vivkr
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Hi,
I am using PSS+Pnoise analysis for simulating the noise in my transistor-level circuit. It is a lossy integrator, with an opamp, 2 caps, and some switches. It does an offset-cancellation by sampling the input with respect to the opamp virtual ground.
When I simulate the noise of this circuit with PSS+Pnoise, I see something strange:
1. There is 1/f noise, although it should have disappeared due to the correlated double-sampling. I am able to verify that the offset is indeed cancelled, if I explicity add a large voltage (few mV) as offset to the opamp input. It cannot be that there is 1/f noise at <1Hz. No shaping of this noise is seen either.
2. The total thermal noise seen is too low, compared to my hand calculations, which are a little pessimistic, but not so large.
I have used very large number of sidebands to verify that increasing sidebands does not have much effect. I expect 5 sidebands, and my circuit shows practically no change going from 10 to 100 sidebands.
The maxacfreq is also set quite high. I see no warning messages in the pnoise log saying that this frequency is too low. Just for reference, the maxacfreq is about 20x of the expect opamp noise bandwidth.
I have set the tstab in PSS so that the cycle ends towards the end of an integration phase. I suppose this is the correct way to do it, since I will get the final output after the last integration cycle.
The integration operation itself is direct charge-transfer in nature. It is only the sampling part where the opamp comes in to help cancel the offset.
Any hints will be greatly appreciated. I have read the paper on using PSS+Pnoise on this website, but I can't see anything which I may have missed, but I am probably wrong about this.
Regards Vivek
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