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SPICE noisefloor vntol (Read 3175 times)

Visjnoe Senior Member Offline Posts: 233	SPICE noisefloor vntol Dec 03^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
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Ken Kundert Global Moderator Online Posts: 2386 Silicon Valley	Re: SPICE noisefloor vntol Reply #1 - Dec 4^th, 2007, 12:55am The simple answer is reltol, but this is not a simple question. -Ken
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Visjnoe Senior Member Offline Posts: 233	Re: SPICE noisefloor vntol Reply #2 - 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
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byang

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Re: SPICE noisefloor vntol
Reply #3 - 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

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