The paper presents a symmetrical passive RC notch filter with two cutoff frequencies used for ripple reduction in chopper offset-stabilized amplifiers. The filter has two cutoff frequencies: the first one is the chopping frequency itself, while the second one is the third harmonic. The rejection of the proposed filter is 60 dB around the cutoff frequencies. The chopping frequency is correlated with the cutoff frequencies by using a current-driven oscillator and a bias circuit using the same type of resistor as the filter itself. A circuit using this technique was fabricated, achieving a low-frequency noise performance of 1.3 μV pp .

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The paper presents three approaches for frequency compensation in chopper offset-stabilized amplifiers with symmetrical passive RC notch filter with two cutoff frequencies. The filter has two cutoff frequencies: the chopping frequency itself and the fifth harmonic of the chopping frequency. The frequency compensation network is connected either at the input of the notch filter, or at its output, or between its two sections. The large signal transient response is evaluated in terms of the overshoot/undershoot and the settling time. Three circuits using these techniques were fabricated and evaluated. The approach that uses the frequency compensation network connected between the sections of the filter has the best results, except for the chopping ripple rejection.