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Design >> Analog Design >> Currents are not steady in AMS simulator ... why?
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Message started by pbennett on Sep 20^th, 2013, 8:07am

Title: Currents are not steady in AMS simulator ... why?
Post by pbennett on Sep 20^th, 2013, 8:07am

I have found that when I plot or measure currents in the AMS simulator that they do not appear to be steady. Why is this?

I have simulated several different designs ... mostly simple current mirrors and several other circuit blocks and I do not find that the current in AMS/spectre to be solid number like 10 uA ... instead it will numerically oscillate between 9.9 and 10.1ua in a very unpredictable and unsteady way ...

Do I have some accuracy setting wrong ?? Have others seen this behavior before?

Title: Re: Currents are not steady in AMS simulator ... why?
Post by pbennett on Sep 20^th, 2013, 10:39am

well it seems that just by tightening RELTOLfrom 1e-3 to 1e-5 ... makes a world of difference.

I would still be interested if anyone has any additional guidance regarding this issue.

The simulator of course runs much slower but at least the answers seem to be correct!

Title: Re: Currents are not steady in AMS simulator ... why?
Post by Ken Kundert on Sep 20^th, 2013, 3:24pm

Sounds like trapezoidal-rule ringing. Is it a point-to-point ringing? If so, switch the transient integration method to gear2.

-Ken

Title: Re: Currents are not steady in AMS simulator ... why?
Post by pbennett on Sep 20^th, 2013, 4:50pm

Yes it seems to be point to point ringing.

I will try your suggestion to move to gear2 method.

TNKS :)