1 introduction
With the development of science and technology, the test measurement under extreme conditions has become an important means of further understanding nature. These tests often measure have some very weak physical quantities, such as weak magnetic, weak sound, weak light, weak vibration, etc. These weak signals are generally converted by the sensor to convert the weak signal to be tested into an electrical signal. When actual measurement, noise and interference cannot be avoided, affecting the sensitivity and accuracy of the measurement. For the purpose of studying the measurement of the PA-level current, the micro-current measuring instrument with a grade 0.5 level was developed, and the minimum range of measurements was 10 Pa. For PA-level current measurements, the measurement circuit cannot directly capture the current signal, and I / U conversion is required. Further enlargement is required for the conversion voltage signal, otherwise it will be interfered with the offset voltage of the op amp. The problem is that the impurity signals such as power frequency interference, noise, and circuit offset are simultaneously amplified while zooming in capturing signals, so it is necessary to design related subsequent circuits to filter, remove. For power frequency interference, filter is filtered by taking shield. For the elimination of these DC impurities signals such as circuit disorders, it is the core of this article, that is, by using a modulation circuit, the differential circuit is filtered away from these impurities DC signals.
2 Overview of Microote Measurement Methods
2.1 Measurement Method
Weak signal detection is to filter out the interference signal from the signal source, enhance / maximize restore useful to be tested, improve the signal-to-noise ratio (SNR), and effectively suppress noise is difficult to measure the micro current measurement. The proposal of the new microtaral detection method and the development of micro-current measuring instruments are a hot spot in this field. In terms of testing methods, mainly: sampling integral method, related detection method, noise analysis method, modulation method, wavelet transform method, high impedance input method, photocoupless method, integrated op amp, computer program control, etc. Sampling resistance method and op amp feedback current method are commonly used in micro current measurements.
Noise interference is an effective pressed interference signal, according to the type and characteristics of noise, there are 2 larger: 1) from the inherent noise inside the electronic system, including the bias current, offset voltage, and electronic components fever generation. Pulse noise generated by noise, digital circuit interference, spike noise generated by the switching circuit, etc.; 2) from the outside of the electronic system, such as power frequency interference, radio frequency noise, atmospheric noise, mechanical noise or the like. In the measurement, the processing of noise is extremely important. This paper proposes that the key to micro-current measurement is to suppress circuit impurity DC signal and power frequency interference.
2.2 Development Status of Microote Measurement Technology
US Ji Shili Company has developed a 6482 dual-channel Pi'an table / voltage source with a 6482 dual-channel Pi'an table / voltage source, and the measurement range 2 Na ~ 20 mA was measured.
3 design theory
3.1 micro-current voltage conversion principle
It is known from Davidan to the network, and any two end networks can be seen as an equivalent voltage source US and the equivalent resistance RS series, ie 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
In the figure: RF is the feedback resistor; R 'is a balance resistor; the UI0 is the transportable disorder voltage; IB-, IB + is the op amp current; IS is the micro current to be tested; UO is the output voltage.
The ideal circuit output is UO = - ISRF. Since there is an offset voltage, the bias current is biased, 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 differential, modulation circuit principle
The measurement method of the DC impurity signal in the circuit of DC impurity signal is proposed, and the interference generation of the measuring instrument itself during the micro current measurement is completely eliminated. Differential, modulation means that the modulation switch is controlled by the central processor, modulating the micro current, by using a modulation circuit, the differential circuit is filtered out of these impurities DC signals, resulting in a microcamparrier signal to be measured to be measured. Differential, the principle of modulation circuit is shown in Figure 2.
Figure 2 Diabal current difference, modulation preamplifier model
When K1 is disconnected, K2 is closed, that is, the output:
U01 = ISRF + UI0 + IB + R '+ IB-RF (3)
When K1 closed, K2 is disconnected, that is, the output:
U02 = UI0 + IB + R '+ IB-RF (4)
The decrease in the formula (3) (4), the system error can be eliminated, namely:
UO = U01- U02 = ISRF (5)
By the formula (5), the DC impurity signal is eliminated, visible, UO is proportional to IS. However, the UO signal is extremely weak, and UO needs to be enlarged by layer, and then differential. The total magnification is k, then the output is: UO = kiSRF; the micro current is measured:
IS = UO / (KRF) (6)
The measurement results are sent to the central processor of the instrument, and finally displayed by the display circuit.
4 system design
4.1 Measuring Circuit Composition
This measurement circuit consists of 3 parts.
1) The pre-amplification stage, the signal is modulated, and the micro-current signal is converted into a micro-voltage signal;
2) The signal amplification stage is composed of a low pass filter circuit, a zero circuit, a switch selection circuit, and a state discrimination circuit.
3) Microote output, consist of sample hold, differential circuit, and the like, the modulation switch is sampled by the modulation switch, respectively, and the system error is removed by the differential circuit, and finally outputs a voltage signal proportional to the measured microote. The measurement circuit is shown in Figure 3.
Figure 3 Measuring circuit system
4.2 Level 1 amplification circuit principle
The amplification process is divided into 8 levels (V1 ~ V8), and the block diagram is from the upper, gradually enlarged as shown in Figure 4. The preamplifier output of the microconrection signal is enlarged by the central processor when magnification is enlarged in the first level. The determination of the series is first closed by the multi-channel switch, and it is judged by the status discrimination circuit. When the output signal exceeds the linear range of the op amp, the level is retroded level 1 and sent to the central processor. To avoid the multi-line interference signal is amplified, each level of magnifying circuits set low-pass filters. The zero circuit is disposed at the end of the amplification circuit to avoid the measurement circuit itself is larger than the linear range of its work.
Figure 4 Level 1 amplification circuit principle
4.3 Level 2 amplification circuit principle
A total of 4 levels are magnified, and the magnification of each level should not be too large, with a saturated voltage of no more than the op amp and the output signal is maximized, as shown in Figure 5.
Figure 5 Level 2 amplification circuit principle
According to different timings of the modulation switch, the results of the signal amplification stage output are stored in two registers, and the differential circuit is used to eliminate the precursor DC signal accompanying the front amplification circuit and the main amplifier circuit.
4.4 Status discrimination circuit principle
The pre-amplifying circuit of 3 V is 3 V, and the J / U conversion signal outputs to the No. 1 status discrimination circuit, and the judgment circuit is determined by the discrimination circuit to send the result to the central processor; the main extension circuit is used to use the power supply The operational amplifier of 15 V, the circuit is output to the 2nd state discrimination circuit, and the result is sent to the central processor as shown in Figure 6.
Figure 6 State discriminant circuit principle
5 installation consideration
In addition to circuit structure design, certain measures should be taken in component selection, circuit installation and process. To reach the micro-current measurement of PA, you must pay attention to the following points:
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