CN111736017A - Circuit and method for realizing alternating current impedance measurement by adopting transconductance shunting structure - Google Patents
Circuit and method for realizing alternating current impedance measurement by adopting transconductance shunting structure Download PDFInfo
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- CN111736017A CN111736017A CN202010563904.0A CN202010563904A CN111736017A CN 111736017 A CN111736017 A CN 111736017A CN 202010563904 A CN202010563904 A CN 202010563904A CN 111736017 A CN111736017 A CN 111736017A
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Abstract
The invention provides a circuit and a method for realizing alternating current impedance measurement by adopting a transconductance shunting structure, which have the advantages of simple structure and high measurement precision. The circuit comprises an alternating current voltage source, an operational amplifier (OP 1), a differential amplifier (OP 2), a measuring resistor (Rs) and a capacitor (Cx), wherein a product to be measured is connected with two ends of the capacitor in parallel, one end of the measuring resistor is connected with one end of the capacitor in common, the other end of the measuring resistor is connected with the output end of the operational amplifier, the output end of the operational amplifier is also connected with the negative input electrode of the differential amplifier, and the positive input electrode of the operational amplifier is connected with the positive input electrode of the differential amplifier; the method comprises the following steps: (1) calculating the resistance of a product to be detected; (2) calculating the AC input voltage VREFAnd a differential output voltage VOA valid value of (a); (3) the ac impedance was calculated under different conditions. The invention can be applied to the field of testing.
Description
Technical Field
The invention relates to the field of testing, in particular to a circuit for realizing alternating current impedance measurement by adopting a transconductance shunting structure and a method for realizing alternating current impedance measurement by utilizing the circuit.
Background
The higher the impedance of the common insulating material is, the higher the voltage endurance is, so the measurement is generally performed by using a high-voltage instrument (0-KV), such as a megger and the like. However, when measuring materials with low withstand voltage (< 2V, even lower) and high impedance, the high voltage testing method cannot be normally applied, because the tested materials or devices have nonlinearity during low voltage measurement, so that the method is not suitable for use in electrometers and the like. Other major factors affecting the measurement accuracy at low voltages include:
1. the influence of noise of the resistor is particularly prominent;
2. exciting self-body noise;
the effects of EMI (electromagnetic interference);
4. and testing the leakage current and the leakage voltage of the network.
At present, when the resistance of more than 1G ohm is measured, the static meter, the SMU, the picoammeter, the voltage source, the high impedance meter and the like are used for measurement. The measurement mode of the electrometer needs to be configured with a voltage source or a current source, so that the precise measurement of the high resistance is realized by using a method of externally connecting a voltage source (source meter) and the electrometer or a picoammeter, so as to obtain the measurement voltage or current, and finally calculating the resistance value by using the ohm's law, as shown in fig. 1 and 2. The meter test method using the source meter and the electrometer can directly obtain the test value. Fig. 1 and 2 show high resistance measurement realized by a general electrometer (electrostatic voltmeter) and a high resistance meter instrument, and the characteristics of the high resistance measurement are that the electrometer (electrostatic voltmeter) and an excitation source (a voltage source or a current source) are separated into two parts. In addition, a source meter and an electrometer are integrated into one instrument by a high-resistance measuring instrument which is popular in the market, such as instrument equipment with the model number of B2985A and the like, and then the high resistance is measured by connecting a special matching adapter for the instrument, so that a test value can be directly read on an instrument panel without additional calculation.
Another method is to use a common digital multimeter to calculate the high resistance by reading the voltage, the principle of which is shown in fig. 3. FIG. 3 shows that an external current source is used to supply a constant current to a device under test, and then an operational amplifier is used as a buffer, wherein V1 ≈ V0, so that a common digital multimeter with lower cost can measure the high resistance of Rx.
In addition, an integrated excitation source (a voltage source or a current source) and a buffer can be built in to form a measuring circuit for high-resistance measurement. In the current circuit measurement technology of many test instrument boards, a current source of a circuit board is used, and then an operational amplifier is used as a buffer of an input signal, as shown in fig. 4 and 5. Fig. 4 is a schematic diagram of high-resistance measurement of an electrometer with a built-in current source, and fig. 5 is a schematic diagram of high-resistance measurement of an electrometer with a protection ohm type.
The measured resistance calculation formulas in fig. 4 and 5 are as follows:
RX=V1/(VS× R) (expression 1)
RX=V1I (expression 2)
The test method shown in fig. 1, 2 and 3 is to realize the measurement of high resistance by the cooperation of instruments and meters. Fig. 4 and 5 are completely the measurement method of the autonomous circuit instrument board card. But the measurement accuracy is very limited by the electrical parameter performance of the operational amplifier in the test system.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide a circuit which is simple in structure, high in measurement accuracy and free from the influence of low withstand voltage of a device to be measured or a material and measurement system factors and adopts a transconductance shunting structure to realize alternating-current impedance measurement.
In addition, the invention also provides a method for realizing alternating current impedance measurement by using the circuit, and the method can realize high-precision measurement of high impedance under the condition of low withstand voltage of a product to be measured.
The technical scheme adopted by the circuit for realizing alternating current impedance measurement by adopting the transconductance shunting structure is as follows: the device comprises an alternating current voltage source, an operational amplifier, a differential amplifier, a measuring resistor and a capacitor, wherein a product to be measured is connected with two ends of the capacitor in parallel, one end of the capacitor is connected with one end of the capacitor, the other end of the measuring resistor is connected with the output end of the operational amplifier, the output end of the operational amplifier is also connected with the negative input electrode of the differential amplifier, the positive input electrode of the operational amplifier is connected with the positive input electrode of the differential amplifier, a protection ring is arranged between the capacitor and the connecting point of the measuring resistor and is connected with the negative input electrode of the operational amplifier, the alternating current voltage source is loaded between the positive input electrode of the operational amplifier and the ground, and the output end of the differential amplifier (OP 2) acquires a differential output voltage Vo.
Furthermore, EMI shielding boxes are arranged on the periphery of the filter capacitor and the product to be tested.
Still further, the alternating voltage source is a direct digital frequency synthesizer.
According to the scheme, as long as the amplitude of the alternating current signal source output to the two ends of the measured object by the operational amplifier is within the range of the measured capacity, the amplitude cannot be changed along with the change of the measured object, and the nonlinear influence of the measured object is not easily caused, so that the sampling working stability of the differential amplifier is greatly stabilized, and the working precision of the measuring circuit is ensured; in addition, through the arrangement of the protection ring, the protected potential at the position is equal to the potential of the measurement signal source, so that the creeping leakage of charges is avoided, the static charges outside the impedance measurement loop are prevented from flowing into the impedance measurement loop or being influenced by the piezoelectric effect of electrode materials near the loop, the low-voltage and high-resistance measurement is not influenced by the low-voltage resistance and measurement system factors of a product or a material to be measured, and the measurement precision is greatly ensured.
In addition, through the setting of EMI shielding box, further shielded the influence of the electromagnetic effect near the measuring loop, guaranteed the quality of measurement.
The method for realizing alternating current impedance measurement by using the circuit comprises the following steps:
(1) calculating the resistance of a product to be detected: firstly, a direct current measuring voltage V is input between the positive input electrode of the operational amplifier and the groundREFThe direct current measuring voltage VREFVoltage drop V between two ends of product to be measured1Equal, the voltage at two ends of the product to be measured is constant, and the current I flowing through the measuring resistorRS= I + Iop, I being the current flowing through the product to be measured, Iop being the bias current of the operational amplifier, the bias current Iop being sufficiently small to be ignored, I = I being obtainedRSAt the moment, the operational amplifier is equivalent to a shunt, and the resistance value of the product to be measured is changedThe conversion causes a voltage V between a positive input electrode and an output electrode of the operational amplifierABAnd differential sampling is performed by the differential amplifier to obtain a change of the differential output voltage value Vo, at which time VAB=VOWhereby the measured value of Rx is deduced to be I = IRS=V0/RS，RX=V1/I，V1=VREF；
(2) Switching between the positive input electrode of the operational amplifier and the ground to be input by an alternating current voltage source, wherein the alternating current voltage source generates a sinusoidal alternating current voltage signal, and the sinusoidal alternating current voltage signal is respectively read by an analog-to-digital converter for an alternating input voltage V in a unit periodREFAnd a differential output voltage VOAnd calculating VREF_RMSAnd VO_RMSTwo effective values, the calculation formula is as follows:
wherein, VREF_RMSFor generating a sine-wave signal V input after generation by an AC voltage sourceREFEffective value of (V)O_RMSSine wave signal V obtained after shunting for operational amplifierOIs identical to the voltage value between the positive and negative inputs of said differential amplifier, VREF_MAXFor an AC input voltage VREFMaximum of n readings, VREF_nFor an AC input voltage VREFThe nth reading of, V0_MAXFor differentially outputting a voltage V0Maximum of n readings, V0_nFor differentially outputting a voltage V0The nth reading, wherein n is the voltage reading frequency and is also the voltage frequency participating in the operation in the effective value calculation;
(3) when the current and voltage in the AC test loop have phase difference, the AC impedance is calculated as follows
Wherein Rs is the resistance of the measuring resistor, Iin is the signal current, and omegaIs the angular frequency, t is the time,
to an initial phase, XCIs an impedance value, CXIs a capacitance value, foIs the frequency of the sine wave;
(4) when the product to be tested has real part impedance in the form of resistance and imaginary part impedance in the form of capacitance, and the phase of the current flowing through the total reactance of the product to be tested leads the phase of the voltage by 90 degrees, the current has
Wherein, XCRepresents impedance, CXRepresenting capacitance, j an imaginary symbol, ω an angular frequency, CSRepresents the capacitance, foAt sine wave frequency, Rs is the resistance of the measuring resistor.
Further, the sinusoidal AC voltage signal generated by the AC voltage source is 1Vpp and has a frequency f0Is a sinusoidal AC signal within the range of 10-100 Hz.
It can be seen from the above scheme that, according to the process of the method of the present invention, because the amplitude of the ac measurement signal at the two ends of the object Rx/Cx to be measured does not change, the factors affecting the measurement of the ac impedance are only the measurement resistor Rs and the differential output voltage V set in the loop0And an AC input voltage VREFThe resistance value of the product to be measured and the current (charge) flowing through the product are irrelevant, so that the measuring result has no error due to the nonlinearity of the product to be measured during low-voltage measurement, the measuring precision is well ensured, the testing process is simple, the measuring stability is high, the cost is low, and the method can be suitable for mass production.
Drawings
FIG. 1 is a simplified schematic diagram of a prior art high resistance measurement using an electrometer and an external voltage source;
FIG. 2 is a simplified schematic diagram of a prior art high resistance measurement using an electrometer and an external current source;
FIG. 3 is a simplified schematic of a prior art high resistance measurement using a real current source and a digital multimeter;
FIG. 4 is a simplified schematic diagram of a prior art electrometer high resistance measurement with a built-in current source;
FIG. 5 is a simplified schematic diagram of a prior art high resistance measurement of an electrometer with protected ohm;
FIG. 6 is a simplified schematic diagram of the circuit of the present invention;
FIG. 7 is an equivalent circuit diagram of the circuit schematic shown in FIG. 6;
FIG. 8 is a waveform of an AC RMS data collection as described in the examples.
Detailed Description
As shown in fig. 6 and fig. 7, the circuit of the present invention includes an ac voltage source, an operational amplifier OP1, a differential amplifier OP2, a measuring resistor Rs and a capacitor Cx, wherein a product Rx to be tested is connected in parallel to two ends of the capacitor Cx and one end of the product Rx is grounded, one end of the measurement resistor Rs is connected to one end of the capacitor Cx, the other end of the measurement resistor Rs is connected to an output terminal of the operational amplifier OP1, the output terminal of the operational amplifier OP1 is also connected to the negative input terminal of the differential amplifier OP2, a positive input electrode of the operational amplifier OP1 is connected with a positive input electrode of the differential amplifier OP2, a guard ring a is provided between the connection point of the capacitance Cx and the measurement resistance Rs to be connected to the negative input pole of the operational amplifier OP1, the alternating voltage source is loaded between the positive input electrode of the operational amplifier OP1 and the ground, and a differential output voltage Vo is obtained at the output end of the differential amplifier OP 2. And an EMI shielding box b is arranged on the periphery of the capacitor Cx and the product to be tested.
In the present embodiment, the operational amplifier OP1 is required to satisfy the following parameter settings: (1) low input bias current: ± 20fA (max, -40 ℃/TA < +85 ℃); (2) low voltage noise density: 14nV/√ Hz (10 kHz); (3) inner guard ring buffer: has a maximum disorder of 100 μ V; (4) offset voltage: 50 uV; (5) power supply voltage: 2.25V to 8V; (6) wide bandwidth: 2MHz unity gain crossover.
The key parameter requirements for measuring the resistance Rs are shown in table 1.
The relationship between current noise density and bandwidth at resistor Rs during measurement is shown in table 2 below, where it is important to use a high quality resistor. Many high value resistors designed for high voltage operation are non-linear at low voltages and are not suitable for electrometer use. If the resistance quality is poor and not satisfactory, the 1/f noise of the resistor can affect the test precision, and finally the measurement result is damaged.
The method for measuring impedance by using the circuit comprises the following steps:
(1) calculating the resistance of the tested product: firstly, a direct current measurement voltage V is input between the positive input electrode of the operational amplifier OP1 and the groundREFThe direct current measuring voltage VREFVoltage drop V between two ends of product to be measured1Equal, the voltage at the two ends of the product Rx to be measured is constant, and the current I flowing through the measuring resistor RsRS= I + Iop, I is the current flowing through the product Rx to be measured, Iop is the bias current of the operational amplifier OP1, the bias current Iop is small enough to be ignored, and I = I is obtainedRSAt this time, the operational amplifier OP1 is equivalent to a shunt, and the resistance value change of the product Rx to be measured causes the voltage V between the positive input electrode and the output electrode of the operational amplifier OP1ABAnd then differentially sampled by the differential amplifier OP2 to obtain a differential output voltage value VOAt this time, VAB=VOWhereby the measured value of Rx is deduced to be I = IRS=V0/RS，RX=V1/I，V1=VREF。
(2) Switching between the positive input electrode of the operational amplifier OP1 and ground to be an AC voltage source input, the AC voltage source generating a sinusoidal AC voltage signal, respectively using an analog-to-digital converter to read the AC input voltage V in a unit periodREFAnd a differential output voltage VOAnd calculating VREF_RMSAnd VO_RMSTwo effective values, the calculation formula is as follows:
wherein, VREF_RMSFor generating a sine-wave signal V input after generation by an AC voltage sourceREFEffective value of (V)O_RMSSine wave signal V obtained after shunting for operational amplifierOIs in accordance with the voltage value between the positive and negative input terminals of the differential amplifier OP2, VREF_MAXFor an AC input voltage VREFMaximum of n readings, VREF_nFor an AC input voltage VREFThe nth reading of, V0_MAXFor differentially outputting a voltage V0Maximum of n readings, V0_nFor differentially outputting a voltage V0N is the number of voltage readings, and is also the number of voltages participating in the calculation of the effective value.
(3) When the current and voltage in the AC test loop have phase difference, the AC impedance is calculated as follows
Wherein Rs is the resistance of the measuring resistor, Iin is the signal current, omega is the angular frequency, t is the time,
to an initial phase, XCIs an impedance value, CXIs a capacitance value, foIs the frequency of the sine wave; x in the above formulaCAnd CXDerived from the circuit original pathThe following formula is calculated to obtain the product,
(4) when the product to be tested has real part impedance in the form of resistance and imaginary part impedance in the form of capacitance, and the phase of the current flowing through the total reactance of the product to be tested leads the phase of the voltage by 90 degrees, the current has
Wherein, XCRepresents impedance, CXRepresenting capacitance, J an imaginary symbol, ω an angular frequency, CSRepresents the capacitance, foAt sine wave frequency, Rs is the resistance of the measuring resistor.
According to the calculation process of the method, the factors influencing the alternating-current impedance are only related to the measuring resistor arranged in the loop, the differential output voltage value and the alternating-current input voltage, but are not related to the resistance value and the voltage drop of the product to be measured, so that the measuring result cannot generate errors due to nonlinearity generated in low-voltage measurement of the product to be measured, and the measuring precision is well ensured.
The invention is briefly illustrated by the following specific examples: for example, when an alternating current sine wave signal frequency of 10Hz is adopted as a test signal source, the signal source period is 100mS (100 mS), four and fifty level data of 2 to 3 complete periods are collected by using a sampling rate of about 200Hz to calculate the RMS effective value, as shown in fig. 8, a higher sampling rate can be set to collect more distributed data to calculate a more accurate effective value, and the purpose of using low-frequency sampling here is to control the signal bandwidth, and the low-frequency interference is better suppressed by using the filter characteristics of 50 Hz and 60 Hz of the ADC chip itself.
Claims (5)
1. A circuit for realizing alternating current impedance measurement by adopting a transconductance shunting structure is characterized in that: the circuit comprises an alternating current voltage source, an operational amplifier (OP 1), a differential amplifier (OP 2), a measuring resistor (Rs) and a capacitor (Cx), wherein a product (Rx) to be measured is connected with two ends of the capacitor (Cx) in parallel, one end of the capacitor (Cx) is connected with the common ground, one end of the measuring resistor (Rs) is connected with one end of the capacitor (Cx), the other end of the measuring resistor (Rs) is connected with the output end of the operational amplifier (OP 1), the output end of the operational amplifier (OP 1) is also connected with the negative input electrode of the differential amplifier (OP 2), the positive input electrode of the operational amplifier (OP 1) is connected with the positive input electrode of the differential amplifier (OP 2), a protection ring (a) is arranged between the connection point of the capacitor (Cx) and the measuring resistor (Rs) and connected with the negative input electrode of the operational amplifier (OP 1), the alternating current voltage source is loaded between the positive input electrode of the operational amplifier (OP 1) and the ground, a differential output voltage Vo is obtained at the output of the differential amplifier (OP 2).
2. The circuit according to claim 1, wherein the circuit for measuring ac impedance using the transconductance shunting structure comprises: and an EMI shielding box (b) is arranged on the periphery of the capacitor (Cx) and the tested product.
3. The circuit according to claim 1, wherein the circuit for measuring ac impedance using the transconductance shunting structure comprises: the alternating current voltage source is a direct digital frequency synthesizer.
4. A method of performing an ac impedance measurement using the circuit of claim 1, the method comprising the steps of:
(1) calculating the resistance of the tested product: firstly, a direct current measuring voltage V is input between the positive input electrode of the operational amplifier (OP 1) and the groundREFThe direct current measuring voltage VREFVoltage drop V between two ends of product to be measured1Equal, the voltage at both ends of the product (Rx) to be measured will be constant, and the current I flowing through the measuring resistor (Rs)RS= I + Iop, I being the current flowing through the product to be tested (Rx), Iop being the bias current of said operational amplifier (OP 1), the bias current Iop being sufficiently small to be negligible, resulting in I = IRSAt this time, the operational amplifier (OP 1) is equivalent to a shunt, and the resistance value change of the product (Rx) to be tested causes the voltage V between the positive input pole and the output pole of the operational amplifier (OP 1)ABIs detected, and a change in the differential output voltage value Vo is obtained by differential sampling of said differential amplifier (OP 2), in which case VAB=VOWhereby the measured value of Rx is deduced to be I = IRS=V0/RS，RX=V1/I，V1=VREF；
(2) Switching between the positive input of the operational amplifier (OP 1) and ground to an AC voltage source input, the AC voltage source generating a sinusoidal AC voltage signal, each using an analog-to-digital converter to read the AC input voltage V per unit periodREFAnd a differential output voltage VOAnd calculating VREF_RMSAnd VO_RMSTwo effective values, the calculation formula is as follows:
wherein, VREF_RMSFor generating a sine-wave signal V input after generation by an AC voltage sourceREFEffective value of (V)O_RMSSine wave signal V obtained after shunting for operational amplifierOEffective value of (a), the value andthe voltage values between the positive and negative input electrodes of the differential amplifier (OP 2) are identical, VREF_MAXFor an AC input voltage VREFMaximum of n readings, VREF_nFor an AC input voltage VREFThe nth reading of, V0_MAXFor differentially outputting a voltage V0Maximum of n readings, V0_nFor differentially outputting a voltage V0The nth reading, wherein n is the voltage reading frequency and is also the voltage frequency participating in the operation in the effective value calculation;
(3) when the current and voltage in the AC test loop have phase difference, the AC impedance is calculated as follows
Wherein Rs is the resistance of the measuring resistor, Iin is the signal current, omega is the angular frequency, t is the time,
to an initial phase, XCIs an impedance value, CXIs a capacitance value, foIs the frequency of the sine wave;
(4) when the product to be tested has real part impedance in the form of resistance and imaginary part impedance in the form of capacitance, and the phase of the current flowing through the total reactance of the product to be tested leads the phase of the voltage by 90 degrees, the current has
Wherein, XCRepresents impedance, CXRepresenting capacitance, J an imaginary symbol, ω an angular frequency, CSRepresents the capacitance, foAt sine wave frequency, Rs is the resistance of the measuring resistor.
5. The method of ac impedance measurement according to claim 4, wherein: the sine alternating voltage signal generated by the alternating voltage source is 1Vpp and has the frequency f0Is a sinusoidal AC signal within the range of 10-100 Hz.
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| CN114264225A (en) * | 2021-12-28 | 2022-04-01 | 石家庄数英仪器有限公司 | Real-time fault discrimination circuit and method for potentiometer type corner sensor |
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| CN114264225A (en) * | 2021-12-28 | 2022-04-01 | 石家庄数英仪器有限公司 | Real-time fault discrimination circuit and method for potentiometer type corner sensor |
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