Symmetrical passive RC notch filter with two cutoff frequencies for ripple reduction in chopper offset-stabilized amplifiers

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.