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Simulators >> Circuit Simulators >> SPICE noisefloor vntol
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Message started by Visjnoe on Dec 3^rd, 2007, 11:36pm

Title: SPICE noisefloor vntol
Post by Visjnoe on Dec 3^rd, 2007, 11:36pm

Dear all,

assuming I want to monitor a certain voltage on a node, what sets the SPICE numerical noisef loor?
I would think this is set by vntol, but with a default value of vntol = 1E-6 this would mean that SPICE has a default noise floor 120dB, which seems like a lot....

Any insights more than welcome.

Kind Regards

Peter

Title: Re: SPICE noisefloor vntol
Post by Ken Kundert on Dec 4^th, 2007, 12:55am

The simple answer is reltol, but this is not a simple question.

-Ken

Title: Re: SPICE noisefloor vntol
Post by Visjnoe on Dec 4^th, 2007, 1:53am

Dear Ken,

if I understand you correctly, the SPICE noisefloor is much more dependent on reltol and further on, it is hard to make calculations based on this parameter to predict the noise floor? How can one than derive the correct simulator settings for e.g. simulating a 24b ΔΣ modulator?

Regards

Peter

Title: Re: SPICE noisefloor vntol
Post by byang on Dec 4^th, 2007, 2:04pm

Usually it is more accurate to use reltol. vntol is absolute error tolerance. vntol kicks in only if reltol is too tight. If reltol is 1e-3 and the overall voltage level is 3v, then the real numerical error is about 3*1e-3 = 3e-3, i.e. the error is much bigger than 1e-5. If reltol is 1e-4, then the error is 3e-4, and so on. This is a very rough estimate. Unfortunately Spice simulation is much more complicated than this. So the real numerical noise floor is hard to estimate accurately.

Usually one doesn't need to set reltol to be smaller than 1e-4 though.

byang
http://www.gemini-da.com