Principle and application of ICL7650 chopper-zero-stabilized operational amplifier

　　 ICL7650 is a chopper-zero-stabilized high-precision operational amplifier produced by Intersil using dynamic zero calibration technology and CMOS technology. It has small input bias current, small offset, high gain, strong common-mode suppression capability, fast response, low drift, Stable performance and low price.

　　1 Chip structure

　　ICL7650 comes in two package forms: 14-pin dual in-line and 8-pin metal shell. Figure 1 shows the pinout diagram of the most commonly used 14-pin dual in-line package. The function description of each pin is as follows:

　　CEXTB: external capacitor CEXTB;

　　CEXTA: external capacitor CEXTA;

　　-IN: inverting input terminal;

　　+IN: Non-inverting input terminal;

　　V-: negative power supply terminal;

　　CRETN: the common terminal of CEXTA and CEXTB;

　　OUTCLAMP: clamp end;

　　OUTPUT: output terminal;

　　V+: positive power terminal;

　　INTCLKOUT: clock output;

　　EXTCLKIN: clock input;

　　Clock control terminal, through which you can choose to use the internal clock or external clock. When the external clock is selected, this terminal is connected to the negative power terminal (V-), and the external clock signal is introduced at the clock input terminal (EXTCLKIN). When this end is open or connected to V+, the circuit will use the internal clock to control the work of other circuits.

　　2 Working principle

　　ICL7650 uses dynamic zero calibration technology to eliminate the inherent offset and drift of CMOS devices, thereby getting rid of the constraints of traditional chopper-zero-stabilized circuits and overcoming these shortcomings of traditional chopper-zero-stabilized amplifiers.

　　The working principle of ICL7650 is shown in Figure 2. In the figure, MAIN is the main amplifier (CMOS operational amplifier), and NULL is the zeroing amplifier (CMOS high-gain operational amplifier). The circuit works in two stages through the conversion of electronic switches. The first is in the first half cycle of the internal clock (OSC). Electronic switches A and B are turned on and C is turned off. The circuit is in the error detection and registration stage; secondly, electronic switches A and B are turned on and C is turned off. In the second half cycle of the internal clock, the electronic switch and C are turned on, A and B are turned off, and the circuit is in the dynamic zero calibration and amplification stage.

　　Since the gain A0N of the NULL operational amplifier in the ICL7650 is generally designed to be around 100dB, even if the offset voltage VOSN of the main operational amplifier MAIN reaches 100mV, the offset voltage of the entire circuit is only 1μV. Since the above two stages continue to alternate, the capacitors CN and CM send the results of the previous stage stored in them to the zero-adjusted terminals of the operational amplifier MAIN and NULL. This makes the circuit shown in Figure 2 almost non-existent offset and drift. It can be seen that, The ICL7650 is a high-gain, high common-mode rejection ratio, and dual-terminal input operational amplifier.

　　3 Application circuit

　　In addition to the characteristics and application range of ordinary operational amplifiers, ICL7650 also has the characteristics of high gain, high common mode rejection ratio, small offset and low drift, so it is often used in measurements of thermocouples, resistance strain bridges, charge sensors, etc. in the preamplifier for weak signals.

　　The circuit shown in Figure 3 is the preamplifier circuit of an earthquake precursor signal acquisition system. The natural ground potentials Vi1 and Vi2 induced between the carbon electrode and the floating ground of the signal conditioner in the system are added to the two input terminals of I-CL7650 respectively. The weak signals Vi1 and Vi2 will be amplified from the 10th pin of ICL7650. After a series of processing, the output and amplified signals can be sent to the monitor and recorder for display and recording for earthquake research and prediction.

　　In order to prevent the ICL7650 from being damaged due to excessive input signal amplitude, the circuit in Figure 3 adds four protective diodes D1 to D4 to the input ends of the two signals. Since the amplitude of the input signals Vi1 and Vi2 during normal operation of the circuit is very small, the diodes do not conduct, which will not affect the normal operation of the circuit. The gain of the circuit is high. In order to prevent high-frequency oscillation, the resistor R4 is connected in parallel with the capacitor C3 during the design. Because of its small capacity, the impact on the signal amplification factor is also very small. In order to increase the DC component of the op amp's output signal, the clamp terminal can be connected to the inverting input of the op amp.

　　More than 30 sets of earthquake precursor signal acquisition systems produced using ICL7650 have been installed at nearly ten seismic stations in four provinces and cities across the country, and a large number of earthquake precursor signals have been collected. Therefore, it can be proved that the entire system has stable performance and is anti-interference. strong ability.

1. ICL7650 datasheet．From http://www. intersil. com

2. Chen Guojie, et al. Design based on ICL7650 program-controlled microcurrent amplifier. Journal of Foshan University of Science and Technology. 2001.12(8)