HdrChopper
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Hi,
the only way I know of to reduce folding is to use and AAF. Actually, your amplifier is acting as a "poor" AAF, and I say poor since the cut-off frequency should be half of your sampling frequency in order to totally avoid folding. So, if further lowering the BW in your amp is a concern due to other aspects of the circuit (as you suggested in your post), then the only option is to make a better (lower noise) amplifier.
To lower the noise, you have to increase the power / area of your circut. By increasing gm1 you certainly decrease the noise, but also increase the BW (BW is proportional to gm/Ccomp). You'll come to the conclusion that the only way to lower the amp noise is by increasing Ccomp (thus by reducing its BW).
Regards tosei
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