3.2 A Chopper-Stabilized Amplifier with a Relaxed Fill-In Technique and 22.6pA Input Current

In chopper amplifiers, the interaction between the input signal and the chopper clock can cause intermodulation distortion (IMD). This is due to amplifier delay, which causes signal transitions generated by the input chopper to arrive at the amplifier's output slightly later than the corresponding clock transitions of the output chopper. This causes large signal-dependent spikes in the final output, which can significantly degrade amplifier linearity, especially at input frequencies near even multiples of the chopping frequency F
cH
, which will cause IMD tones near DC. In [2–4], spread-spectrum clocks are used to convert such tones into noise-like signals. However, this increases the noise floor, without solving the underlying problem. Recently, it has been shown that such spikes can be eliminated by using the fill-in technique [1], in which two identical OTAs are chopped in quadrature, allowing a spike-free output to be generated by switching between their outputs in a ping-pong fashion.