Design and simulation of weak signal amplification circuit based on Multisim 8

1 Introduction

Operational amplifier (op-amp) is abbreviated as op amp, which was named because it was originally used for mathematical operations of analog quantities. It is a direct-coupled multi-stage amplifier circuit with high voltage gain, high input resistance and low output resistance. It is also the most basic, most representative and most widely used analog integrated circuit . With the rapid development of integrated circuit technology and the improvement of circuit performance design, integrated op amps are penetrating into various fields with incomparable excellence. Ordinary integrated op amps generally have an offset voltage of mV and a temperature drift of microvolts per degree, so it is very difficult to use integrated op amps directly for amplifying weak signals. However, in industrial automation control and process control, op amps are often used to amplify low-level signals from sensors , which requires the integrated op amp used as a preamplifier to have high input impedance, low output impedance, low offset voltage and temperature drift, as well as precise feedback characteristics and high common mode rejection ratio capability. Otherwise, the drift problem caused will make the system unable to work properly. IC L7650 was successfully developed to meet the above requirements.

2 ICL7650 Performance Introduction

The chopper-stabilized operational amplifier ICL7650 chip is Intersil's fourth-generation operational amplifier, which has extremely superior and stable performance. Therefore, it is widely used as a preamplifier in precision instruments , weak signal detection and process control systems .

ICL7650 has the following main features:

1) Extremely low input offset voltage: only ±1μV over the entire operating temperature range (approximately 100°C); Low input bias current: 15pA (typical);

2) The temperature drift and long-term drift of the offset voltage are extremely low: 0.01t, v/℃ and 100 nV/Month respectively;

3) Extremely high open-loop gain, CMRR, PSRR are ≥130 dB; Higher conversion rate: SR = 0.5 V/μs;

4) Unity gain bandwidth BWG = 2 MHz, with internal compensation, phase margin ≥ 80;

5) There is an internal clamping circuit to reduce the recovery time during overload; there is only very small chopping spike leakage at the input and output ends.

3 Design of weak signal preamplifier circuit using ICL7650

According to the above analysis, combined with the principle of instrument amplifier, the actual circuit design is shown in Figure 1:

Figure 1 Schematic diagram of amplifier circuit

R0 is the input current limiting protection resistor of ICL 7650. In the peripheral circuit of ICL7650, a 0.1μF (104) capacitor is connected between the power supply voltage input terminal and the ground to filter out the interference caused by the power supply. Sampling capacitors C2 and C3 play a key role in dynamic zero calibration, which directly affects the accuracy of the automatic zero stabilization of the op amp. Therefore, high-impedance, porcelain dielectric, and high-quality capacitors of polyvinyl chloride are selected, and their value can be 0.1μF. R3 and C6 form a filtering network to filter out the chopping spike noise caused by the commutation of the ICL7650 analog switch and reduce the overshoot in the output voltage.

The first stage of the circuit is two symmetrical ICL7650 integrated operational amplifiers, which have high input impedance and common mode rejection ratio, and change the double-ended input to single-ended output. Since the offset voltage and drift of the entire circuit are closely related to the first stage, A1 and A2 use ICL7650 integrated operational amplifiers with ultra-low offset voltage and ultra-low drift. As a high-precision, low-drift amplifier, ICL7650 can work normally with an input voltage of only a few hundred microvolts or even tens of microvolts.

4 Simulation using Multisim 8 software

4.1 Multisim 8 Software Introduction

Multisim 8 software is developed from EWB (Electronics WorkBench), an electronic circuit simulation software launched by Interactive Image Technology of Canada. It inherits the intuitive circuit simulation and design interface of EWB, and develops the device library and virtual instrument library of EWB. Multisim 8 is an upgraded version of Multisim 7. Its user-friendly interface, huge device instrument library and perfect analysis method can be competent for most occasions of circuit design and simulation. It can easily simulate analog, digital or mixed circuits, and most of them use actual models to ensure the authenticity and practicality of simulation and design results. Since this design is to amplify μV-level voltage signals, it is very difficult to amplify and process such weak electrical signals. For example, zero drift, noise, external interference, channel transmission, etc. of operational amplifiers will seriously affect the fidelity and extraction of signals. Therefore, although the actual hardware circuit has been successfully built, there may still be interference and noise. Therefore, Multisim 8 software is used to simulate and analyze the performance of the designed circuit.

4.2 Create a simulation circuit schematic

According to the circuit schematic diagram designed in FIG1, a simulation circuit diagram is created, and a simulation circuit diagram as shown in FIG2 is obtained.

Figure 2 Simulation circuit

4.3 Function signal generator settings

In order to simulate the weak signal sent by the sensor, the frequency of the input signal is set to 20Hz and the amplitude is set to 10uV, as shown in Figure 3.

Figure 3 Signal generator settings

4.4 Output signal waveform

The 10uV AC signal is amplified by the amplifier circuit in Figure 2 to obtain a 30mV voltage AC signal, as shown in the dual-channel oscilloscope .

Figure 4 Dual-channel oscilloscope

4.5 Circuit amplitude-frequency characteristics Test

The amplifier circuit designed in this paper mainly amplifies low-frequency weak signals and also has a certain degree of suppression on high frequencies. Double-click the icon of the XBP1 Bode plotter, set reasonable parameters, and display the amplitude-frequency characteristic curve of the circuit, as shown in Figure 5. In the middle horizontal line of the curve in the figure, it can be clearly seen that the gain of the circuit is 69.589dB. By dragging the reading axis with the mouse, the upper limit frequency fH = 294.963Hz, the lower limit frequency fL = 0, and the bandwidth B = fH - fL = 294.963Hz4.6 Circuit distortion test Double-click the distortion meter icon of XDA1, and the interface shown in Figure 6 will appear. After adjusting the parameters, the distortion of the circuit is zero and the signal-to-noise ratio is 100dB. Therefore, this circuit has a high signal-to-noise ratio and low noise.

Figure 5 Bandwidth test

Figure 6 Distortion test

5 Conclusion

This paper designs a weak electrical signal preamplifier circuit based on the characteristics of the sensor input signal, and uses Multisim 8 software for functional simulation. Since the chopper-stabilized zero amplifier ICL7650 has extremely low offset voltage and drift, the circuit has good performance. After simulation, all parameters basically meet the design requirements. This amplifier circuit has a simple structure and good effect, and has certain use value for preamplification of weak DC and low-frequency signals.