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Design >> Analog Design >> Spectre Warnings
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Message started by mateenfl on Oct 14^th, 2003, 11:18am

Title: Spectre Warnings
Post by mateenfl on Oct 14^th, 2003, 11:18am

Hello All,

I get the following two message from the Spectre simulator

"Warning from spectre during initial setup.

M1: Cdscd = -28.83e-6 is negative.
M1: Parasatic resistor 'rs' has been deleted because its value of 26 mOhm (R/MFactor) was smaller than minr"

Can any one guide me on the reasons for the warnings and their implications if any (e.g during transient, noise simulations etc)

Kind Regards,
Faisal Mateen

Title: Re: Spectre Warnings
Post by Andrew Beckett on Oct 18^th, 2003, 8:08am

Well, the first one is fairly evident. You have a negative
capacitance in the device model.

The second is because small resistors are not a good idea
for spectre, because it leads to convergence difficulty
(put shortly, this is because of trying to ensure Kirchoff's
laws are maintained, and so when the normal numerical
methods are used to compute the voltages at the resistor
terminals, these would have to be done to excessive
precision in order to get the current across this small resistor
to be computed correctly to meeting Kirchoff's law). I
think this is covered rather more eloquently/clearly in Ken's
book, if I remember rightly.

Regards,

Andrew.