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Regarding PSS and PNOISE analysis: Getting a peak at 1KHz in Noise plot

kapiljainwal

I am simulating a chopper stabilized opamp circuit in which I need to do noise analysis. As the biasing point is changing with time periodically, I need to do PSS analysis and for Noise PNoise analysis. As I am changing my minimum frequency of the signal beat frequency is getting changed but every time I am getting a peak in noise plot at 1KHz only. It is irrespective of all frequencies in circuit. In circuit signal frequency is around 500Hz and clock frequency is arounf 1MHz. 

This peak is coming only at 1KHz only. What is wrong, being done by me  during simulation ?

Please help.

  • It's hard to tell what your setup is. Maybe a picture of the PSS form would help? (you can attach it via the Options tab when posting). Also if you can paste the options and analysis statements from the bottom of your input.scs, that would help.

    Regards,

    Andrew.

  • Generally speaking when analysing the noise of a clocked system, you would do the PSS analysis with just the clock present, and not a large-signal input present too. This means it will then do a small-signal analysis of the noise with the aliasing and noise folding caused by the sampled data system itself - and will also be a lot faster as you then don't need to include the 1kHz signal in the PSS solution.

    If you do include the 1kHz signal as a large signal input, you'll be able to see the intermodulation of the noise with this large 1kHz signal too - normally that's only important if the large signal input is large enough to create distortion - because then you might get intermodulation with these distortion harmonics which could impact the noise performance - but for most normal situations there's no need to have the large signal input. Think about how you'd analyse the noise for an opamp; you wouldn't normally have a large-signal input present at the same time.

    If you do have 1kHz signal input, this will cause flicker noise to mix with the 1kHz signal and its harmonics. Given that flicker noise is infinite at 0Hz (as it is 1/f), you'll end up with an "infinite" (or very large) noise at multiples of 1kHz. Since your frequency sweep may not exactly hit the multiples of 1kHz, you'll probably just see peaks.

    So I'd try again without the 1kHz large signal being present.

    Regards,

    Andrew. 

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