CN103383404B - Current measurement circuit - Google Patents

Abstract

The present invention relates to a current measurement circuit, which includes an integration circuit, an analog-to-digital conversion circuit, a processor unit and a switch control circuit, the integration circuit converts the input current signal to be measured into a voltage signal, and the analog-to-digital conversion circuit converts the input current signal into a voltage signal The voltage signal is converted into a digital signal, and the processor unit is used to control the current measurement circuit and calculate the value of the input current signal according to the data transmitted by the analog-to-digital conversion circuit; the integration circuit includes an operational amplifier and a feedback Network, the operational amplifier has an inverting input terminal, a non-inverting input terminal and an output terminal, one end of the feedback network is connected to the inverting input terminal, the other end is connected to the output terminal, and the non-inverting input terminal grounding; the feedback network includes a first feedback circuit and a second feedback circuit connected in parallel with the first feedback circuit, and the switch control circuit is controlled by the processor unit to switch the first feedback circuit and the second feedback circuit .

Description

current measurement circuit

technical field

The invention relates to a current measurement circuit, in particular to a measurement circuit which can be used for real-time measurement of a weak current with a large dynamic range.

Background technique

When detecting radioactive rays (such as gamma, beta rays, neutrons, etc.) The current is amplified and measured. When detecting weak signals, the dynamic range of the weak current output by the detector is sometimes very large. For example, when an ionization chamber is used to measure γ-rays, the current signal output by the ionization chamber can range from 10 ^-14 A to 10 ^-6 A, which belongs to the category of weak current. The weak current measurement circuit not only needs to be able to measure the weak current with such a large dynamic range, but also needs to have the characteristics of small size and real-time measurement, so that it can be integrated with the detector to form a weak signal measurement device. Ordinary ammeters cannot meet the above requirements, and it is generally necessary to design a special weak current measurement circuit to meet the above requirements.

At present, there are mainly two measurement methods for weak currents: transimpedance method and integral capacitance method. The transimpedance method usually converts the weak current signal into a voltage signal by using one or more parallel feedback resistors with different resistance values, and then converts the voltage signal into a digital signal through an analog-to-digital conversion (ADC) circuit. However, the transimpedance method has high requirements on the anti-interference ability of the feedback resistor, amplifier and the whole system, and the detection of weak current is limited by the resistance of the feedback resistor itself, so it is difficult to measure very weak currents of 10 ^-15 to 10 ^-13 A current. The integral capacitance method can be used to detect weaker current signals than the transimpedance method. The integral capacitance method converts the weak current signal into a voltage signal by integrating on the capacitance, and then calculates the current value by measuring the voltage change per unit time through the ADC circuit. The integral capacitance method can measure very weak current by increasing the measurement time, but the integral capacitance method is limited by the capacitance capacity, the measurement range of weak current is limited, and the response time is slow.

Contents of the invention

In view of this, it is indeed necessary to provide a current measurement circuit capable of measuring weak currents with a large dynamic range.

A current measurement circuit, comprising an integrating circuit, an analog-to-digital conversion circuit, a processor unit, and a switch control circuit, the integrating circuit converts an input current signal to be measured into a voltage signal, and the analog-to-digital conversion circuit converts the voltage signal converted into a digital signal, the processor unit is used to control the current measurement circuit and calculate the value of the input current signal according to the data transmitted by the analog-to-digital conversion circuit; the integration circuit includes an operational amplifier and a feedback network, so The operational amplifier has an inverting input end, a non-inverting input end and an output end, one end of the feedback network is connected to the inverting input end, the other end is connected to the output end, and the non-inverting input end is grounded; The feedback network includes a first feedback circuit and a second feedback circuit connected in parallel with the first feedback circuit, the first feedback circuit includes a feedback resistor and a first switch connected in series with the feedback resistor; the second feedback circuit includes An integrating capacitor, a second switch, and a third switch, the integrating capacitor is connected in series with the second switch, the circuit in which the integrating capacitor is connected in series with the second switch is connected in parallel with the third switch, and the switch controls The circuit is controlled by the processor unit to switch the first feedback circuit and the second feedback circuit.

Compared with the prior art, the embodiment of the present invention measures the current by combining the integrating capacitor and the feedback resistor in the integrating circuit. Since the integrating capacitor can integrate a relatively weak current, by measuring the integral The rate of change of the voltage can be used to calculate the value of the current signal to be measured. The larger weak current can be measured by using the feedback resistance, thereby realizing the real-time measurement of the weak current with a large dynamic range. The current measuring circuit of the present invention can measure the current with a large dynamic range of 1fA-1μA in real time.

Description of drawings

FIG. 1 is a functional block diagram of a current measurement circuit provided by an embodiment of the present invention.

FIG. 2 is a circuit diagram of a current measurement circuit provided by an embodiment of the present invention.

Explanation of main component symbols

Integrator circuit U1 Analog-to-digital conversion circuit U2 processor unit U3 switch control circuit U4 pre-integration circuit U5 temperature sensor circuit U6 digital interface circuit U7 Operational Amplifier A Feedback resistor _f transfer resistance R _p first switch S ₁ second switch S ₂ third switch S ₃ fourth switch S ₄ Integrating capacitance C _i Pre-integration capacitor C _p

The following specific embodiments will further illustrate the present invention in conjunction with the above-mentioned drawings.

Detailed ways

The current measurement circuit provided by the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

Please refer to FIG. 1 , an embodiment of the present invention provides a current measurement circuit, which can measure a current with a large dynamic range of 1fA-1μA in real time. The current measuring circuit includes an integrating circuit U1 , an analog-to-digital conversion (ADC) circuit U2 , a processor unit U3 and a switch control circuit U4 . The integrating circuit U1 converts the input current signal I _in into a voltage signal, and the ADC circuit U2 converts the voltage signal into a digital signal and sends it to the processor unit U3, and the processor unit U3 receives the The digital signal is calculated and the magnitude of the current signal I _in is obtained.

Please also refer to FIG. 2 , the integrating circuit U1 further includes an operational amplifier A and a feedback network.

The operational amplifier A has an inverting input terminal (-), a non-inverting input terminal (+) and an output terminal (Vo). The current signal I _in to be detected is input from the inverting input terminal, and the non-inverting input terminal is grounded. One end of the feedback network is connected to the inverting input end of the operational amplifier A, and the other end is connected to the output end of the operational amplifier A.

The operational amplifier A may be a FET-input operational amplifier with low leakage current, and the leakage current is less than 1 picoampere (pA). The type of operational amplifier A described in the implementation of the present invention is AD549.

The feedback network includes a first feedback circuit and a second feedback circuit connected in parallel with the first feedback circuit. The first feedback circuit includes a feedback resistor R _f and a first switch S ₁ connected in series with the feedback resistor R _f . The resistance value of the feedback resistor R _f can be 100KΩ to 100MΩ. In the embodiment of the present invention, the resistance value of the feedback resistor R _f is 10 MΩ. The first feedback circuit can be used to measure a weak current with a large current value. In an embodiment of the present invention, the first feedback circuit can measure a current greater than 1 nA. In addition, the feedback network may include a plurality of first feedback circuits connected in parallel, and the resistance values of the feedback resistors R _f in the plurality of first feedback circuits are different from each other, so that the measurement of current signals in different ranges can be realized.

The second feedback circuit includes an integrating capacitor C _i , a second switch S ₂ , and a third switch S ₃ . The integrating capacitor C _i is connected in series with the second switch S ₂ , and the integrating capacitor C _i is connected in parallel with the third switch S ₃ after being connected in series with the second switch S ₂ . The _second switch S2 is used to control opening and closing of the second feedback circuit. The _third switch S3 is used to control the discharge of the charge accumulated on the integrating capacitor C _i . Preferably, the integral capacitor C _i has a capacitance ranging from 1pF to 1000pF. In the embodiment of the present invention, the capacitance value of the integrating capacitor C _i is 50pF. The second feedback circuit can be used to measure weak current with relatively small current value. In an embodiment of the present invention, the second feedback circuit can measure a current less than 10 nA.

The ADC circuit U2 is used to convert the voltage signal output by the integrating circuit U1 into a digital signal. Specifically, the ADC circuit U2 can collect and convert the integral voltage accumulated on the integral capacitor C _i and the voltage on the feedback resistor R _f into digital signals. The ADC circuit can be realized by an ADC chip. The ADC circuit described in the embodiment of the present invention adopts a 24-bit Σ-Δ ADC chip.

The switch control circuit U4 can be used to switch the first feedback circuit and the second feedback circuit to realize real-time detection of current signals in different ranges. Specifically, the switch control circuit U4 is used to control the closing and opening of the first switch S ₁ , the second switch S ₂ and the third switch S ₃ . The first switch S ₁ , the second switch S ₂ and the third switch S ₃ are all high insulation switches. The insulation resistance of the first switch S ₁ , the second switch S ₂ and the third switch S ₃ is not lower than 10 ¹² Ω.

Further, the current measurement circuit includes a pre-integration circuit U5, which can integrate the input current signal I _in during the process of discharging the charge of the integration capacitor C _i , so as to avoid the current The dead time of the current detection circuit is measured to achieve the purpose of real-time detection of the input current.

The pre-integration circuit U5 includes a pre-integration capacitor C _p , a transmission resistor R _p and a fourth switch S ₄ . One end of the pre-integrating capacitor C _p is grounded, and the other end is connected to the transmission resistor R _p . One end of the _fourth switch S4 is connected to the transfer resistor _Rp , the other end is the output end of the pre-integration circuit U5, which is connected to the input end of the integration circuit U1, specifically, the inverting phase of the operational amplifier A input.

The capacitance range of the pre-integration capacitor C _p is 1pF to 10nF. The _fourth switch S4 is a switch with high insulation. The insulation resistance of the fourth switch S ₄ is not lower than 10 ¹² Ω. The _fourth switch S4 is controlled by the switch control circuit U4.

When the integrating capacitor C _i in the integrating circuit U1 discharges charges, the _fourth switch S4 is turned off, and the input current signal I _in is integrated on the pre-integrating capacitor C _p . After the integration circuit U1 completes the discharge of the charge, the _fourth switch S4 is closed, and the charge accumulated on the pre-integration capacitor _Cp is released through the transfer resistor _Rp , flows to the integration circuit U1, and then passes through the The ADC circuit U2 and the processor unit U3 collect and calculate the magnitude of the current signal, thereby solving the problem of dead time in the traditional integral capacitance method. It can be understood that the pre-integration circuit U5 is an optional component in the current measurement circuit when the dead time caused by the discharge of the charge of the integration circuit U1 is not considered. When the current measurement circuit does not include the pre-integration circuit U5, the current signal I _in to be detected is directly input into the integration circuit U1, and in the embodiment of the present invention, the current signal I _in to be detected is directly passed through The inverting input of the operational amplifier A is input.

The processor unit U3 is used to control the switch control circuit U4, read the data at the output end of the ADC circuit U2 and process and obtain the measurement result of the input current signal I _{in in} real time. The processor unit U3 may be a programmable logic device, such as a microcontroller, a microprocessor or an FPGA (Field Programmable Gate Array). The programmable logic device can automatically complete the control of the measurement and the real-time processing of the measurement result to obtain the final measurement result of the input current. In the embodiment of the present invention, the processor unit U3 is implemented by the microcontroller.

The current measurement circuit may further include a temperature sensor circuit U6 for collecting ambient temperature to correct the measurement result of the current signal I _in so as to improve the measurement accuracy of the current measurement circuit. The temperature sensor circuit U6 can be realized by a temperature sensor and a measurement circuit connected to the temperature sensor. The temperature sensor may be a thermistor. In addition, the temperature sensor circuit U6 can also be realized by using a temperature measurement integrated chip. The ambient temperature collected by the temperature sensor circuit U6 is transmitted to the processor unit U3 for correction.

The current measurement circuit may further include a digital interface circuit U7 for transmitting the measurement result of the current signal I _in to other electronic devices for recording, analysis or real-time display. Specifically, the digital interface circuit U7 can communicate with other electronic devices through Ethernet, USB communication or serial communication. The electronic equipment can be a computer, a liquid crystal display or a digital tube.

The working process of the current measurement circuit is as follows: First, the second feedback circuit starts to work, that is, the _second switch S2 and the _fourth switch S4 are closed, and the _first switch S1 and the third switch S3 are opened , the current signal I _in to be detected is integrated on the integrating capacitor C _i to generate an integral voltage, and the ADC circuit U2 converts the integral voltage into a digital signal and transmits it to the processor unit U3 for processing to obtain a current signal The measurement result of I _in . When it is detected that the value of the current signal I _in is too large (that is, the measured current signal value approaches or exceeds the upper limit value of the current signal that the second feedback circuit itself can measure), the second feedback circuit stops work, the first feedback circuit starts to work, that is, the _first switch S1 and the _fourth switch S4 are closed, the _second switch S2 and the _third switch S3 are open, and the input current signal I _in detects the magnitude of its current signal I _in through the voltage generated on the feedback resistor R _f . Similarly, when the current signal I _in obtained by the processor unit U3 is small, that is, when the measured current signal value is close to or smaller than the lower limit value of the current range that can be measured by the first feedback circuit, the processor The unit U3 switches the first feedback circuit to the second feedback circuit to detect the current signal. Therefore, the current signal value within a large dynamic range can be accurately detected in real time.

When the second feedback circuit is working, the input current signal I _in is integrated on the integrating capacitor C _i to generate an integrated voltage, and when the integrated voltage exceeds the threshold value of the integrating capacitor C _i , the second switch S ₂ and S ₃ are turned on at the same time, and the _fourth switch S4 is turned off, so that the integrating capacitor C _i performs charge discharge. At this time, the input current signal I _in is integrated on the pre-integrating capacitor C _p , and after the charge discharge of the integrating capacitor C _i is completed, the second feedback circuit works normally, that is, the _third switch S3 is turned off, The second switch S ₁ is closed, and the fourth switch S ₄ is closed at the same time, the charge accumulated on the pre-integration capacitor C _p is transferred to the integration capacitor C _i through the transfer resistor R _p for integration, and the generated integrated voltage is passed through the ADC The circuit U2 and the processor unit U3 obtain the magnitude of the current signal value input by the integrating capacitor C _i during the time period of discharging the charge, thereby avoiding the existence of the dead time of the current measurement circuit and realizing the measurement of the large dynamic range current Real-time detection.

In the embodiment of the present invention, the current is measured by combining the integrating capacitor and the feedback resistor in the integrating circuit. Since the integrating capacitor can integrate relatively weak current, by measuring the rate of change of the integrated voltage, that is The value of the current signal to be measured can be calculated. The larger weak current can be measured by using the feedback resistance, thereby realizing the measurement of the weak current with a large dynamic range. The current measuring circuit of the present invention can measure the current with a large dynamic range of 1fA-1μA in real time. In addition, since the feedback resistor is used in combination with the integral capacitor, conventional low temperature coefficient resistors can be selected as the feedback resistor, which can greatly reduce the difficulty of circuit design and feedback resistor selection. In addition, real-time switching between the first feedback circuit and the second feedback circuit, that is, switching of different measurement ranges, can be realized through the switch control circuit, so that real-time measurement of weak currents with a large dynamic range can be realized. Finally, the components and parts in the current measuring circuit of the present invention are commonly used components and parts, therefore, it is easy to be practically applied, and the current measuring circuit has a small volume and can be embedded in devices that need to measure weak currents.

In addition, those skilled in the art can also make other changes within the spirit of the present invention. Of course, these changes made according to the spirit of the present invention should be included within the scope of protection claimed by the present invention.

Claims (6)

1. a current measurement circuit, comprise integrating circuit, analog to digital conversion circuit, processor unit and ON-OFF control circuit, input current signal to be measured is converted into voltage signal by described integrating circuit, described voltage signal is converted to digital signal by analog-digital conversion circuit as described, and described processor unit is for controlling this current measurement circuit and calculating the value of current signal of input according to the data of analog-digital conversion circuit as described transmission; It is characterized in that,

Described integrating circuit comprises an operational amplifier and a feedback network, described operational amplifier has an inverting input, a normal phase input end and an output terminal, inverting input described in described feedback network one termination, output terminal described in another termination, described positive input end grounding; Described feedback network comprises the first feedback circuit and second feedback circuit in parallel with this first feedback circuit, the first switch that described first feedback circuit comprises a feedback resistance and connects with this feedback resistance; Described second feedback circuit comprises an integrating capacitor, second switch and one the 3rd switch, described integrating capacitor is connected with described second switch, described integrating capacitor connect with second switch after circuit and described 3rd switch in parallel, described ON-OFF control circuit controls to switch described first feedback circuit and the second feedback circuit by described processor unit;

Described current measurement circuit comprises a pre-integration circuit further, described pre-integration circuit is used for carrying out integration to the current signal of input in the process of described integrating capacitor electric charge bleed off, described pre-integration circuit comprises a pre-integration electric capacity, transmission resistance and one the 4th switch, described pre-integration electric capacity one end ground connection, the other end is electrically connected with one end of described transmission resistance, described transmission resistance is connected with the 4th switch, resistance is transmitted, the inverting input of operational amplifier described in another termination described in one termination of described 4th switch.

2. current measurement circuit as claimed in claim 1, it is characterized in that, the resistance of described feedback resistance is 100K Ω to 100M Ω.

3. current measurement circuit as claimed in claim 1, it is characterized in that, the capacitance scope of described integrating capacitor is 1pF to 1000pF.

4. current measurement circuit as claimed in claim 1, is characterized in that, comprise one further for the temperature sensor circuit of testing environment temperature, the described environment temperature detected is input to described processor unit and corrects the current signal value detected.

5. current measurement circuit as claimed in claim 1, is characterized in that, comprise a digital interface circuit further, and this digital interface circuit is used for the current signal value detected to be transferred to an electronic equipment.

6. current measurement circuit as claimed in claim 1, it is characterized in that, the capacitance scope of described pre-integration electric capacity is 1pF to 10nF.