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Message started by vm511 on Nov 25th, 2019, 12:10pm

Title: Noise sims for switched capacitor amplifiers
Post by vm511 on Nov 25th, 2019, 12:10pm

Hi all,
I am simulating a switched capacitor circuit with very low capacitances (feedback capacitor is 5fF). I am observing issues with my simulations that I wanted to clarify:
1. On running the transient noise and PSS-PNoise analysis --> the results match with each other, and are lower than what I expect theoretically.
2. However, when I do noise analysis in the two phases of the circuit separately, the values match my theoretical prediction and are more than that.
This especially happens when I'm using capacitive autozeroing (that is in the offset storage phase, a capacitor is connected in feedback instead of connecting the opamp in unity gain)
For my .noise analysis, I'm running transient analysis and then using .prevoppoint I'm doing the noise analysis at that point.
For effect of offset storage phase I'm calculating the charge stored at the input of the opamp (where the input capacitance and feedback capacitance are connected) by multiplying the rms voltage by the total capacitance at that node. And then I'm dividing this charge by the feedback capacitance to get the effect on the output.
I'm unsure why .noise analysis and PSS-PNoise/Transient noise are giving me different results and completely lost on which simulation to trust.
I'd be extremely grateful if someone could help!

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