RE: [SI-LIST] : The correct way to short tow (two) nodes in SPIC E is?

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Fri Oct 20 2000 - 11:44:14 PDT


Unfortunately, there is a tendency for circuit designers to think that a 0
ohm resistor is more economical (in $, watts, board space, etc.) than a 0
volt DC source, so many people do use the 0 (or 0.001 or ?) ohm resistor
approach. I see it a lot.

As I understand it, the reality of SPICE is that voltage sources are
mathematically simpler / less troublesome than resistors.

It requires a change of mindset from old ways that some people find
difficult to overcome.

I personally saw cases where small (~milohm or micro-ohm) resistors in
circuits introduced significant errors. Years ago I was using a derivative
of generic SPICE2 and simulating ECL logic. All the voltages "became
unhinged" when those small resistors were included. SPICE converged, but I
was seeing steady-state node voltages up around +4V rather than the normal
~-1.4V, even though there was no applied voltage greater than 0V. A clear
departure from reality. With no better explanation and little knowledge
about the inner workings of SPICE, I chalked it up to some sort of
mathematical dynamic range problem.

Because I haven't seen these kinds of obvious errors lately, I am less
likely to insist on 0V voltage sources, even though it is probably the more
correct thing to do.

Regards,
Andy

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