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Simulators >> Circuit Simulators >> Different results for same netlist
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Message started by Cri Azzolini on Jan 26^th, 2006, 1:44am

Title: Different results for same netlist
Post by Cri Azzolini on Jan 26^th, 2006, 1:44am

Hi all,

I would like to submit a question regarding Spectre simulator.
Sometimes it happens to me that an analog design netlisted as a flat circuit (flat netlist) produces a certain output whereas the SAME circuit netlisted using hierarchical schematics produces different results. I heard this queer behaviour happened also to colleagues of mine but I would like to understand the reason.
Can anyone explain this point?

Thanks in advance,
Cri

Title: Re: Different results for same netlist
Post by Paul on Jan 27^th, 2006, 2:21pm

Hi Cri,

as I am using hierarchical netlists by default, I haven't experienced such problems. Intuitively, I would tend to blame the netlister and not the simulator. In any case, make sure the netlister renetlists all the blocks, these guys having a bad habit to keep older parts somewhere in memory to reduce the netlisting duration.

Paul

Title: Re: Different results for same netlist
Post by Cri Azzolini on Jan 28^th, 2006, 12:42pm

Hi Paul,

I am going to follow your advice and I will check for a complete re-netlisting. I am also quite sure that the netlister is the cause but this means that the simulator (in this case Spectre) can solve the circuit's equations depending on how the circuit is described (i.e. flat vs hierarchical)! It seems to me very strange since I can not imagine how this is possible: I mean, the equations are the same and the initial condition are (I suppose) the same.

In the meanwhile I will try to find an idea but I would be grateful if anybody out there had an explanation.

Bye,
Cri

Title: Re: Different results for same netlist
Post by Paul on Jan 29^th, 2006, 1:12pm

Hi Cri,

what I was trying to say is that IMHO the netlister doesn't generate a correct flat netlist and that's why the simulation results differ. I don't think Spectre does any difference on this...

Paul

Title: Re: Different results for same netlist
Post by Andrew Beckett on Jan 30^th, 2006, 2:09am

There is always the possibility that you have a circuit that has multiple operating points - and that the difference in order of processing is causing the numerical algorithms to prefer one solution over the other (i.e. both are correct, but there's an element of luck to which solution you'll get).

However, the most likely explanation is that the circuit is not in fact identical. I don't know how you're generating a flat netlist (this used to be supported in the spectreS interface, but not in the newer spectre interface) - it might just be that you have two different schematics - in which case perhaps you entered something wrongly.

I'd compare the netlists and see if you can spot any difference.

Best Regards,

Andrew.