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QPSS on a switched capacitor amplifier giving incorrect harmonic amplitudes

samini

Hi,


I am trying to run QPSS on a switched capacitor amplifer to see the harmonics of the circuit.  The motivation is to save time instead of running a long transient and FFT.

I have set both the sampling and feedback capacitors for the SC amplifier to to be the same and have an input sinewave at 250Hz with an amplitude of 225mV.  My clock is 256KHz. 

For QPSS, I set the clock as the large signal and the input as the moderate signal.  I have set 3 harmonics for the input and 0 harmonics for the large signal.  Accuracy is set to conservative.

The problem is that the results show the amplitude at 250Hz to be 112mV rather than the expected 250mV (so about half of expected).  I have tried playing around with many of the parameters including tstab, maxstep, maxperiod etc and the output does not change at all.

 Am I missing something fundamental here?  Is there something that can throw off the results by so much?

Parents

  •  Hi Andrew,

    Thanks for the quick reply.

    1.  I have done transient simulations for sanity check, the output has a peak of 225mV as expected.

    2.  As you suggested, I re-ran the simulation with a clock harmonics set to 10, still same results.  I am using Shooting method as suggested by Spectre manual for SC circuits.

    Do you still suggest customer service?  One thing that does bother me from the spectre output log is that the simulation does not report any Newton Iterations.  There are 2 PSS iterations, 3 Stabilization iterations but no mention of Newton iterations which puzzles me quite a bit.  The simulation still completes without error or warning...
Reply

  •  Hi Andrew,

    Thanks for the quick reply.

    1.  I have done transient simulations for sanity check, the output has a peak of 225mV as expected.

    2.  As you suggested, I re-ran the simulation with a clock harmonics set to 10, still same results.  I am using Shooting method as suggested by Spectre manual for SC circuits.

    Do you still suggest customer service?  One thing that does bother me from the spectre output log is that the simulation does not report any Newton Iterations.  There are 2 PSS iterations, 3 Stabilization iterations but no mention of Newton iterations which puzzles me quite a bit.  The simulation still completes without error or warning...
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