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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
Parents
  • Thanks, Lawrence. I'm actually quite familiar with the operation of IC and nodeset and I believe I am using them correctly. In fact, IC's are ignored during DC simulations, so they would have no affect. I think that the problem does have something to do with a metastable state, as you mentioned. However, the DC Sweep should have the exact same result when simulating the same point as the DCOp - I think they're equivalent by definition (a DC Sweep is a sweep of DCOps)... Adi
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  • Thanks, Lawrence. I'm actually quite familiar with the operation of IC and nodeset and I believe I am using them correctly. In fact, IC's are ignored during DC simulations, so they would have no affect. I think that the problem does have something to do with a metastable state, as you mentioned. However, the DC Sweep should have the exact same result when simulating the same point as the DCOp - I think they're equivalent by definition (a DC Sweep is a sweep of DCOps)... Adi
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