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Irregularity during DC Sweep Simulation

aditeman
Hi all (or specifically to Andrew), I have noticed a strange irregularity during a DC Sweep simulation scenario and I would like to know if anyone knows what could be wrong. I ran DCOp simulations on a test circuit changing a parameter (the supply voltage, VDD) and got certain results. I used nodeset statements to aid convergence. (values from 0.1V to 1.1V) Then I reran the simulations using a DC Sweep on the same parameter (VDD) from 0.1V to 1.1V, however the results were completely different, specifically at the same points I simulated independently. For reference, I measured the DC voltage at a certain node with VDD=1.1V. In case A it was 513mV and in case B it was 81mV. I played around a bit - for example, I marked "restart=yes" (even though this should be the default) thinking that maybe it was using the former solution as the next nodeset. Same result. Then - and this is what completely throws me off - I changed the sweep region to 0.2V to 1.1V and I got the expected results (i.e. with VDD=1.1V I got 513mV). The only thing I can think of is that the 0.1V point is used as a nodeset for subsequent sweeps even though restart=yes. If that is the reason - is there a workaround? Thanks, Adi Teman
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  • Adi,

    It may be a bug, although I don't expect it is. If you have multiple operating points, it is very hard to guarantee which you will get - the slightest little thing can cause it to roll down either side of the mountain into the two valleys (to use an analogy).

    Restart does start the op point from scratch, but it is using the matrix built from the previous run, and so the ordering within the matrix can affect the solution. Rebuilding the matrix usually then forces the sim to start from scratch.

    That said, if you are doing a dc sweep vs a parametric sweep of dcOp (I can't see the previous posts in the threads as I'm travelling), that could explain it. The restart won't cause it to reset within a DC sweep (I think - it's primarily for how it handles a dc sweep within a parametric analysis or sweep analysis, rather than between the points in the sweep) and rebuilding the matrix won't help.

    I don't think there's an easy solution other than the one you've done.

    Andrew
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  • Adi,

    It may be a bug, although I don't expect it is. If you have multiple operating points, it is very hard to guarantee which you will get - the slightest little thing can cause it to roll down either side of the mountain into the two valleys (to use an analogy).

    Restart does start the op point from scratch, but it is using the matrix built from the previous run, and so the ordering within the matrix can affect the solution. Rebuilding the matrix usually then forces the sim to start from scratch.

    That said, if you are doing a dc sweep vs a parametric sweep of dcOp (I can't see the previous posts in the threads as I'm travelling), that could explain it. The restart won't cause it to reset within a DC sweep (I think - it's primarily for how it handles a dc sweep within a parametric analysis or sweep analysis, rather than between the points in the sweep) and rebuilding the matrix won't help.

    I don't think there's an easy solution other than the one you've done.

    Andrew
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