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switched cap and PSS/PAC analysis

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Hello all!!

I'm designing a bandpass filter using switched-cap (100Hz center frequency and Q=5 with two low-Q and one high Q biquad).
At typical case, the pss simulation converges well and the AC results are similar to what is expected.
However at corners, especially at lowest temperature, the pss simulation do not converge or converge with difficulty.
If pss finally converges, the AC results given are wrong and the transient results (of one period) that can be seen with pss, shows big oscillations (some signals go down to zero with a fixed period). This issue is solved if opamps are replaced by ideal ones. Also, this oscillation cannot be seen with traditional transient simulation.

Does anyone have some ideas on that issue?
Any idea is welcome.

Thks in advance

AXP


Originally posted in cdnusers.org by axp

  • I will extend the question...

    When a time plot of a voltage node using PSS shows oscillations whereas the initial transient (launched before iteration of pss) do not show them... What can I trust? Can I trust that the timeplot using pss is very accurate or should I assume there is a simulation issue?

    In my switched cap design, according to PSS timeplot it seems that oscillations take place (and hence cannot converge) but the transient with small time step do not show any oscillation. Further phase margin simulations and settling time give excellent results for stability. This is quite strange.

    I would really appreciate any idea on how do I have to tune pss options or if someone would have any suggestion if this issue could be involved by simulator or from design.

    Thks for your help!

    Regards,

    Axel


    Originally posted in cdnusers.org by axp

  • Bear in mind that PSS finds the steadystate - it accelerates settling, and so if the oscillation takes a long time to build up, you wouldn't see it in the initial transient unless that initial transient is very long.

    Try running a very long transient to see what happens.

    Regards,

    Andrew.


    Originally posted in cdnusers.org by adbeckett

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