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The chopper frequency should be above the 1/f corner frequency, in order to filter 1/f noise out.
The higher the chopper frequency, the easier it will be to filter out chopper ripple. You could maybe use a sample-and-hold synchronised with the chopper clock.
However a higher chopper frequency will also give an increase in residual offset due to extra charge injection. So be careful when you layout the amplifier! The bandwidth of the chopped amplifier stages should also be able to cope with the modulated signals.
So I would say that your chopper frequency depends on 1/f corner and residual offset.
Are you sure you need chopping? Ususally for regular BGR this is not necessary unless you need very low noise or very high accuracy. Just use large input MOSFETS to minimize offset and 1/f noise.
It sounds like you are trying to design a reference for an audio adc type of circuit. If this is the case, your chopper frequency should be well above the 20kHz audio frequency, I'd use 100kHz to 300kHz.
The offset still contribute to the PTAT-loop but it does not matter because in a PTAT-loop there are also offsets. The loop amplifier offset is then part of the total offset.
The key to lower offset contribution or better noise is to use kT-multiplier-less PTAT's. They have enough voltage to be combined direct with a diode voltage.
You can see it from a noise voltage spectral density plot. It generally depends on used CMOS technology. CMOS processes specialized for low frequency analog applications, e. g. instrumentation or audio OP have achieved great improvements in this regard. While standard processes have still above 10 or even 100 khz noise corner, they approached to e. g. below 1 kHz.
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