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Design >> Analog Design >> too low thermal noise in 5.1.41 version
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Message started by imagelimit on Aug 25^th, 2004, 9:33am

Title: too low thermal noise in 5.1.41 version
Post by imagelimit on Aug 25^th, 2004, 9:33am

Somehow MOS in triode mode gives me 3 times less thermal noise than an equivalent resistor. This was not a case in older versions of spectre.
Anybody can reproduce this problem?

imagelimit.

Title: Re: too low thermal noise in 5.1.41 version
Post by Andrew Beckett on Aug 26^th, 2004, 12:55am

Which MOS model are you using? Can you post the model card here (I understand it might be proprietary, so don't break any rules)?

Andrew.

Title: Re: too low thermal noise in 5.1.41 version
Post by Vivek on Aug 31^st, 2004, 12:27am

I have a similar problem trying to simulate the effect of opamp noise while using an ideal OTA. I get exactly the amount of noise I expect when I use a CT circuit and the OTA, but when I try to use the OTA in a switched-cap circuit, I dont see any noise at all (almost none). I was just trying to check if the simulator would be able to help me compute the overall thermal noise in my SC circuit, accounting for effects that I might have overlooked. However, I am unable to make it behave and so have resigned myself to using my hand-calculations, which are not bad thankfully.

I even tried using hundreds of sidebands and simulating far beyond my clock frequency range to allow for enough noise aliasing.

You should try to do handcalculations as well. I think pac might work well in terms of predicting the NTF from any node to the output but for some curious reason, pnoise is not behaving well for OTA noise even though it works well for kT/C noise.

If anyone knows how to fix this problem, I would be glad to know. There are some bigger circuits that are harder to analyze at times.

Sorry, I cant post the modelcard for proprietary reasons, but I doubt if that has anything to do with this problem, especially since I see the exact amount of noise while using a CT circuit.

Vivek

Title: Re: too low thermal noise in 5.1.41 version
Post by Frank Wiedmann on Aug 31^st, 2004, 2:33am

Could you show us the problem on a simple example circuit that does not use any proprietary models?

Title: Re: too low thermal noise in 5.1.41 version
Post by 100questions on Sep 1^st, 2004, 1:01pm

Vivek, does this happen in older versions too?