, and its high accuracy further ensures the accuracy of the resistance measurement and improves the measurement accuracy. Preferably, the adjustable micro resistor array has resistor modules that match each other and are respectively located at its front and rear ends, thereby further reducing temperature drift and improving the stability of measurement.

[0047] In a preferred embodiment of the circuit according to the present application, the constant current source has a current gear switch for switching to the corresponding sub-constant current source. The current gear switch and the gear switch mentioned above are adaptively switched according to the size of the resistance to be measured, and this switching is automatically performed according to the program. Specifically, the gear switch switches between the constant voltage source and the constant current source according to the size of the resistance to be measured. If the gear switch is switched to the constant voltage source, the reference voltage generating circuit of the constant voltage generator outputs the constant voltage, see Figure 3 (b). If the gear switch is switched to the constant current source, then further according to the size of the resistance to be measured, the current gear switch is switched to the corresponding sub-constant current source, which can belong to a small current constant current source module below 500μA, or a larger current constant current source module of 500μA-1A. The current gear switch can be, for example, a single-pole multi-throw switch module, which can be implemented using a mechanical relay, an electronic switch, etc. Similarly, the gear switch for switching between the constant voltage source and the constant current source can also be implemented using a mechanical relay, an electronic switch, etc.

[0048] In summary, according to the present application, when measuring a large resistor such as more than 200 kilo-ohms, a voltage divider method is used, a constant voltage source is used, and the DC power supply voltage provided is required to be constant. When measuring a small resistor such as less than 200 kilo-ohms, a combination of a four-wire method and a voltage divider method is used, and a combined constant voltage source and a constant current source are used, and the DC power supply current provided is required to be less than or equal to 1 ampere. In this case, the voltage on the resistor to be measured and the reference resistor in series through which the same DC current flows is first measured by a four-wire method, and then the two measured voltage signals are processed accordingly using corresponding electronic devices, so as to improve measurement accuracy, noise resistance, amplify measurement signals, improve measurement stability, etc., and according to the principle of the voltage divider method, the resistance value of the resistor to be measured is finally measured. The present application thus improves the measurement accuracy of the resistor and expands its measurement range. Therefore, the circuit of the present application is an ultra-wide range high-precision resistance measurement circuit in an adaptive constant current source and constant voltage source combination mode.

[0049] Some embodiments of the present application have been introduced for the purpose of illustration, but the present application is not limited to these embodiments. Those skilled in the art may also think of many modifications and variations. Therefore, these embodiments are selected and described in order to better illustrate the principles and practical applications of the present application and to enable those skilled in the art to understand the content thereof, that is, without departing from the spirit of the present application, all modifications and variations made will fall within the scope of protection of the present application as defined by the appended claims.

Claims

1. A circuit for measuring resistance, characterized in that The circuit comprises: A combined constant current source and constant voltage source for providing a constant power supply current and a constant power supply voltage, respectively, wherein the power supply current is less than or equal to 1 ampere; A resistor to be measured and a reference resistor connected in series with each other, wherein the resistor to be measured and the reference resistor are electrically connected to the constant current source or to the constant voltage source respectively; A load signal conditioner and a reference signal conditioner, wherein the load signal conditioner and the reference signal conditioner are electrically connected to the resistor to be measured and the reference resistor respectively; A dual-channel synchronous AD conversion module, wherein the dual-channel synchronous AD conversion module is electrically connected to the load signal conditioner and the reference signal conditioner respectively; A controller, the controller being electrically connected to the dual-channel synchronous AD conversion module; and A gear switcher, the gear switcher is electrically connected to the controller and is used to switch between the constant current source and the constant voltage source, wherein when the resistance to be measured is greater than 200kΩ, it switches to the constant voltage source, otherwise it switches to the constant current source.

2. The circuit according to claim 1, characterized in that The constant current source has an adjustable micro resistor array composed of a plurality of resistors, and the temperature drift of the plurality of resistors is 2-5ppm.

3. The circuit according to claim 2, characterized in that The constant current source comprises a small current constant current source module below 500 μA and a large current constant current source module of 500 μA-1A. The small current constant current source module and the large current constant current source module respectively comprise a plurality of sub-constant current sources.

4. The circuit according to claim 3, characterized in that The constant current source has a current gear switch for switching to the corresponding sub-constant current source.

5. The circuit according to claim 4, characterized in that The current gear switch and the gear switch are adaptively switched according to the size of the resistance to be measured.

6. The circuit according to claim 2, characterized in that The adjustable micro resistor array has resistor modules that match each other and are located at the front and rear ends of the adjustable micro resistor array.

7. The circuit according to any one of claims 1 to 6, characterized in that: The constant current source adopts an enhanced HOWLAND design with a buffer and a filter.

8. The circuit according to any one of claims 1 to 6, characterized in that: The load signal conditioner and the reference signal conditioner are electrically connected to the dual-channel synchronous AD conversion module through their own instrument amplifiers.

9. The circuit according to claim 8, characterized in that The instrumentation amplifier is provided with matched impedance.

10. The circuit according to any one of claims 1 to 6, characterized in that: The dual-channel synchronous AD conversion module operates using an isolated external clock signal.

11. The circuit according to any one of claims 1 to 6, characterized in that: The dual-channel synchronous AD conversion module is electrically connected to the controller via a digital isolator.

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