Research and Design of a Chopper Offset Stabilized Instrumentation Amplifier
Instrumentation amplifiers integrate key components inside the amplifier, which are derived from op amps, but are superior to op amps. Its low noise, low offset, high common-mode rejection ratio, and high input impedance are important indicators for instrumentation amplifiers.
Current methods for reducing 1/f noise and offset are: fine-tuning techniques, auto-zeroing techniques, and chopping techniques. Fine-tuning techniques do not reduce the amplifier's 1/f noise and temperature drift. The auto-zero technique is a sampling technique that reduces the 1/f noise and offset by sampling the low-frequency noise and offset and then subtracting them from the instantaneous value of the signal at the input or output of the op amp. Because this technology is an undersampling process for broadband white noise, it will cause white noise aliasing [1]. The chopping technique uses modulation and demodulation methods to modulate the 1/f noise and offset to the high frequency end, and then filter it out through the low-pass filter. After the useful signal is modulated, it is demodulated to the baseband. This technique does not. The disadvantage of white noise aliasing, but its chopping frequency limits its bandwidth.
The instrumentation amplifier designed in this paper uses both chopper stabilization technology [2] and auto-zero technology [3] to reduce the effects of 1/f noise and offset voltage, with high common-mode rejection ratio, low offset voltage, and dynamic Compensation for the characteristics of the offset voltage.
1 The basic principle of chopper technology
The chopping schematic is shown in Figure 1. The chopping technique modulates the input signal and the square wave signal, and then obtains the desired signal through synchronous demodulation and low-pass filtering. It does not substantially eliminate the offset, but modulates the offset voltage and the low frequency noise to the high frequency. The offset voltage and noise at high frequencies are then filtered out through a low pass filter. Ideally, chopper op amps can completely eliminate dc offset and low frequency noise (mainly 1/f noise). The principle of chopping modulation is shown in Figure 1. It is assumed that Vin and Vout are the input and output signal voltages respectively, A is the gain of the amplifier, and Vch is the periodic square wave signal, fch
2 Chopper Offset Stabilization Technology
The chopping process produces a lot of mixing products, including the sum of the chopping frequency and the input signal, and the difference. These mixing products cause a lot of distortion, especially when the signal frequency is close to the chopping frequency. And low pass filtering reduces the bandwidth of the available signal. The best solution to suppress noise and offsets without reducing the signal bandwidth is to use a chopper-stabilized op amp. This circuit structure provides signal bandwidth in the main path, and the auxiliary path reduces offset. The circuit structure is shown in Figure 2. The auxiliary path includes a chopper-stabilized amplifier and an integrator, and the main path has only one amplifier.
Assume that the offset voltages of the main and auxiliary inputs of the main amplifier are Vosm (main), Vosm, 1 (auxiliary), the gains of the main and auxiliary inputs are Am, Am, and 1, respectively; the equivalent offset voltage of the auxiliary op amp is Vosn, The gain is An, and the overall offset voltage of the entire amplifier is Vos, then:
3 Chopper Offset Stabilization Amplifier Design and Simulation
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