Ultra-low frequency alternating current weak signal current measuring instrument and measuring method

Abstract

The invention relates to an ultra-low-frequency alternating current weak signal current measuring instrument and a measuring method, which are used for measuring parameters of ultra-low-frequency weak alternating current, and comprise a conversion device used for converting the ultra-low-frequency weak alternating current to be measured into low-frequency voltage; the reference voltage chip is used for providing stable reference voltage for the analog-to-digital conversion chip; the analog-to-digital conversion chip is used for converting the low-frequency voltage into a digital signal by comparing the low-frequency voltage with a reference voltage; and the control module is used for resolving the voltage amplitude and frequency of the ultralow-frequency weak alternating current according to the digital signal and the information of the conversion device. Current-voltage conversion is realized by adopting a virtual ground operational amplifier and a trans-impedance mode, the converted signal is an alternating-current voltage signal which has the same frequency as a to-be-measured current signal and is in a certain proportion, an error introduced by an alternating-current impedance component of a feedback resistor can be negligible, the linearity is good, the frequency consistency is good, and the accuracy is high. And the method is more suitable for measuring ultralow-frequency weak alternating current to be measured.

Description

Technical Field

[0001] The invention relates to the technical field of weak current signal measurement, and in particular to an ultra-low frequency AC weak signal current measuring instrument and a measuring method. Background Art

[0002] There are a large number of weak current signals in production practice and scientific research, such as ultra-low frequency AC weak current, which is widely used in deep geological exploration, deep space exploration, earthquake prediction research and medical fields. Ultra-low frequency AC weak current refers to a small AC current with a frequency lower than 10Hz and an amplitude lower than 1mA.

[0003] At present, the measurement of weak AC current is mainly achieved through capacitors, frequency recorders and AC voltmeters. However, when this method is applied to low-frequency AC weak current testing, the lowest measurable frequency of the existing AC voltmeter is 1Hz, and there is a problem with measurement bandwidth, especially for current measurement with a frequency lower than 10Hz.

[0004] The current AC current-to-voltage converters are usually resistive feedback current-to-voltage converters. If an AC-to-DC shunt is used as an AC current-to-voltage converter, the problem of AC resistance tracing cannot be solved. Moreover, because the AC current has a low amplitude, it needs to be used in conjunction with an impedance converter. In addition, since there is currently no traceability method for AC resistance and impedance converters, the use of AC resistance to measure small AC currents is technically difficult and will involve a series of traceability verification issues.

[0005] The electromagnetic weak current sensor type AC and DC current measurement method has a wide measurement frequency and can achieve AC current measurement in the range of DC to 100kHz. However, its minimum measurable AC current is 1mA, which cannot meet the needs of measuring small AC currents below 1mA.

[0006] For example, the Chinese invention patent with publication number CN117929855A discloses a zero-load effect AC micro-current measurement device and method, which mainly solves the problem of high-frequency loss compensation during weak signal conversion, but has high requirements on the output impedance of the current source to be measured, and is not suitable for the characteristic parameter measurement of semiconductor devices, such as the emitter current of transistors, the piezoelectric current generated by the piezoelectric effect of special crystals, etc. There are many restrictions and requirements on the signal to be measured, which is suitable for the measurement of nanoampere-level high-frequency AC current, but cannot solve the problem of low-frequency AC weak current measurement. The final test result needs to be calculated through intermediate quantities, and the test result is not intuitive; there are many factors affecting the error in the connection process, and other influencing quantities in the test process cannot be quantified, so the test result is not accurate. Summary of the invention

[0007] In order to solve the technical problems existing in the above-mentioned prior art, the purpose of the present invention is to provide an ultra-low frequency AC weak signal current measuring instrument and measuring method, which adopts virtual ground current-voltage conversion, reduces the error introduced by the AC impedance component of the feedback resistor, reduces the load capacity requirement for the ultra-low frequency weak AC current to be measured, and makes the measurement of the ultra-low frequency weak AC current more accurate.

[0008] To achieve the above-mentioned purpose, the present invention provides an ultra-low frequency AC weak signal current measuring instrument, which is used to measure the parameters of ultra-low frequency weak AC current, including:

[0009] A conversion device, used to convert the ultra-low frequency weak alternating current to be measured into a low frequency voltage;

[0010] A reference voltage chip is used to provide a stable reference voltage to the analog-to-digital conversion chip;

[0011] The analog-to-digital conversion chip is used to convert the low-frequency voltage into a digital signal by comparing the low-frequency voltage with the reference voltage;

[0012] A control module is used to calculate the voltage amplitude and frequency of the ultra-low frequency weak alternating current according to the digital signal and information of the conversion device.

[0013] According to a technical solution of the present invention, the conversion device includes an operational amplifier, a relay, and a feedback resistor. The non-inverting input terminal of the operational amplifier is connected to a reference ground, and a combination of multiple groups of the relays and the feedback resistors connected in series are connected in parallel between the inverting input terminal and the output terminal of the operational amplifier.

[0014] According to a technical solution of the present invention, the ultra-low frequency weak alternating current to be measured is input into the inverting input terminal of the operational amplifier, and the output terminal of the operational amplifier is connected to the analog-to-digital conversion chip.

[0015] According to a technical solution of the present invention, a combination of multiple groups of the relays and the feedback resistors connected in series is divided into multiple ranges according to the characteristics of the feedback resistors, and the selection of the range is controlled by the relays.

[0016] According to a technical solution of the present invention, the feedback resistor is used to adjust the output voltage amplitude after passing through the feedback resistor to within the range of 0.1V-2V, and the feedback resistor is a metal foil resistor with no inductance and no capacitance design.

[0017] According to a technical solution of the present invention, the bias current of the operational amplifier is less than 0.1% of the minimum amplitude of the ultra-low frequency weak alternating current to be measured, and the sampling rate of the analog-to-digital conversion chip is greater than 1k sps and the bit number is not less than 16 bits.

[0018] According to a technical solution of the present invention, the reference voltage chip is selected according to the reference voltage value, temperature coefficient, and long-term stability of the analog-to-digital conversion chip.

[0019] According to a technical solution of the present invention, the control module calculates the voltage amplitude and frequency of the ultra-low frequency weak alternating current to be measured by a sampling four-parameter measurement method.

[0020] According to one aspect of the present invention, a measurement method based on the above-mentioned ultra-low frequency AC weak signal current measuring instrument is provided, comprising the following steps:

[0021] Step S1, selecting a suitable measuring range;

[0022] Step S2, converting the ultra-low frequency weak alternating current to be measured into a low frequency voltage through the conversion device;

[0023] Step S3, comparing the low-frequency voltage with the reference voltage provided by the reference voltage chip through the analog-to-digital conversion chip, and converting the low-frequency voltage into the digital signal;

[0024] Step S4: calculating the voltage amplitude and frequency of the ultra-low frequency weak alternating current to be measured according to the digital signal and the information of the conversion device through the control module.

[0025] According to a technical solution of the present invention, the current value of the ultra-low frequency weak alternating current to be measured is calculated as follows: in =V out / R f

[0026] Among them, V out is the voltage amplitude of the ultra-low frequency weak alternating current to be measured, I in is the current value of the ultra-low frequency weak alternating current to be measured, R f is the resistance value of the corresponding feedback resistor.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1. The ultra-low frequency weak AC current-to-voltage conversion device adopts virtual ground current-to-voltage conversion. The conversion feedback resistor is located between the inverting input and output terminals of the operational amplifier, so that the ultra-low frequency AC weak signal current measuring instrument is close to the zero input impedance of the ideal ammeter. The power consumption during measurement is small. Because the measured signal frequency is low, the influence of the capacitance at the input terminal of the operational amplifier on the test result can be ignored. The influence on the ultra-low frequency weak AC current to be measured is small, and the load capacity requirement for the ultra-low frequency weak AC current to be measured is low. It is more suitable for the measurement of ultra-low frequency weak AC current to be measured.

[0029] 2. The ultra-low frequency weak AC current-to-voltage conversion device adopts a transimpedance method, and the feedback resistor uses a metal foil resistor with a non-inductive and non-capacitive design, which has good linearity and frequency consistency. The converted voltage signal is an AC voltage signal with a certain proportion and the same frequency as the current signal to be measured. The error introduced by the AC impedance component of the feedback resistor can be ignored, which makes it more convenient to calculate the amplitude and frequency of the current signal to be measured based on the digital signal of the analog-to-digital conversion chip.

[0030] 3. The ultra-low frequency voltage measurement link adopts three hardware circuits: reference voltage chip, analog-to-digital conversion chip and control module. The four-parameter method is built into the control module to realize AC voltage parameter calculation. It does not require additional frequency measurement circuit, nor does it require the known frequency of the signal to be measured to achieve ultra-low frequency voltage measurement. It is simple, convenient and highly accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0032] Figure 1 The schematic diagram schematically shows the principle of the ultra-low frequency AC weak signal current measuring instrument of the present invention;

[0033] Figure 2 The schematic diagram schematically shows the circuit principle of the ultra-low frequency weak alternating current voltage conversion device of the present invention. DETAILED DESCRIPTION

[0034] The description of the embodiments of this specification should be combined with the corresponding drawings, which should be considered as part of the complete specification. In the drawings, the shape or thickness of the embodiments may be enlarged and indicated for simplification or convenience. Furthermore, the parts of each structure in the drawings will be described separately. It is worth noting that the elements not shown in the drawings or not described in words are in a form known to ordinary technicians in the relevant technical field.

[0035] The description of the embodiments herein and any reference to directions and orientations are only for the convenience of description and are not to be construed as any limitation on the scope of protection of the present invention. The following description of the preferred embodiments may involve combinations of features, which may exist independently or in combination, and the present invention is not particularly limited to the preferred embodiments. The scope of the present invention is defined by the claims.

[0036] like Figure 1 and Figure 2As shown, the present invention proposes an ultra-low frequency AC weak signal current measuring instrument, the ultra-low frequency AC weak signal current measuring instrument is as follows Figure 1 As shown, it includes: a conversion device, an analog-to-digital conversion chip, a reference voltage chip, a control module, a communication interface and a display.

[0037] The ultra-low frequency weak AC current I to be measured in Input from the conversion device, converted into a low-frequency voltage V out The output is to the analog-to-digital conversion chip, the reference voltage chip output is connected to the reference voltage terminal of the analog-to-digital conversion chip, the other control signals and digital signal output terminals of the analog-to-digital conversion chip are connected to the control module, the control signal input and output terminals of the conversion device are connected to the control module, and the external device can also be connected to the device of the present invention through the communication interface to obtain the specific information of the ultra-low frequency weak AC current to be measured.

[0038] The ultra-low frequency AC weak signal current measuring instrument is based on the resistance negative feedback current conversion technology and the ultra-low frequency voltage fast measurement technology, which can achieve precise current measurement in the range of 1mHz-10Hz, 2nA-1mA, with an accuracy better than 0.5%.

[0039] The conversion device is used to convert the ultra-low frequency weak AC current to be measured into a low frequency voltage, such as Figure 2 As shown, it includes an operational amplifier U1, a relay, and a feedback resistor.

[0040] The ultra-low frequency weak alternating current to be measured is input to the inverting input terminal of the operational amplifier U1, and the output terminal of the operational amplifier U1 is connected to the analog signal input terminal of the analog-to-digital converter.

[0041] The non-inverting input terminal of the operational amplifier U1 is connected to the reference ground, and multiple groups of relays k and feedback resistors R f The combination is connected in parallel between the inverting input and output of the operational amplifier U1.

[0042] Multiple sets of relays k and feedback resistors R connected in series f The combination of the ultra-low frequency weak AC current amplitude range to be measured is different, and the resistance value of the feedback resistor is different. The measurement range is divided into multiple ranges. The resistance value connected to the feedback loop is controlled by the relay. The feedback resistor value is selected, that is, the relay connected in series with it is controlled to be closed, and the other relays are disconnected, so as to obtain the best measurement performance.

[0043] The present invention adopts virtual ground current-voltage conversion, the non-inverting input terminal of the operational amplifier U1 is connected to the reference ground, the conversion resistor is located between the inverting input terminal and the output terminal of the operational amplifier U1, the potentials of the inverting input terminal and the non-inverting input terminal of the operational amplifier U1 are not only equal, but also equal to 0, close to the zero input impedance of the ideal ammeter, the power consumption during measurement is small, the influence on the current signal to be measured is small, the load capacity requirement of the current signal to be measured is low, and it is suitable for the measurement of weak current signals.

[0044] When selecting the operational amplifier U1, its bias current should be less than 0.1% of the minimum amplitude of the current to be measured. The operational amplifier U1 selects an electrostatic amplifier with high input impedance and small bias current.

[0045] The method for selecting the resistance value of the feedback resistor is: on the premise of ensuring that the output voltage amplitude after passing through the feedback resistor is within the range of 0.1V-2V, use a metal foil resistor with a non-inductive and non-capacitive design.

[0046] The conversion device adopts a resistor feedback current-to-voltage conversion method, and converts the input current into a voltage through a feedback loop of an operational amplifier U1 with a high gain. The voltage is proportional to the current to be measured. The voltage burden at the input end of the ammeter is small, the current conversion accuracy is high, the frequency response consistency is good within the range of 1mHz-10Hz to be measured, the circuit has good stability and linearity, the differential input voltage of the operational amplifier U1 is small, and the voltage-to-current conversion accuracy is high;

[0047] A reference voltage chip is used to provide a stable reference voltage to the analog-to-digital conversion chip;

[0048] The voltage value of the reference voltage chip should be consistent with the reference voltage value of the analog-to-digital converter, and should have a small temperature coefficient and good long-term stability.

[0049] The reference voltage chip can provide a stable output voltage when the process, power supply voltage and temperature change.

[0050] The analog-to-digital conversion chip is used to convert the low-frequency voltage into a digital signal by comparing it with a reference voltage; the analog-to-digital converter should have good linearity, a sampling rate greater than 1k sps, and 16 bits or above.

[0051] The control module is used to calculate the voltage amplitude and frequency of the ultra-low frequency weak alternating current to be measured according to the digital signal and the information of the ultra-low frequency weak alternating current voltage conversion device.

[0052] The control module calculates the voltage amplitude and frequency of the ultra-low frequency weak AC current to be measured through a sampling four-parameter measurement method. Compared with the traditional method, it can measure the ultra-low frequency AC weak current without knowing the frequency in advance.

[0053] According to Ohm's law, the current value to be measured is calculated from the measured low-frequency voltage amplitude and the value of the conversion resistor in the current-voltage conversion device.

[0054] During the use of the ultra-low frequency AC weak signal current measuring instrument, the ultra-low frequency weak AC current to be measured is connected to the conversion device and converted into a low-frequency voltage of the same frequency and a certain proportion, and then input into the input end of the analog-to-digital conversion chip. By comparing with the reference voltage, the low-frequency voltage signal is converted into a digital signal and output to the control module. The control module calculates the amplitude and frequency of the ultra-low frequency current according to the digital signal sequence and the information of the current-voltage conversion module, and transmits the current amplitude and frequency of the signal to be measured to the display for display, and can also be transmitted to other devices through the communication interface.

[0055] The ultra-low frequency AC weak signal current measuring instrument of the present invention has low requirements on the output impedance of the current source to be measured. It is not only adaptable to current source devices with various output impedances, but also suitable for measuring characteristic parameters of semiconductor devices, such as the emitter current of a transistor, the piezoelectric current generated by the piezoelectric effect of a special crystal, etc.

[0056] The ultra-low frequency AC weak signal current measuring instrument of the present invention can measure the frequency value and current amplitude of the signal to be measured by simply connecting the signal to be measured to the input terminal, without the need to pre-determine the frequency of the signal to be measured, and without the need for the tester to convert according to the intermediate test value. The ultra-low frequency AC weak signal current measuring instrument is a desktop device with a fixed connection, an integrated design, and a strong anti-interference ability.

[0057] In another embodiment of the present invention, a measurement method based on the ultra-low frequency AC weak signal current measuring instrument is proposed, comprising the following steps:

[0058] Step S1, selecting a suitable measuring range;

[0059] Step S2, converting the ultra-low frequency weak alternating current to be measured into a low-frequency voltage through an ultra-low frequency weak alternating current voltage conversion device;

[0060] Step S3, using an analog-to-digital conversion chip to compare the low-frequency voltage with a reference voltage provided by a reference voltage chip, and converting the low-frequency voltage into a digital signal;

[0061] Step S4: The voltage amplitude and frequency of the ultra-low frequency weak alternating current to be measured are calculated by the control module according to the digital signal and the information of the ultra-low frequency weak alternating current voltage conversion device.

[0062] The calculation formula for the current value of the ultra-low frequency weak AC current to be measured is as follows: I in =V out / R f

[0063] Among them, Vout is the voltage amplitude of the ultra-low frequency weak alternating current to be measured, Iin is the current value of the ultra-low frequency weak alternating current to be measured, R f is the resistance value of the corresponding feedback resistor.

[0064] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. The above embodiments and descriptions are only preferred examples of the present invention and are not intended to limit the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. An ultra-low frequency AC weak signal current measuring instrument, used to measure the parameters of ultra-low frequency weak AC current, characterized in that: include: A conversion device, used to convert the ultra-low frequency weak alternating current to be measured into a low frequency voltage; A reference voltage chip is used to provide a stable reference voltage to the analog-to-digital conversion chip; The analog-to-digital conversion chip is used to convert the low-frequency voltage into a digital signal by comparing the low-frequency voltage with the reference voltage; A control module is used to calculate the voltage amplitude and frequency of the ultra-low frequency weak alternating current according to the digital signal and information of the conversion device.

2. The ultra-low frequency AC weak signal current measuring instrument according to claim 1, characterized in that: The conversion device includes an operational amplifier, a relay, and a feedback resistor. The non-inverting input terminal of the operational amplifier is connected to a reference ground, and a combination of multiple groups of the relays and the feedback resistors connected in series are connected in parallel between the inverting input terminal and the output terminal of the operational amplifier.

3. The ultra-low frequency AC weak signal current measuring instrument according to claim 2, characterized in that: The ultra-low frequency weak alternating current to be measured is input into the inverting input terminal of the operational amplifier, and the output terminal of the operational amplifier is connected to the analog-to-digital conversion chip.

4. The ultra-low frequency AC weak signal current measuring instrument according to claim 2, characterized in that: The combination of multiple groups of the relays and the feedback resistors connected in series is divided into multiple ranges according to the characteristics of the feedback resistors, and the selection of the range is controlled by the relays.

5. The ultra-low frequency AC weak signal current measuring instrument according to claim 2, characterized in that: The feedback resistor is used to adjust the output voltage amplitude after passing through the feedback resistor to be within the range of 0.1V-2V. The feedback resistor is a metal foil resistor with no inductance and no capacitance design.

6. The ultra-low frequency AC weak signal current measuring instrument according to claim 2, characterized in that: The bias current of the operational amplifier is less than 0.1% of the minimum amplitude of the ultra-low frequency weak alternating current to be measured, and the sampling rate of the analog-to-digital conversion chip is greater than 1k sps, 16 bit.

7. The ultra-low frequency AC weak signal current measuring instrument according to claim 1, characterized in that: The reference voltage chip is selected according to the reference voltage value, temperature coefficient, and long-term stability of the analog-to-digital conversion chip.

8. The ultra-low frequency AC weak signal current measuring instrument according to claim 1, characterized in that: The control module calculates the voltage amplitude and frequency of the ultra-low frequency weak alternating current to be measured by a sampling four-parameter measurement method.

9. A measurement method based on the ultra-low frequency AC weak signal current measuring instrument according to any one of claims 1 to 8, characterized in that: The following steps are involved: Step S1, selecting a suitable measuring range; Step S2, converting the ultra-low frequency weak alternating current to be measured into a low frequency voltage through the conversion device; Step S3, comparing the low-frequency voltage with the reference voltage provided by the reference voltage chip through the analog-to-digital conversion chip, and converting the low-frequency voltage into the digital signal; Step S4: calculating the voltage amplitude and frequency of the ultra-low frequency weak alternating current to be measured according to the digital signal and the information of the conversion device through the control module.

10. The measuring method according to claim 9, characterized in that: The calculation formula of the current value of the ultra-low frequency weak alternating current to be measured is as follows: in =V out / R f Among them, V out is the voltage amplitude of the ultra-low frequency weak alternating current to be measured, I in is the current value of the ultra-low frequency weak alternating current to be measured, R f is the resistance value of the corresponding feedback resistor.

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