In the signal detection design, the problem of DC drift often occurs, and the general interference processing uses filtering. However, if it is a low frequency signal, it is easily biased by external factors. How to solve this kind of problem, we need a servo circuit. The input signal can be fed back to the output signal to adjust the input.
In the signal detection design, we usually use filtering for the processing of interference. The filtering is mainly for the frequency characteristic parameters, and the useful signal is selected by setting the passband. Then if the interference signal is a low frequency signal, the signal to be tested also contains a low frequency signal. Or the DC offset of the signal to be tested is easily offset by external factors, and even the common mode input voltage is too large. How does this handle such DC offset?
In the "Measurement of Electronic Circuit Design - Simulation", a servo circuit that specifically solves this problem is mentioned. The so-called servo is that the input signal can get feedback of the output signal to adjust the input. The principle of the circuit is actually very simple: the integration circuit is used to feed back the offset to the input and cancel it through the op amp.
We can verify this by simulation:
The DC input value is normally amplified without using the servo circuit.
After adding the feedback servo circuit, it was found that the DC value was eliminated. It can be seen that the DC value is integrated and fed back to the input negative terminal 300mv.
Some instrument op amps are equipped with a feedback voltage input pin VREF, which can be used as follows:
However, it is obvious that this type of circuit is used to eliminate direct current. The so-called unacceptable DC signal is mainly used to handle DC offset processing of AC or modulated signals.
A few notes:
1. DC offset problem used to process AC signals.
2. The integrator is equivalent to a low-pass filter with reasonable parameter settings and no signal loss.
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