1 Introduction
The operational amplifier (op-amp) is referred to as an op amp, and is named for the mathematical operation originally used for 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, representative and widely used analog integrated circuit. In industrial automation control and process control, op amps are often used to amplify low-level signals from sensors, which requires integrated op amps as preamplifiers with high input impedance, low output impedance, and low offset voltage. And temperature drift and precision feedback characteristics and high common mode rejection ratio, otherwise the drift problem will make the system not work properly, ICL7650 is developed to meet the above requirements.
The performance of the ICL7650 chopper integrated op amp is introduced, and a weak signal preamplifier circuit is designed. The mulTIsim 8 software is used for simulation and testing. The gain, amplitude-frequency characteristics, signal-to-noise ratio and other performance indicators can be Meet the design requirements. The circuit has a simple structure and has certain reference use value for DC and low frequency weak electrical signal amplification.
2 ICL7650 Performance Introduction
The chopper-stabilized operational amplifier ICL7650 is Intersil's fourth-generation operational amplifier with exceptional performance and stability, making it a preamplifier for precision instrumentation, weak signal detection, and process control systems.
The ICL7650 has the following main features:
1) Very low input offset voltage: ±1μV over the entire operating temperature range (approximately 100 °C); low input bias current: 15pA (typ);
2) The temperature drift of the offset voltage and the long-term drift are extremely low: 0.01 t, v/°C and 100 nV /Month, respectively;
3) Very high open-loop gain, CMRR, PSRR are ≥130 dB; higher slew rate: SR = 0. 5 V/μs;
4) unity gain bandwidth BWG = 2 MHz, with internal compensation, phase margin ≥ 80;
5) There is a clamping circuit inside, which can reduce the recovery time during overload; there is only a very small chopping peak leakage at the input and output terminals.
3 Designing a weak signal preamplifier circuit with ICL7650
According to the above analysis, combined with the principle of the instrument amplifier, the actual circuit design is shown in Figure 1:
Figure 1 Schematic diagram of the amplifier circuit
R0 is the ICL 7650 input current-limiting protection resistor. In the peripheral circuit of the ICL7650, a 0.1μF (104) capacitor is connected between the power supply voltage input and 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 zeroing of the op amp. Therefore, high-impedance, porcelain dielectric, and high-quality capacitors of polyethylene are used. The values ​​can be 0.1μF.R3 and C6. It forms 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 op amps with high input impedance and common-mode rejection ratio, and the dual-ended input is a single-ended output. Since the offset voltage and drift of the entire circuit are closely related to the first stage, A1 and A2 select the ICL7650 integrated op amp with ultra-low offset voltage and ultra-low drift. As a high-precision, low-drift amplifier, the ICL7650 typically operates with a few hundred microvolts or even tens of microvolts.
4 Simulation with MulTIsim 8 software
4.1 Introduction to MulTIsim 8 Software
Developed by Interactive Image Technology's electronic circuit simulation software EWB (Electronic WorkBench), MulTIsim 8 software inherits EWB's intuitive circuit simulation and design interface and develops EWB's device library and virtual instrument library. Multisim 8 is an upgraded version of Multisim 7. Its user-friendly interface, large device instrument library and sophisticated analysis methods are suitable for most of the circuit design and simulation. It can easily simulate analog, digital or hybrid circuits. Most of them use actual models to ensure the authenticity and practicability of simulation and design results. Since this design is to amplify the μV-level voltage signal, it is very difficult to amplify and process such a weak electrical signal. For example, the zero drift of the operational amplifier, noise, external interference, channel transmission, etc., will seriously affect the signal protection. Real and extract. Therefore, although the actual hardware circuit has been successfully built, there may be interference and noise. Therefore, the performance simulation analysis of the designed circuit is performed using Multisim 8 software.
4.2 Creating a Simulation Circuit Schematic
The simulation circuit diagram is created according to the circuit schematic designed in Fig. 1, and the simulation circuit diagram shown in Fig. 2 is obtained.
Figure 2 Simulation circuit
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