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will the phase of the pss period affect sampled pnoise result? can I control the pss phase?

Ring Lewis

Hello, everyone. I understand the sampling/aliasing/ktc/chop-modulation.Here is a ping-ping choped amplifer. When OPA1 is during auto-zero, only OPA2 drives output, and vice versa.

Auto-zero will introduce KT/C noise (also including OPA's RMS noise) which is manifested as low frequency noise, this kt/c noise could be modulated to high frequency by chopping.

SprefreRF's simulation perfectly confirms this analysis (top plot). pss period = 1/f(AZ), sampled time = very end of chopping phase. sample ratio=4;

But what happens if a PSS period dosen't include a correct ping-ping's period (bottom plot). In this situation, OPA2's KTC noise is being completely choped, while OPA1 seems to have 2 differnet KT/C noise: (KT/C@last time) is half chopped, then AZ, then  (KT/C@this  time) is half chopped.

The KT/C(@last time) and KT/C(@this time) is not the same KT/C and they are uncorrelated? will the OPA1's KT/C noise be correctly modulated? If not, how can I control pss period phase? thank you!

  • I see you've altered the picture since first posting it because the relative phases of AZ and CHP were different in your original post (which was what I was thinking might have been your problem).

    A phase shift should not make a difference; the PSS is capturing the periodic steady state, and so the period includes whatever happened in the previous period. You can adjust the phase by adding some tstab time - but I wouldn't expect it to (significantly) change the results. I say "significantly" because of course there could be small numerical differences. Put another way, using sampled pnoise is computing the time-averaged periodic noise at the output of a strober added to the output of the circuit. It does not just behave as if the circuit started at time zero in the PSS period with no history of what went on in previous periods.

    Andrew

  • in reply to Andrew Beckett

    Thank you, Andrew. Yes, phase shift makes no difference. One of my previous pnoise result didn't show modulated KT/C noise was due to unsettled sampling. This should be calculated by auto-correlation...while, sim-tool saves me time to do this tedious calculation

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