vivkr
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Hi,
The continuous-time filter noise can be estimated separately from that of the sampler, and then you can combine the 2 noise values. For the sampler, you correctly use the pnoise analysis.
As for what upper frequency limit to use for computing the noise from the continuous-time filter, you need to decide how much of the total noise falls in the signal band of interest for you.
There are several different scenarios and methods, all depending on your system definition, and the applicable bandwidth where noise is of interest to you.
If your sampling rate is much higher than the cutoff of the filter, then things are easy. Just take the signal band of interest to you (which is obviously less than the cutoff of the filter),
If you cannot ignore out-of-band noise in your system for some reason, then taking all the noise contained within the "noise bandwidth" of your filter is a good first guess. For a first-order lowpass filter, this is about (pi/2)*cutoff frequency.
The other extreme is to consider all the noise generated by your filter. This is typically limited and a function of the dominant capacitance (s) of your filter. Since noise simulations of continuous-time filters are trivial, this is very easy to do.
Regards, Vivek
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