Discussion
Why does the chopper technique help a low-level direct-coupled measurement?
Answer: B
Chopping translates the slowly varying signal up to the chopping frequency, where flicker noise has fallen away and only the flat thermal and shot contributions remain, and the signal is recovered by synchronous detection afterwards. It does nothing to the input resistor's thermal noise, which is flat anyway and so cannot be escaped by moving in frequency. It does not itself narrow the bandwidth, though the synchronous detector that follows it does. And shot noise is likewise flat. The whole technique is an application of the one-over-f spectrum, and it makes sense only once that shape is understood.
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