Measurement scheme of micro current

Introduction: With the development of science and technology, experimental measurement under extreme conditions has become an important means to further understand nature. These experiments often measure some very weak physical quantities, such as weak magnetic field, weak sound, weak light, weak vibration, etc. , Because these weak signals are generally converted by the sensor to convert the weak signals to be measured into electrical signals.

1 Introduction

In actual measurement, noise and interference cannot be avoided, which affects the sensitivity and accuracy of the measurement. For the purpose of researching and measuring pA-level current, a micro-current measuring instrument with an accuracy of 0.5 was developed and designed with a measurement range of 10 pA. For pA-level current measurement, the measurement circuit cannot directly capture the current signal, and I/U conversion is required. The converted voltage signal needs to be further amplified, otherwise it will be interfered by DC signals such as the offset voltage and bias current of the operational amplifier. The problem is that while amplifying and capturing the signal to be measured, impurity signals such as power frequency interference, noise, and circuit offset are also amplified at the same time, so it is necessary to design related follow-up circuits to filter and remove. For power frequency interference, just use shielding and filtering. The elimination of these DC impurity signals, such as circuit offset, is what this article will explain, that is, filtering out these impurity DC signals by using a modulation circuit and a differential circuit.

2 Overview of micro current measurement methods

2.1 Measurement method

Weak signal detection is to filter out the interference signal from the signal source, enhance/limit to restore the useful signal to be measured, improve the signal-to-noise ratio (SNR), and effectively suppress noise is the difficulty and key point of micro-current measurement. The proposal of a new micro-current detection method and the development of a micro-current measuring instrument are currently a hot spot in this field. As far as detection methods are concerned, currently there are: sampling integration method, correlation detection method, noise analysis method, modulation and demodulation method, wavelet transform method, high impedance input method, photoelectric coupling method, integrated operational amplifier, computer program control, etc., but Sampling resistance method and operational amplifier feedback current method are commonly used methods for micro current measurement.

Noise interference is an effective suppressive interference signal. According to the type and characteristics of noise, there are mainly two major ones: 1) The inherent noise from the electronic system, including the bias current of the operational amplifier, the offset voltage, and the heat generated by the heating of electronic components. Noise, impulse noise generated by digital circuit interference, spike noise generated by switching circuit, etc.; 2) From outside the electronic system, such as power frequency interference, radio frequency noise, atmospheric noise, mechanical noise, etc. In the measurement, the processing of noise is extremely important. This article proposes that the key to micro-current measurement is to suppress the impurity DC signal and power frequency interference of the circuit.

2.2 Development status of micro current measurement technology

U.S. Keithley Company has developed the 6482 dual-channel picoammeter/voltage source based on its technical advantages in sensitive current measuring instruments, with a measurement resolution of up to 1 fA, 6 and a half digits, and a measurement range of 2 nA to 20 mA.

3 Design theory

3.1 Principle of micro-current-voltage conversion

According to Thevenin's theorem, any two-terminal network can be regarded as an equivalent voltage source Us in series with the equivalent resistance Rs, that is, Rs=Us/Is. The measurement principle of the op amp feedback current method is shown in Figure 1.

Figure 1 Principle of op amp feedback current measurement method
Figure 1 Principle of op amp feedback current measurement method

In the figure: Rf is the feedback resistance; R'is the balance resistance; UI0 is the op amp offset voltage; Ib-, Ib+ are the op amp bias current; Is is the micro current to be measured; Uo is the output voltage.

The ideal circuit output is Uo=-IsRf. Because the op amp has offset voltage and bias current, the actual circuit output is:

U＇o=-IsRf+UI0+Ib+R＇+Ib-Rf (1)

The voltage output error is:

△Uo=UI0+Ib+R'+Ib-Rf (2)

3.2 Principles of Differential and Modulation Circuits

A measurement method that uses differential and modulation circuits to filter out the DC impurity signals in the circuit is proposed to completely eliminate the interference generated by the circuit of the measuring instrument itself in the process of micro-current measurement. Differential and modulation means that the modulation switch is controlled by the central processing unit to modulate the micro-current. By using a modulation circuit and a differential circuit to filter out these impurity DC signals, a micro-voltage signal proportional to the signal to be measured is obtained. The principle of the differential and modulation circuit is shown in Figure 2.

Figure 2 Weak current differential, modulation preamplifier model
Figure 2 Weak current differential, modulation preamplifier model

When K1 is open and K2 is closed, the output is:

U01= IsRf+UI0+Ib+R'+Ib-Rf (3)

When K1 is closed, K2 is disconnected, that is, output:

U02= UI0+Ib+R'+Ib-Rf (4)

Eq. (3) minus Eq. (4), the system error can be eliminated, namely:

Uo=U01- U02= IsRf(5)

According to formula (5), the DC impurity signal is eliminated, and it can be seen that Uo is proportional to Is. However, the Uo signal is extremely weak, and Uo needs to be amplified layer by layer and then differentiated. Suppose the total magnification is K, then the output is: Uo=KIsRf; the measured micro-current is:

Is=Uo/(KRf) (6)

The measurement results are sent to the central processing unit of the instrument and displayed through the display circuit.

4 System design

4.1 Measuring circuit composition

This measurement circuit is composed of 3 parts.

1) In the pre-amplification stage, the signal is modulated and amplified, and the micro-current signal is converted into a micro-voltage signal at the same time;

2) The signal amplification stage is composed of a low-pass filter circuit, a zero-adjusting circuit, a switch selection circuit, and a state discrimination circuit respectively;

3) The micro current output is composed of sample and hold, differential circuit, etc., the amplified voltage signal is sampled and held by the modulation switch, and the system error is removed through the differential circuit, and the voltage signal proportional to the measured micro current is output. The structure of the measurement circuit is shown in Figure 3.

Figure 3 Measurement circuit system composition
Figure 3 Measurement circuit system composition

4.2 Principle of the first stage amplifying circuit

The amplification process is divided into 8 small stages (V1～V8) to complete, the block diagram is gradually enlarged from top to bottom as shown in Figure 4. When the micro-voltage signal output by the preamplifier circuit is amplified in the first stage, the central processor controls the number of amplification stages. The determination of the number of stages is firstly closed by the multiple switches in turn, and the judgment is made by the state discrimination circuit. When the output signal exceeds the linear range of the operational amplifier for the first time, the number of stages is reversed by one stage and sent to the central processing unit. In order to avoid the power frequency interference signal being amplified several times, each stage of the amplifying circuit is equipped with a low-pass filter. The zero-adjusting circuit is set at the final stage of the amplifying circuit to prevent the measurement circuit's own offset signal from being amplified several times and possibly exceeding its linear range of work.

Figure 4 Principle of the first stage amplifying circuit
Figure 4 Principle of the first stage amplifying circuit

4.3 Principle of the second stage amplifying circuit

There are 4 levels of amplification, each level of amplification should not be too large, and the saturation voltage of the operational amplifier and the output signal shall prevail, as shown in Figure 5.

Figure 5 Principle of the second-stage amplifying circuit
Figure 5 Principle of the second-stage amplifying circuit

According to the different time states of the modulation switch, the output result of the signal amplification stage is stored in two registers, and the differential circuit is used to eliminate the impurity DC signal accompanying the preamplifier circuit and the main amplifying circuit.

4.4 Principle of State Discrimination Circuit

The preamplifier circuit with a power supply of 3 V is used, and the signal after the J/U conversion is output to the No. 1 state discriminating circuit, and the discriminating circuit makes the judgment and sends the result to the central processing unit; the main amplifying circuit adopts the power supply as 15 V operational amplifier, the circuit outputs to the No. 2 state discrimination circuit, and sends the result to the central processing unit as shown in Figure 6.

Figure 6 Principle of state discrimination circuit
Figure 6 Principle of state discrimination circuit

5 Installation precautions

In addition to the circuit structure design, certain measures must be taken in the selection of components, circuit installation and technology. In order to achieve pA-level micro-current measurement, the following points must be noted:

1) In order to avoid interference as much as possible, the input terminal should be completely surrounded by a shielding ring, and the shielding layer should be connected to the shell, substrate and signal ground], and the protection ring should be set on both sides of the printed board.

2) Each loop of the circuit should use the ground as the current return channel. In view of the fact that the impedance on the ground wire is not zero and forms a potential difference, the capacitive coupling between the ground wire and the signal wire will further increase the noise interference, so try to set Fewer ground points or reduce the distance between ground points.

3) When PCB wiring, pay attention to the placement of various components. Each chip must be equipped with decoupling capacitors. High-power components must be close to the power supply. Minimize the length of the wire as much as possible. Apply a large area to the output part of the power supply and amplifier. copper. When wiring the circuit board, take the ground wire and power wire first.

6 Test simulation

6.1 Power frequency interference test

Power frequency noise can enter through space radiation and conduction. By adding a metal shielding layer to the measuring instrument, when the tester's hand touches the instrument casing, the output waveform of the test circuit is shown in Figure 7; the metal shielding layer is removed and the tester's hand approaches the instrument In the case of the shell, the output waveform of the test circuit is shown in Figure 8. From the comparison of the two figures, it can be seen that the 50 Hz noise is effectively suppressed.

Figure 7 Circuit output waveform when shielded
Figure 7 Circuit output waveform when shielded

Figure 8 Circuit output waveform without shielding
Figure 8 Circuit output waveform without shielding

6.2 Verify the effectiveness of modulation sampling circuit and differential circuit

In order to filter out DC impurity signals such as circuit offset, a modulation circuit and a differential circuit are used. In order to verify the effectiveness of the circuit, the oscilloscope was used to measure the sample-and-hold input waveform and the differential circuit output waveform, as shown in Figure 9. Obviously, the direct current impurities are effectively filtered.

Figure 9 Output waveform after differential circuit
Figure 9 Output waveform after differential circuit

6.3 Test data

Test data, as shown in Table 1 with 5 measurement results of different values.

 


For 100 pA, measure the average value:

=100.156 pA, measurement error is 0.16%, measurement repeatability s=0.24 pA;

For 10 pA, measure the average value:

=9.993 pA, the measurement error is -0.07%, and the measurement repeatability s=0.04 pA.

The measurement accuracy and repeatability reach the expected purpose and meet the 0.5 level requirements.

7 Conclusion

With the further development of electronic measurement technology, pA-level current measurement has an extremely important position in many fields, and micro-current measurement is extremely susceptible to environmental conditions and the noise of the measuring instrument itself. The measuring instrument designed according to the proposed measuring method has undergone tests of high, low temperature, electromagnetic interference, etc., for a current of 10 pA, the accuracy of the instrument can reach 0.5 level, which has high accuracy and good measurement repeatability and stability. . The test data shows that removing the influence of power frequency interference and DC error is the main factor to reduce the measurement error of micro current.