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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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  • Hi Adi,

    perhaps the circuit has two or more DC operating points, in other words the solutions are metastable states? One solution is a dcOp and the other starts from a dcOp and the voltage is swept - this can lead to different solutions.  What kind of circuit is it?  Have you tried using initial conditions rather than nodesets?  I believe the former is a "suggested starting point" whereas the latter is more of a fixed quantity - if the nodeset values are not correct, this will force the simulation in a certain direction which may be why you arrive at a different solution?

    Andrew will have more useful input I'm sure (and may even correct me if I'm wrong), but hopefully the above might be useful to you?

    Regards,

    Lawrence.

Reply

  • Hi Adi,

    perhaps the circuit has two or more DC operating points, in other words the solutions are metastable states? One solution is a dcOp and the other starts from a dcOp and the voltage is swept - this can lead to different solutions.  What kind of circuit is it?  Have you tried using initial conditions rather than nodesets?  I believe the former is a "suggested starting point" whereas the latter is more of a fixed quantity - if the nodeset values are not correct, this will force the simulation in a certain direction which may be why you arrive at a different solution?

    Andrew will have more useful input I'm sure (and may even correct me if I'm wrong), but hopefully the above might be useful to you?

    Regards,

    Lawrence.

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