CN111766447A - Circuit and method for realizing direct current impedance measurement by adopting transconductance shunting structure - Google Patents
Circuit and method for realizing direct current impedance measurement by adopting transconductance shunting structure Download PDFInfo
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- CN111766447A CN111766447A CN202010563899.3A CN202010563899A CN111766447A CN 111766447 A CN111766447 A CN 111766447A CN 202010563899 A CN202010563899 A CN 202010563899A CN 111766447 A CN111766447 A CN 111766447A
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R27/00—Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
- G01R27/02—Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant
- G01R27/025—Measuring very high resistances, e.g. isolation resistances, i.e. megohm-meters
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R1/00—Details of instruments or arrangements of the types included in groups G01R5/00 - G01R13/00 and G01R31/00
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Abstract
The invention provides a circuit and a method for realizing direct current impedance measurement by adopting a transconductance shunting structure, which have the advantages of simple structure, high measurement precision and no influence by the low voltage resistance of a device to be measured or a material and the factors of a measurement system. The circuit comprises a constant voltage source, an operational amplifier (OP 1), a differential amplifier (OP 2), a measuring resistor (Rs) and a filter capacitor (Cx), wherein a product to be measured is connected with two ends of the filter capacitor in parallel, one end of the product to be measured is grounded, one end of the measuring resistor is connected with one end of the filter 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, and a protection ring (a) is arranged between the connection point of the filter capacitor and the measuring resistor and connected with the negative input electrode of the operational amplifier; the method comprises the following steps: (1) calculating the electricity of the tested productBlocking; (2) calculated at a constant measurement voltage VREFImpedance in the state of positive voltage, negative voltage and 0V voltage respectively; (3) and obtaining the direct current impedance by averaging. 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 direct current impedance measurement by adopting a transconductance shunting structure and a method for realizing direct 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 direct-current impedance measurement.
In addition, the invention also provides a method for realizing direct 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 device or material to be measured.
The technical scheme adopted by the circuit for realizing the direct current impedance measurement by adopting the transconductance shunting structure is as follows: the circuit comprises a constant voltage source, an operational amplifier, a differential amplifier, a measuring resistor and a filter capacitor, wherein a product to be measured is connected with two ends of the filter capacitor in parallel, one end of the product to be measured is grounded, and the measuring resistor is connected with the two ends of the filter capacitor in parallelThe 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 guard ring is arranged between the connection points of the filter capacitor and the measuring resistor and is connected with the negative input electrode of the operational amplifier, the constant voltage source is loaded between the positive input electrode of the operational amplifier and the ground, and the output end of the differential amplifier obtains differential output voltage V0。
Further, an EMI shielding box is arranged on the periphery of the filter capacitor and the tested product.
The above scheme shows that a constant measurement voltage V is setREFThe constant measurement voltage VREFPressure drop V between two ends of the product to be measured1When the voltage across the product to be measured is equal, the voltage across the product to be measured is constant, and the current I flowing through the measuring resistorRS=I+IOPI is the current flowing through the product to be tested, IOPIs a bias current of the operational amplifier, bias current IOPSmall enough to be ignored, resulting in I = IRSAt this time, the operational amplifier is equivalent to a shunt, and the resistance value change of the product to be detected causes the voltage V between the positive input electrode and the output electrode of the operational amplifierABAnd differential output voltage value V is obtained by differential sampling of the differential amplifier0The change of the voltage and the current relationship can be used for obtaining the resistance value of a product to be measured, then the direct current impedance can be obtained through an impedance calculation formula, similarly, the impedance under different current states can be obtained by measuring the impedance values under the conditions of positive current, negative current and zero potential current, so the direct current impedance can be quickly obtained through the circuit, the circuit structure is simple, the high measurement precision can be ensured through the cooperation of an operational amplifier and a differential amplifier, in addition, the protected potential is equal to the potential of a measurement signal source through the arrangement of a protection ring, the creeping leakage of charges is avoided, and the static charges outside an impedance measurement loop are prevented from flowing into a resistance circuitThe resistance measurement loop or the resistance measurement loop is influenced by the piezoelectric effect of the electrode material near the loop, so that the low-voltage high-resistance measurement is not influenced by the low voltage resistance of the product to be measured or the material and the factors of a measurement system, 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 direct current impedance measurement by using the circuit comprises the following steps:
(1) calculating the resistance of the tested product: setting a constant measurement voltage VREFThe constant measurement voltage VREFPressure drop V between two ends of the 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+IOPI is the current flowing through the product to be tested, IOPIs a bias current of the operational amplifier, bias current IOPSmall enough to be ignored, resulting in I = IRSAt this time, the operational amplifier is equivalent to a shunt, and the resistance value change of the product to be detected causes the voltage V between the positive input electrode and the output electrode of the operational amplifierABAnd differential output voltage value V is obtained by differential sampling of the differential amplifier0At this time, VAB=VOWhereby the measured value of Rx is deduced to be I = IRS=V0/RS，RX=V1/I；
(2) Calculated at a constant measurement voltage VREFImpedances in a positive voltage, a negative voltage and a 0V voltage state, respectively, at which a constant measurement voltage V in the positive voltage and the negative voltage state is setREFAre equal in absolute value, and the impedances in the positive voltage, negative voltage and 0V voltage states are defined as XR_P、XR_N、XR_0Setting up
Constant measurement voltage VREFAt positive voltage, the expression is VREF_P1V，V0Is expressed by the expression VO_P1V，
Constant measurement voltage VREFWhen it is a negative voltage, the expression is VREF_N1V，V0Is expressed by the expression VO_N1V，
Constant measurement voltage VREFAt 0V, the expression is VREF_0V，V0Is expressed by the expression VO_0V，
Setting the power supply of the operational amplifier and the differential amplifier to adopt complementary symmetrical positive and negative power supply rails, and calculating a formula according to direct current impedance
To obtain
A forward current impedance of
,
Negative current impedance of
,
Zero potential current impedance of
;
(3) Averaging the positive current impedance value, the negative current impedance value and the zero potential current impedance value to obtain a direct current impedance XRIs composed of
And n is the number of times of reading the impedance value.
Further, when the tested product is sensitive to reverse voltage or current, the DC impedance X is obtainedRIs XR=XR_P。
Still further, when the product to be tested is sensitive to a forward voltage or current, the DC impedance X is setRIs XR=XR_N。
According to the scheme, the calculation process of the method disclosed by the invention shows that the factors influencing the direct current impedance are only related to the measurement resistor arranged in the loop, the differential output voltage value and the constant measurement voltage, but are not related to the resistance value of the product to be measured, so that the measurement result cannot generate errors due to nonlinearity generated when the product to be measured has low voltage measurement, the measurement precision is well ensured, the test process is simple, 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.
Detailed Description
As shown in fig. 6 and 7, the circuit of the present invention includes a constant voltage source, an operational amplifier OP1, a differential amplifier OP2, a measurement resistor Rs and a filter capacitor Cx, wherein a product Rx to be measured is connected in parallel to two ends of the filter capacitor Cx and one end of the product Rx is connected to the common ground, one end of the measurement resistor Rs is connected to one end of the filter capacitor Cx, the other end of the measurement resistor Rs is connected to an output end of the operational amplifier OP1, an output end of the operational amplifier OP1 is further connected to a negative input electrode of the differential amplifier OP2, a positive input electrode of the operational amplifier OP1 is connected to a positive input electrode of the differential amplifier OP2, a guard ring a is disposed between a connection point of the filter capacitor Cx and the measurement resistor Rs and connected to the negative input electrode of the operational amplifier OP1, the constant voltage source is loaded between the positive input electrode of the operational amplifier OP1 and the ground, obtaining a differential output voltage V at an output of the differential amplifier OP20. And an EMI shielding box b is arranged on the periphery of the filter capacitor Cx and the tested product.
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: setting a constant measurement voltage VREFThe constant measurement voltage VREFPressure drop V between two ends of the 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+IOPI is the current flowing through the product Rx to be tested, IOPIs a bias current of the operational amplifier OP1, a bias current IOPSmall enough to be ignored, resulting in I = IRSAt 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 outputVoltage value V0At this time, VAB=VOWhereby the measured value of Rx is deduced to be I = IRS=V0/RS，RX=V1/I。
(2) Calculated at a constant measurement voltage VREFImpedance in the state of positive voltage, negative voltage and 0V voltage is set three times respectively: in the present embodiment, the measurement voltage V is constantREFThe level value of the voltage value can be set according to the actual measurement range through the DAC digital-to-analog conversion chip. The positive voltage and the negative voltage are set in the range of-2V to + 2V. At this time, a constant measurement voltage V in a positive voltage and a negative voltage state is setREFAre equal in absolute value, i.e. | + VREF|=|- VREFAnd setting the absolute value error of the positive and negative voltages to be less than 0.05%, setting the voltage to be higher than 0.1%, and keeping the output impedance of the positive and negative signal sources consistent. Defining impedances in the states of positive voltage, negative voltage and 0V voltage as XR_P、XR_N、XR_0Setting up
Constant measurement voltage VREFAt positive voltage, the expression is VREF_P1V，V0Is expressed by the expression VO_P1V，
Constant measurement voltage VREFWhen it is a negative voltage, the expression is VREF_N1V，V0Is expressed by the expression VO_N1V，
Constant measurement voltage VREFAt 0V, the expression is VREF_0V，V0Is expressed by the expression VO_0V，
The operational amplifier OP1 and the differential amplifier OP2 are set to be supplied with power by adopting complementary symmetrical positive and negative power supply rails according to a direct current impedance calculation formula
To obtain
A forward current impedance of
,
Negative current impedance of
,
Zero potential current impedance of
。
(3) Averaging the positive current impedance value, the negative current impedance value and the zero potential current impedance value to obtain a direct current impedance XRIs composed of
And n is the number of times of reading the impedance value.
When the tested product is sensitive to reverse voltage or current, the DC impedance XRIs XR=XR_P. When the tested product is sensitive to forward voltage or current, the DC impedance XRIs XR=XR_N。
According to the calculation process of the method, the factors influencing the direct current impedance are only related to the measuring resistor arranged in the loop, the differential output voltage value and the constant measuring voltage, but are not related to the resistance value of the product to be measured, so that the measuring result cannot generate errors due to nonlinearity generated during low-voltage measurement of the product to be measured, and the measuring precision is well ensured.
Claims (5)
1. A circuit for realizing direct current impedance measurement by adopting a transconductance shunting structure is characterized in that: the circuit comprises a constant voltage source, an operational amplifier (OP 1), a differential amplifier (OP 2), a measuring resistor (Rs) and a filter capacitor (Cx), wherein a product (Rx) to be measured is connected with two ends of the filter capacitor (Cx) in parallel, one end of the filter capacitor (Cx) is connected with the ground, one end of the measuring resistor (Rs) is connected with one end of the filter 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), and the positive input electrode of the differential amplifier (OP 2) isA guard ring (a) is arranged between the connection point of the filter capacitor (Cx) and the measuring resistor (Rs) and is connected with the negative input pole of the operational amplifier (OP 1), the constant voltage source is loaded between the positive input pole of the operational amplifier (OP 1) and the ground, and the output end of the differential amplifier (OP 2) obtains a differential output voltage V0。
2. The circuit according to claim 1, wherein the circuit for implementing dc impedance measurement by using the transconductance shunting structure comprises: and an EMI shielding box (b) is arranged on the periphery of the filter capacitor (Cx) and the tested product.
3. A method for performing dc impedance measurements using the circuit of claim 1, the method comprising the steps of:
(1) calculating the resistance of the tested product: setting a constant measurement voltage VREFThe constant measurement voltage VREFPressure drop V between two ends of the 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+IOPI is the current flowing through the product (Rx) to be measured, IOPIs a bias current of said operational amplifier (OP 1), a bias current IOPSmall enough to be ignored, 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 differential output voltage value V is obtained by differential sampling of said differential amplifier (OP 2)0At this time, VAB=VOWhereby the measured value of Rx is deduced to be I = IRS=V0/RS，RX=V1/I；
(2) Calculated at a constant measurement voltage VREFImpedances in a positive voltage, a negative voltage and a 0V voltage state, respectively, at which a constant measurement voltage V in the positive voltage and the negative voltage state is setREFAre equal in absolute value and are defined as being positively chargedThe impedances under the states of voltage, negative voltage and 0V are respectively XR_P、XR_N、XR_0Setting up
Constant measurement voltage VREFAt positive voltage, the expression is VREF_P1V，V0Is expressed by the expression VO_P1V，
Constant measurement voltage VREFWhen it is a negative voltage, the expression is VREF_N1V，V0Is expressed by the expression VO_N1V，
Constant measurement voltage VREFAt 0V, the expression is VREF_0V，V0Is expressed by the expression VO_0V，
Setting the power supply of the operational amplifier (OP 1) and the power supply of the differential amplifier (OP 2) to adopt complementary symmetrical positive and negative power supply rails according to a direct current impedance calculation formula
A forward current impedance of
，
Negative current impedance of
，
Zero potential current impedance of
；
(3) Averaging the positive current impedance value, the negative current impedance value and the zero potential current impedance value to obtain a direct current impedance XRIs composed of
And n is the number of times of reading the impedance value.
4. The method of claim 3The method is characterized in that: when the tested product is sensitive to reverse voltage or current, the DC impedance XRIs XR=XR_P。
5. The method of claim 3, wherein: when the tested product is sensitive to forward voltage or current, the DC impedance XRIs XR=XR_N。
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| CN118112328A (en) * | 2024-04-30 | 2024-05-31 | 博最科技(苏州)有限公司 | Active detection method for shunt mounting point impedance |
| CN119382474A (en) * | 2024-12-27 | 2025-01-28 | 芯洲科技(北京)股份有限公司 | Equivalent capacitance circuit and device thereof, equivalent capacitance calculation method, system and Buck circuit |
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