High-precision measurement system for ultra-high impedance of low-voltage devices

Abstract

The present invention provides a high-precision measurement system for ultra-high impedance of low-voltage devices with simple structure, low cost, high efficiency and strong performance. The present invention comprises an MCU (1), an AC source signal generating device (2), a serial port isolator (U9), a port isolator (U16), a sinusoidal signal isolating device (U18), a DC source signal generating device (3), a DC source switching device (4), an impedance measuring device (5), a data collector (6), a test resistance analog switch (7) and a power supply. A shielding device (8) is arranged around the product to be tested. The MCU (1) is connected to the impedance measuring device (5), the data collector (6) and the test resistance analog switch (7) respectively through the port isolator (U16). A protection ring (9) is arranged between the impedance measuring device (5) and the product to be tested. The power supply supplies power to the entire measurement system. The present invention can be applied to the field of testing.

Description

Technical Field

[0001] The present invention relates to the field of testing, and in particular to a method for testing GΩ (1×10 14 >X R >1×10 9 It is a high-precision measurement system for ultra-high impedance of low-voltage devices, which can measure the DC impedance and AC impedance of precision devices with a voltage tolerance of 0.024 V (<2-5 V) or even T ohms and low voltage tolerance of the material itself. Background Art

[0002] The higher the impedance of ordinary insulating materials, the higher their voltage tolerance will be, so high-voltage instruments (0~KV) are generally used for measurement, such as megohmmeters. However, when measuring low-voltage (<2V, or even lower) and high-impedance materials, this high-voltage test method cannot be used normally. The main factors that affect measurement accuracy under low voltage include:

[0003] 1. The influence of the noise of the resistor itself is particularly prominent;

[0004] 2. Noise from the excitation source itself;

[0005] 3. The impact of EMI (electromagnetic interference);

[0006] 4. Test the leakage current and leakage voltage of the network.

[0007] At present, when measuring resistances > 1G ohms, electrometers, SMUs, picoammeters + voltage sources, high resistance meters, etc. are basically used together for measurement. The measurement method of the electrometer requires the configuration of a voltage source or a current source, so the precision test of high resistance is achieved by using an external voltage source (source meter) plus an electrometer or picoammeter to obtain the measured voltage or current, and finally calculate the resistance value using Ohm's law, such as Figure 1 and Figure 2 As shown. Using the instrument test method of source meter and electrometer, the test value can be obtained directly. Figure 1 , Figure 2 The figure shows the high resistance measurement achieved by the general electrometer (electrostatic voltmeter) and high resistance meter. Their characteristics are that the electrometer (electrostatic voltmeter) and the excitation source (voltage source or current source) are two separate parts. In addition, the more popular high resistance measuring instruments on the market, such as the B2985A and other instruments, integrate the source meter and the electrometer in one instrument, and then connect the high resistance through the instrument's special matching adapter to measure the high resistance. The test value can be read directly on the instrument panel without additional calculation.

[0008] Another method is to use a common digital multimeter to calculate the high resistance by reading the voltage. The principle is as follows Figure 3 shown. Figure 3The figure shows that an external current source is used to provide a constant current to the device under test, and then an operational amplifier is used as a buffer, where V1≈V0, so that a low-cost ordinary digital multimeter can measure the high resistance of Rx.

[0009] In addition, the integrated excitation source (voltage source or current source) and buffer can be built in to form a measurement circuit for high resistance measurement. At present, many test instrument board circuit measurement technologies use the circuit board's own current source and then use the op amp as the input signal buffer, such as Figure 4 and Figure 5 shown. Figure 4 This is a simplified schematic diagram of an electrometer high resistance measurement with a built-in current source. Figure 5 This is a simplified schematic diagram of high resistance measurement using an electrometer with guarded ohm type.

[0010] Figure 4 , 5 The measured resistance calculation formula is as follows:

[0011] R X =V1 / (V S ×R) (Expression 1)

[0012] R X =V1 / I (Expression 2)

[0013] The above and Figure 1 , Figure 2 , Figure 3 The test method shown is to achieve high resistance measurement through the coordination of instruments. Figure 4 , Figure 5 This is a completely independent circuit instrument board measurement method. However, its measurement quality is greatly limited by the electrical parameter performance of the operational amplifier itself.

[0014] In the above-mentioned prior art, when measuring high resistance of >100G and the test voltage is ≤1V, the directional movement of charges in the electric field is very susceptible to interference from power frequency radiation because the voltage loaded at both ends of the object being measured is too low. The object being measured itself has extremely high impedance, and the radiation can be easily transmitted to both ends of the measuring instrument through space and metal or low-resistance non-conductors, causing the measured electrical signal to be loaded with low-frequency harmonics of 0 to 150Hz, and it takes a long time to reach a stable test state. In order to ensure faster and more accurate testing, the voltage of the current source or voltage source must be adjusted to a higher level, generally adjusted to >1V, or even higher, to reduce the impact of thermal noise generated by the resistor itself. And the test conditions must be tested in a very high shielding environment (isolation >60dB). Despite this, since this type of voltage divider test has no dynamic response capability, it is still necessary to wait for the electric field at both ends of the object being measured to stabilize before the resistance value can be accurately tested, and the test time is still long.

[0015] Since current measurement technology is basically based on voltage division testing, high-precision instruments such as picoammeters, electrometers, voltage sources, etc. are required. In addition, it is easily affected by EMI (electromagnetic interference) and electrostatic fields in the external environment, temperature and humidity in the natural environment, etc., so a good shielding environment must be built, resulting in very high testing costs and unnecessary waste. The test instruments and measuring equipment occupy a relatively large volume, making it difficult to introduce them into quantitative production.

[0016] In addition, there are many materials with low withstand voltage and high impedance on the market. The above existing test methods often cause the tested object to break down or the material to fission due to withstand voltage problems, thereby reducing the properties and service life of the material itself. When the insulation DC impedance and AC impedance of the tested device (material) exceed GΩ (1×10 9 Ohms), when measuring ultra-high impedance, when the test voltage for these materials is greater than 2 volts, the electrical parameters of the materials under test will change sensitively, presenting a nonlinear low-voltage working state. Summary of the invention

[0017] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a high-precision measurement system for ultra-high impedance of low-voltage devices with simple structure, low cost, high efficiency and strong performance. The system is scientifically designed, accurately tested and suitable for mass production.

[0018] The technical solution adopted by the present invention is that the system comprises

[0019] An independently configured MCU for outputting test data to the measurement system and receiving test results.

[0020] An AC source signal generating device independently provided and used to output an AC sinusoidal excitation signal,

[0021] A serial port isolator and a port isolator for isolating and coupling the data transmission of the MCU,

[0022] a sinusoidal signal isolating device for isolating and coupling the AC alternating current sinusoidal excitation signal output by the AC source signal generating device,

[0023] A DC source signal generating device for generating a DC power supply signal,

[0024] A DC source switching device for switching positive and negative phase DC sources,

[0025] An impedance measuring device for measuring the impedance of a product to be tested,

[0026] Data logger for collecting impedance measurement data of the product to be tested,

[0027] Test resistor analog switch for setting different resistance values ​​of test resistor, and power supply,

[0028] A shielding device is arranged around the product to be tested, the output end of the test resistor analog switch is connected to the product to be tested, the test resistor analog switch is provided with a plurality of test resistors, the DC source signal generating device is connected to the impedance measuring device through the DC source switching device, the output end of the impedance measuring device is connected to the input end of the test resistor analog switch, and is connected to the data collector after the impedance measuring device takes samples, the serial port isolator is connected to the MCU and the data collector respectively, the output end of the sinusoidal signal isolation device is connected to the impedance measuring device and the data collector respectively through a third relay, the MCU is connected to the impedance measuring device, the data collector and the test resistor analog switch respectively through the port isolator, a protection ring is arranged between the impedance measuring device and the product to be tested, and the power supply supplies power to the entire measurement system.

[0029] The serial port isolator, the port isolator, the sinusoidal signal isolating device, the DC source signal generating device, the DC source switching device, the impedance measuring device, the data collector and the test resistance simulation switch are all arranged on the same PCB board.

[0030] The DC source signal generating device includes a DAC digital-to-analog converter, a twenty-second low-voltage difference linear regulator, a high-precision voltage divider, an inverter and a buffer. The twenty-second low-voltage difference linear regulator and the high-precision voltage divider are connected to form a high-precision voltage source. The DAC digital-to-analog converter provides a set value voltage source for the DC impedance measuring device. The high-precision voltage source and the set value voltage source are switched through a channel switch.

[0031] The impedance measuring device includes a third low-power amplifier, a measuring op amp, and an instrumentation amplifier. The output end of the third low-power amplifier is connected to the negative input end of the measuring op amp, the positive input end of the third low-power amplifier, the positive input end of the measuring op amp, and the negative input end of the instrumentation amplifier are connected, the output end of the instrumentation amplifier is connected to the data collector, one end of the test resistance analog switch is connected to the output end of the measuring op amp, and the other end is connected to the input end of the product to be tested, the guard ring is connected to the negative input end of the measuring op amp and the input end of the product to be tested, a fourth relay is arranged between the positive input end of the measuring op amp and the output ends of the inverter and the buffer, and a first relay is arranged between the output end of the third low-power amplifier and the negative input end of the measuring op amp.

[0032] A second relay and a three-coaxial connector are also arranged between the product to be tested and the test resistance simulation switch. The product to be tested is connected to the three-coaxial connector, and the three-coaxial connector is connected to the second relay. The shielding device is an EMI shielding box, and the EMI shielding box is connected to a voltage source through a reference voltage source device.

[0033] The power supply is an isolated power supply with a single power input and a dual power output. The power supply includes a dual-output isolated power supply, a first low-voltage dropout linear regulator, a second low-voltage dropout linear regulator, a third low-voltage dropout linear regulator, a fifth low-voltage dropout linear regulator and a sixth linear regulator. The first low-voltage dropout linear regulator, the third low-voltage dropout linear regulator and the sixth linear regulator are all connected to one output of the dual-output isolated power supply, and the second low-voltage dropout linear regulator and the fifth low-voltage dropout linear regulator are all connected to the other output of the dual-output isolated power supply.

[0034] The beneficial effect of the present invention is as follows: in the present invention, a DC source signal generating device outputs a set fixed voltage value to the in-phase input terminal of a measuring amplifier of a DC impedance measuring device, thereby completing the establishment of a positive voltage for DC measurement. At this time, a series closed loop is formed between the product to be measured, the DC measuring resistor, the measuring amplifier, the constant measuring voltage source and the ground. In the closed loop, a change in the resistance value of the product to be measured will cause a change in the voltage at the input terminal of the measuring amplifier. The changed voltage is defined as a differential voltage. After amplifying the differential voltage, the differential voltage value is transmitted to the MCU using a channel switching device. The voltage value under the forward current condition is obtained; then, by connecting the fourth relay, the voltage is input to the in-phase terminal of the measuring amplifier through the inverter to complete the establishment of the negative voltage of the DC measurement, and the two ends of the product to be measured obtain a negative voltage source with a reverse current. A series closed loop is formed between the product to be measured, the DC measuring resistor, the measuring amplifier, the constant measuring voltage source and the ground. In the closed loop, the reverse current is shunted by the measuring amplifier, that is, the reverse negative voltage value is obtained. After amplifying the differential voltage, the differential voltage value is transmitted to the MCU using the channel switching device, that is, the voltage under the reverse current condition is obtained. voltage value; the DAC digital-to-analog converter of the DC source signal generating device is set to 0V output again, and a series closed test loop is formed between the product to be tested, the DC measuring resistor, the measuring amplifier and the ground, and finally the zero potential voltage value is read from the channel switching device; since the DC measuring resistor is a known set value, the relationship between the known resistance value and the voltage drop is used to obtain the impedance values ​​under the conditions of positive current, negative current and zero potential current, respectively, and then the impedance of the product to be tested is obtained by averaging; and the measurement circuit that can be switched to the AC impedance is switched on by the relay, and the signal generated by the AC source signal generating device is coupled to the AC amplifier through the isolation coupling device, and then passes through the AC measuring resistor, the second relay and the product to be tested to form a test loop, and then the AC measuring voltage signal is read through the channel switching device, and then the AC impedance of the product to be tested can be obtained through the known AC measuring resistor; therefore, the system of the present invention can quickly and accurately complete the high-precision measurement of the product to be tested, and has excellent working performance. The design of the system is very scientific, and its volume is small, and it is easy to achieve such high-precision and accurate testing of high impedance measurement requirements in quantitative production, and is also suitable for verification testing and scientific research in the product development stage. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a simple schematic diagram of high resistance measurement using an electrometer and an external voltage source in the prior art;

[0036] Figure 2 It is a simple schematic diagram of high resistance measurement using an electrometer and an external current source in the prior art;

[0037] Figure 3It is a simple schematic diagram of high resistance measurement using a real current source and a digital multimeter in the prior art;

[0038] Figure 4 It is a simplified schematic diagram of high resistance measurement of an electrometer with a built-in current source in the prior art;

[0039] Figure 5 It is a simplified schematic diagram of high resistance measurement of electrometer with protection ohm type in the prior art;

[0040] Figure 6 It is a simplified structural block diagram of the circuit of the present invention;

[0041] Figure 7 is a circuit schematic diagram of the AC signal isolation circuit;

[0042] Figure 8 is a circuit schematic diagram of the communication data isolation circuit;

[0043] Fig. 9 is a circuit schematic diagram of the DAC digital-to-analog converter;

[0044] Fig.10 is a circuit schematic diagram of the high-precision voltage source;

[0045] Fig.11 is a circuit schematic diagram of the fifth relay;

[0046] Fig.12 is a circuit schematic diagram of the buffer and the inverter;

[0047] Fig.13 It is a schematic diagram of a high resistance measurement circuit of the present invention;

[0048] Fig.14 is a circuit schematic diagram of the shielding device;

[0049] Fig.15 is a circuit schematic diagram of the fourth relay;

[0050] Fig.16 is a circuit schematic diagram of the ADC analog-to-digital converter;

[0051] Fig.17 is a circuit schematic diagram of the dual-output isolated power supply;

[0052] Fig.18 is a circuit schematic diagram of the first low-dropout linear regulator;

[0053] Fig.19 is a circuit schematic diagram of the second low dropout linear regulator;

[0054] Fig. 20is a circuit schematic diagram of the third low voltage dropout linear regulator;

[0055] Fig.21 is a circuit schematic diagram of the fifth low voltage dropout linear regulator;

[0056] Fig. 22 is a circuit schematic diagram of the sixth linear regulator;

[0057] Fig.23 This is a diagram of data collected in an AC impedance measurement state according to an embodiment of the present invention. DETAILED DESCRIPTION

[0058] like Figure 6 As shown, the system of the present invention includes

[0059] An MCU 1 independently provided and used to output test data to the measurement system and receive test results,

[0060] An AC source signal generating device 2 is independently arranged and used to output an AC alternating current sinusoidal excitation signal,

[0061] A serial port isolator U9 and a port isolator U16 for isolating and coupling the data transmission of the MCU 1,

[0062] A sinusoidal signal isolation device U18 for isolating and coupling the AC sinusoidal excitation signal output by the AC source signal generating device 2,

[0063] A DC source signal generating device 3 for generating a DC power supply signal,

[0064] A DC source switching device 4 for switching positive and negative phase DC sources,

[0065] An impedance measuring device 5 for measuring the impedance of the product to be measured,

[0066] A data collector 6 for collecting impedance measurement data of the product to be tested,

[0067] A test resistor analog switch 7 for setting different resistance values ​​of the test resistor, and a power supply,

[0068] A shielding device 8 is arranged around the product to be tested, the output end of the test resistance analog switch 7 is connected to the product to be tested, the test resistance analog switch 7 is provided with a plurality of test resistances Rs, the DC source signal generating device 3 is connected to the impedance measuring device 5 through the DC source switching device 4, the output end of the impedance measuring device 5 is connected to the input end of the test resistance analog switch 7, and the impedance measuring device 5 is connected to the data collector 6 after sampling, the serial port isolator U9 is respectively connected to the MCU 1 and the data collector 6, the output end of the sinusoidal signal isolating device U18 is respectively connected to the impedance measuring device 5 and the data collector 6 through the third relay K3, the MCU1 is respectively connected to the impedance measuring device 5, the data collector 6 and the test resistance analog switch 7 through the port isolator U16, a protection ring 9 is arranged between the impedance measuring device 5 and the product to be tested, and the power supply supplies power to the entire measurement system. The model of the MCU is selected from STM32F103ZET6.

[0069] The serial port isolator U9, the port isolator U16, the sinusoidal signal isolating device U18, the DC source signal generating device 3, the DC source switching device 4, the impedance measuring device 5, the data collector 6 and the test resistance simulation switch 7 are all arranged on the same PCB board.

[0070] The DC source signal generating device 3 includes a DAC digital-to-analog converter U10, a twenty-second low-voltage difference linear regulator U22, a high-precision voltage divider U21, an inverter U20A and a buffer U20B. The twenty-second low-voltage difference linear regulator U22 and the high-precision voltage divider U21 are connected to form a high-precision voltage source. The DAC digital-to-analog converter U10 provides a set value voltage source for the DC impedance measuring device 5. The high-precision voltage source and the set value voltage source are switched through a channel switch U15.

[0071] The impedance measuring device 5 includes a third low-power amplifier U14A, a measuring amplifier U12, and an instrument amplifier U11. The output end of the third low-power amplifier U14A is connected to the negative input end of the measuring amplifier U12, the positive input end of the third low-power amplifier U14A, the positive input end of the measuring amplifier U12 and the negative input end of the instrument amplifier U11 are connected, the output end of the instrument amplifier U11 is connected to the data collector 6, one end of the test resistance simulation switch 7 is connected to the output end of the measuring amplifier U12, and the other end is connected to the input end of the product to be tested, the protection ring 9 is connected to the negative input end of the measuring amplifier U12 and the input end of the product to be tested, a fourth relay K4 is arranged between the positive input end of the measuring amplifier U12 and the output ends of the inverter U20A and the buffer U20B, and a first relay K1 is arranged between the output end of the third low-power amplifier U14A and the negative input end of the measuring amplifier U12.

[0072] A second relay K2 and a three-coaxial connector J4 are also arranged between the product to be tested and the test resistance simulation switch 7. The product to be tested is connected to the three-coaxial connector J4, and the three-coaxial connector J4 is connected to the second relay K2. The shielding device 8 is an EMI shielding box, and the EMI shielding box is connected to a voltage source through a reference voltage source device U8.

[0073] The power supply is an isolated power supply with a single power input and dual power outputs, and the power supply includes a dual-output isolated power supply U4, a first low-voltage dropout linear regulator U1, a second low-voltage dropout linear regulator U2, a third low-voltage dropout linear regulator U3, a fifth low-voltage dropout linear regulator U5 and a sixth linear regulator U6. The first low-voltage dropout linear regulator U1, the third low-voltage dropout linear regulator U3 and the sixth linear regulator U6 are all connected to one output of the dual-output isolated power supply U4, and the second low-voltage dropout linear regulator U2 and the fifth low-voltage dropout linear regulator U5 are all connected to the other output of the dual-output isolated power supply U4.

[0074] Here, considering the signal acquisition accuracy and the number of independent signal channels required, the present invention selects a chip model AD7175 as a data collector. The chip has a 24-bit, 250KSPS data sampling rate, and the integrated Σ-Δ analog-to-digital converter is suitable for low-bandwidth high-impedance measurement signal input, and the built-in digital filter can synchronously suppress 50 Hz / 60 Hz and the multiple of the power frequency (100Hz / 120Hz) at a 27.27 SPS output data rate, and its suppression capability reaches 86dB, which effectively reduces the radiation effect of the power frequency noise during the measurement process. Specifically, the signal for DC impedance measurement is read from the DC differential signal input channels ADC_AIN0 and ADC_AIN1, and the signal for AC impedance measurement is read from the AC differential signal input channels ADC_AIN2 and ADC_AIN3. The V setting REF Both the DC level and AC sine wave amplitude signals are read from the signal reading channel ADC_AIN4.

[0075] like Fig.13 and Fig.14 As shown, the shielding device 8 is provided for the following reasons: Since the measuring circuit itself has the inherent characteristic of high impedance, it is very easy to absorb the surrounding piezoelectric electromagnetic field, so whether it is the power frequency electric field or the electrostatic field, it will disturb the measuring circuit in the form of electromagnetic field, and these interferences will appear in the effective signal being measured in the form of noise superposition, thereby causing measurement errors. In order to solve the EMC problem in measurement, electromagnetic interference is generally through conduction and radiation, so a shielding device is required to isolate and shield electromagnetic interference.

[0076] like Figures 18 to 22 As shown, the power supply is an isolated power supply with a single power input and dual power outputs, and the power supply includes a dual-output isolated power supply U4, a first low-voltage dropout linear regulator U1, a second low-voltage dropout linear regulator U2, a third low-voltage dropout linear regulator U3, a fifth low-voltage dropout linear regulator U5 and a sixth linear regulator U6, the first low-voltage dropout linear regulator U1, the third low-voltage dropout linear regulator U3 and the sixth linear regulator U6 are all connected to one output of the dual-output isolated power supply U4, and the second low-voltage dropout linear regulator U2 and the fifth low-voltage dropout linear regulator U5 are all connected to the other output of the dual-output isolated power supply U4.

[0077] The present invention takes into account the noise problems caused by anti-interference and conducted disturbances, and selects an isolated power supply module with a single power input, dual power supply ±12V, and 500mA output as the power supply. After testing, the voltage rejection ratio of the operational amplifier in the circuit in the DC working area is very good. However, considering the characteristics that the performance of AC signals decreases due to the increase in frequency, in order to reduce the noise of the power rail, low-voltage dropout linear regulators LT3042 and LT3094 with excellent common-mode rejection ratio are selected as the voltage regulated output of the dual power supply to suppress the switching noise generated from the isolated power supply end. The second low-voltage dropout linear regulator U2 also serves to isolate the unknown multi-spectrum noise outside the high-resistance measurement circuit.

[0078] Here, the protection ring is described as follows. Fig.13 As shown, since the current in the measured impedance loop is extremely small, the charge in the current leaks to the electrodes with unequal potentials, which can easily cause the electrostatic charge outside the high-resistance measurement loop to flow into the high-resistance measurement loop or be affected by the piezoelectric effect of the electrode material near the loop. These charges flowing into the high-resistance measurement loop will seriously affect the measurement. In order to avoid the creeping leakage of charge, a guard ring is designed around the measurement main loop to make the potential of the protection equal to the potential of the measurement signal source, thereby solving the creeping leakage problem of charge. In the present invention, the 8th pin of the measuring amplifier U12 and the AC amplifier U16 are directly connected to the high-impedance device (including the DC measurement resistor Rz (R36), the AC measurement resistor Rj (R52) and the product to be tested), and the test cable is connected in the middle, such as the J4 output triaxial connector and the coaxial cable. A longer network needs to establish a guard ring. In the present invention, the VREF signal input to the measuring operational amplifier U12 and the first pin of the AC operational amplifier U16 is also input to the third low-power amplifier U14A (model ADA4522). The third low-power amplifier U14A forms a follower circuit. The output signal of the first pin of the third low-power amplifier U14A serves as a protection ring 9, so that the potential of the protected part is equal to the point position of the measuring signal source, thereby avoiding external noise interference.

[0079] In the present invention, the process of DC impedance measurement is as follows:

[0080] like Fig.13 As shown, first, a 1V voltage is set by the DAC digital-to-analog converter U10 in the DC source signal generating device 4, or a fixed voltage value of 1.024V is set by connecting the twenty-second low voltage difference linear regulator U22 and the high precision voltage divider U21 to form a high precision voltage source, and the set constant measurement voltage source V is controlled by the relay. REF Input to the non-inverting terminal of the measurement op amp U12, thus completing V REFThe positive voltage is established. Using the virtual short characteristic of the measuring op amp U12, the common mode level of IN+\IN- at both ends of the 8-pin is kept at 1V (or 1.024V), so the level at both ends of the product under test is also kept at 1V (or 1.024V). In this way, the product under test (Rx), the DC measuring resistor Rz (R36), the measuring op amp U12, and the constant measuring voltage source V REF , analog ground AGND form a series closed loop. The current change caused by the resistance change of the product to be tested will eventually be reflected in the voltage division of the 1st and 6th pins of the measuring amplifier U12. That is, the current change caused by different impedance values ​​will cause the voltage change of the 1st and 6th pins of the measuring amplifier U12. This voltage is defined as the forward voltage value V under the forward current condition. O_P1V ;Forward voltage value V O_P1V The differential voltage is input to the DC instrument amplifier U11 (model AD8422) for amplification. The amplified differential voltage is transmitted to the input end of the DC differential signal input channel AIN0 and AIN1 of the channel switching device U7 through the 32nd resistor R32 and the 40th resistor R40 as the first channel. After digital-to-analog conversion, the microprocessor can read V O_P1V The voltage value is used for impedance calculation.

[0081] The forward voltage V O_P1V After that, just turn on the fourth relay K4, and the previously set 1V voltage is input to the in-phase terminal (pin 1) of the measuring op amp U12 through the inverter U20A, thus completing the constant measuring voltage source V REF The two ends of the product to be tested will obtain a -1V voltage source with the opposite current direction, so that the product to be tested Rx, the DC measurement resistor Rz (R36), the measurement amplifier U12, the constant measurement voltage source V REF , analog ground AGND to form a series closed loop. The reverse current is shunted by the measuring op amp U12 (pin 1, pin 6) to obtain the reverse voltage value V under the reverse current condition. O_N1V Here, the DC instrument amplifier U11 and the follower U13 form a circuit with a common-mode reference voltage point of 2.5V. The voltage level of the 6th pin REF of the DC instrument amplifier U11 is raised and stabilized at 2.5V, so that the common-mode reference point voltage of the DC instrument amplifier U11 amplification circuit is 2.5V, and the voltage of the reference point of the 4th pin (REFOUT) of the ADC analog-to-digital converter U7 in the channel switching device is also set to 2.5V (the 4th pin of the ADC analog-to-digital converter U7 and the 3rd pin of the follower U13 are the same network signal), and the AIN0 and AIN1 input terminals of the ADC analog-to-digital converter U7 can read the negative voltage value V O_N1VThis can ensure that the absolute values ​​of the forward impedance measurement voltage and the reverse impedance measurement voltage are consistent, thereby ensuring the consistency of the output impedance of the positive and negative signal sources.

[0082] Again, the above channel settings remain unchanged, set the DAC digital-to-analog converter U10 to 0V, the test circuit is the same as above, and the AIN0 and AIN1 input terminals of the ADC analog-to-digital converter U7 can read the zero potential voltage value V O_0V .

[0083] By measuring the forward voltage value V O_P1V , reverse voltage value V O_N1V and zero potential voltage value V O_0V , and the known DC measurement resistance Rz (R36), the impedance values ​​of the product under test in three cases can be calculated respectively, and then the average of these impedance values ​​can be obtained to obtain the impedance of the product under test.

[0084] The measurement process of the AC impedance of the present invention is as follows:

[0085] Set the AC source signal generator (i.e., direct digital frequency synthesizer, hereinafter referred to as DDS) to a 10Hz AC sine wave with an amplitude of 1Vpp (set according to the impedance of the product to be tested), and switch the first relay K1, the second relay K2, and the third relay K3 to the AC impedance test circuit. The 10Hz AC sine wave is coupled to the first pin of the AC op amp U16 through the linear isolator U18 and the first relay K1, and then passes through the AC measurement resistor Rj (R52, 100M ohm resistor), the second relay K2, the three-coaxial connector J4, and the product to be tested to form a test circuit. The measured voltage signal is read from the input end of the AC differential signal input channel (ADC_AIN2, ADC_AIN3) of the ADC analog-to-digital converter U7 and the input end of the signal reading channel (ADC_AIN4). At the set frequency, the AC impedance is much smaller than the DC impedance, so the sampling resistance of our AC measurement resistor Rj (R52) is as small as possible, and at the same time, considering the nonlinearity of the AC instrument amplifier U15 in the AC amplification circuit, try to use the circuit model with gain G=1. In this way, the sine wave signal V generated by DDS can be tested and input REF The effective value V REF_RMS And the sine wave signal V obtained after the op amp split O The effective value V O_RMS , and then the AC impedance value is obtained based on the known AC measurement resistance Rj.

[0086] The principles of the AC impedance measurement signal channel and the DC impedance measurement signal channel are basically the same. The biggest difference lies in the different ways of collecting and measuring the sine wave amplitude. For example, when a 10Hz AC sine wave signal frequency is used as the test signal source, the signal source period is 100mS (100 milliseconds), and a sampling rate of about 200Hz is used to collect 40 to 50 level data of 2 to 3 complete cycles to calculate the RMS effective value. Fig.23 The collected data shown can of course be set at a higher sampling rate to collect more distribution data for calculation to obtain a more accurate effective value. In this embodiment, the purpose of using low-frequency sampling is to control the signal bandwidth and utilize the 50 Hz and 60 Hz filter characteristics of the ADC analog-to-digital converter U7 itself to better suppress low-frequency interference.

[0087] The impedance measurement range of the present invention can be adjusted by adjusting the measurement voltage V loaded on both ends of the product to be measured. REF The voltage amplitude can also be changed by changing the resistance of the DC measuring resistor Rz or the AC measuring resistor Rj to achieve a broadband measurement range from 50M ohms to 100T ohms. In addition, the high resistance measurement of the present invention has certain requirements on the test environment and the natural environment. First, the device under test should be kept dry to truly reflect the physical properties of the device.

[0088] The high-precision measurement system of the present invention can be used to measure more than G ohms (1×10 14 >X R >1×10 9 The DC impedance and AC impedance of precision devices with a voltage tolerance of less than 2-5 volts (<2-5 volts), especially those (voltage, current type) sensors, electronic devices, semiconductor devices and other components that can only work normally under low voltage. For example, it can be used in high-precision testing of high input impedance of semiconductor devices, DC and AC high impedance testing of chemical sensors, and AC high impedance testing of capacitive sensors, and is widely used. In addition, the system is also suitable for surface resistance and volume resistance testing of low-voltage insulating materials, and the characteristic parameters of insulation DC impedance and AC impedance of the tested device (material) exceed G ohms, and even reach T ohms. It can also be applied to analytical instruments and measuring equipment, measurement in the process of material research and development, impedance testing of biomedicine, inspection of semiconductor devices, and testing of electronic products. This technology has the characteristics of being very easy to quantify production test applications, which is conducive to obtaining accurate impedance data of the tested materials in laboratories or processing plants, and conducting characteristic analysis and quality monitoring of the tested materials.

Claims

1. A high-precision measurement system for ultra-high impedance of low-voltage devices, characterized by: The system includes An independently arranged MCU (1) for outputting test data to the measurement system and receiving test results, An AC source signal generating device (2) is independently arranged and used to output an AC alternating current sinusoidal excitation signal, A serial port isolator (U9) and a port isolator (U16) for isolating and coupling data transmission of the MCU (1), a sinusoidal signal isolating device (U18) for isolating and coupling the AC sinusoidal excitation signal output by the AC source signal generating device (2), A DC source signal generating device (3) for generating a DC power supply signal, A DC source switching device (4) for switching positive and negative phase DC sources, an impedance measuring device (5) for measuring the impedance of a product to be measured, A data collector (6) for collecting impedance measurement data of the product to be tested, a test resistor analog switch (7) for setting different resistance values ​​of the test resistor, and a power supply, A shielding device (8) is arranged around the product to be tested. The output end of the test resistance analog switch (7) is connected to the product to be tested. The test resistance analog switch (7) is provided with a plurality of test resistors (Rs). The DC source signal generating device (3) is connected to the impedance measuring device (5) through the DC source switching device (4). The output end of the impedance measuring device (5) is connected to the input end of the test resistance analog switch (7). After the impedance measuring device (5) takes samples, the output end is connected to the data acquisition device (6). The serial port isolator (U9) is connected to the MCU (1) and the data acquisition device (6) respectively. The output end of the sinusoidal signal isolating device (U18) is connected to the MCU (1) and the data acquisition device (6) through a third relay. The electrical appliance (K3) is connected to the impedance measuring device (5) and the data acquisition device (6) respectively; the MCU (1) is connected to the impedance measuring device (5), the data acquisition device (6) and the test resistance analog switch (7) respectively through the port isolator (U16); a protection ring (9) is provided between the impedance measuring device (5) and the product to be tested; the power supply supplies power to the entire measurement system; the DC source signal generating device (3) comprises a DAC digital-to-analog converter (U10), a twenty-second low-voltage dropout linear regulator (U22), a high-precision voltage divider (U21), an inverter (U20A) and a buffer (U20B); the twenty-second low-voltage dropout linear regulator (U22) and the The high-precision voltage divider (U21) is connected to form a high-precision voltage source, the DAC digital-to-analog converter (U10) provides a set value voltage source for the impedance measuring device (5), and the high-precision voltage source and the set value voltage source are switched through a channel switcher (U15); the impedance measuring device (5) comprises a third low-power amplifier (U14A), a measuring op amp (U12), and an instrumentation amplifier (U11), the output end of the third low-power amplifier (U14A) is connected to the negative input end of the measuring op amp (U12), the positive input end of the third low-power amplifier (U14A), the positive input end of the measuring op amp (U12), and the negative input end of the instrumentation amplifier (U11) The output end of the instrument amplifier (U11) is connected to the data acquisition device (6), one end of the test resistance analog switch (7) is connected to the output end of the measuring operational amplifier (U12), and the other end is connected to the input end of the product to be tested, the protection ring (9) is connected to the negative input end of the measuring operational amplifier (U12) and the input end of the product to be tested, a fourth relay (K4) is provided between the positive input end of the measuring operational amplifier (U12) and the output ends of the inverter (U20A) and the buffer (U20B), and a first relay (K1) is provided between the output end of the third low-power amplifier (U14A) and the negative input end of the measuring operational amplifier (U12).

2. The high-precision measurement system for ultra-high impedance of low-voltage devices according to claim 1, characterized in that: The serial port isolator (U9), the port isolator (U16), the sinusoidal signal isolating device (U18), the DC source signal generating device (3), the DC source switching device (4), the impedance measuring device (5), the data collector (6) and the test resistance analog switch (7) are all arranged on the same PCB board.

3. The high-precision measurement system for ultra-high impedance of low-voltage devices according to claim 1, characterized in that: A second relay (K2) and a three-coaxial connector (J4) are also provided between the product to be tested and the test resistance simulation switch (7); the product to be tested is connected to the three-coaxial connector (J4); the three-coaxial connector (J4) is connected to the second relay (K2); the shielding device (8) is an EMI shielding box; the EMI shielding box is connected to a voltage source via a reference voltage source device (U8).

4. The high-precision measurement system for ultra-high impedance of low-voltage devices according to claim 1, characterized in that: The power supply is an isolated power supply with a single power input and dual power outputs, and comprises a dual-output isolated power supply (U4), a first low-voltage dropout linear regulator (U1), a second low-voltage dropout linear regulator (U2), a third low-voltage dropout linear regulator (U3), a fifth low-voltage dropout linear regulator (U5) and a sixth linear regulator (U6); the first low-voltage dropout linear regulator (U1), the third low-voltage dropout linear regulator (U3) and the sixth linear regulator (U6) are all connected to one output of the dual-output isolated power supply (U4), and the second low-voltage dropout linear regulator (U2) and the fifth low-voltage dropout linear regulator (U5) are all connected to the other output of the dual-output isolated power supply (U4).

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