sheldon wrote on Mar 30th, 2010, 6:02am:Lunren,
I have built a switched capacitor VGA for DSC AFE. It contains a
number of on and off switches. The I ran pss with time domain noise
and calculated the noise at the several time points. Looking at the
noise summary, the dominant noise sources are the input pair,
the current sources for the folded cascodes, and the input common
mode switches.
Questions:
1) The common-mode feedback circuit seems to contribute a lot
of noise. The common-mode feedback were the largest noise
contributors until I turned them off. This makes sense sine the
the common mode has correlation, that is, equally effects each
of the differential outputs.
2) Have not seen the details of your results, could it be that the
switch noise is stored on the capacitor during one phase and
show up in the noise summary during another phase? That is,
the switch noise is stored on the capacitor during sampling
and show up at the output during hold.
In the next two appends, I will attach the noise summary and
the PSS waveform.
Best Regards,
Sheldon
I should mention that the previous simulation results were gotten with general AC noise sims. However, in Pnoise (option sources or timedomain) sims, it seems that the group of off switches didn't contribute much noise and the dominant noise sources are the input pair and the current mirror (load) of the first stage the amp, the sim results with Pnoise seem to be rational. But in my case, the common mode feedback does not dominate the noise.
I think in switched capacitor circuit, noise indeed will be sampled to cap during one phase and shows up in another phase.
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