Ken Kundert
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In concept, CDS works by sampling the offset and subtracting it off. In your circuit you are getting about 30 dB of cancellation, meaning that your are canceling roughly 97% of the offset. To get rid of the offset completely, the sampling and subtraction must be perfect. For example, you would see the levels of performance you are seeing if the sampled offset were to droop roughly 3% between when the sample was taken and when the correction is applied. Another possible source of error is feed through, a small amount of input signal feeds through the sampler or perhaps directly from the input to the output. In my experience, the capacitance are very small in CDS circuits and impedance levels are high, meaning that small parasitics could create significant feed through.
The situation with choppers is similar but different. With CDS the offsets and low frequency noise are eliminated through a sample and correct process. With choppers is a modulation process. With choppers, the issue is symmetry, the better the symmetry the better the suppression of the offset. Essentially with choppers there are two nearly equal and opposite paths. If the paths are identical yet opposite, then the error suppression will be perfect. In practice you cannot get two identical but opposite paths in the actual circuit, but you conceivably could in a simulation where perfect symmetry is possible. So you might expect that when simulating choppers you could get perfect suppression, but there is another form of asymmetry that you need to be aware of, and that is in the time steps the simulator is taking. For perfect cancellation, the simulator needs to take precisely the same time steps in each half cycle. This is something you don't have a lot of control over. In practice, the timestep asymmetry will prevent you from getting perfect cancellation, but at 30 dB I believe you are well above the simulator imposed accuracy constraints.
-Ken
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