From: Kim Helliwell (firstname.lastname@example.org)
Date: Fri Oct 20 2000 - 08:39:10 PDT
> I would like to discuss one of the big SPICE myths. I have used several
> different spice programs (PSPICE, HSPICE, etc) and I have seen two different
> implementations. The problem is that nobody can agree which is the better
> implementation of the two. I would like to hear your opinion on this.
> The two implementations are:
> 1) Connect two nodes to each other by putting 0 Ohm resistor.
> 2) Connect two nodes to each other by putting 0 volt DC source between them.
> Method 1 typically results in resistance set to some small value other than 0
> (by default HSPICE will set it to 1e-5; this can be changed in Hspice). I have
> trying to convince people using this method to at least put several of
> resistors in parallel(just in case someone changed default values; in HSPICE
> this is easily done my setting multipler M to some high value).
> What is your opinion on this?
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I have supported various versions of SPICE for more years than I care to
mention, and, except in Spectre, I *DO NOT* recommend using 0-ohm resistors.
The correct way to model a short is a voltage source of 0 volts. The
reason for this is that 0-ohm (or very small resistors) will introduce
a large diagonal element in the matrix representing the circuit, which can
cause large roundoff errors during the solution. This is not a good thing
to do. It can cause inaccuracy of your solution or even nonconvergence.
The reason Spectre is different in this regard is that, for small resistances
(including 0 ohms), the simulator uses a branch form for modeling the resistor,
which (guess what?) is exactly the same as putting in a 0-volt voltage source.
So in Spectre, you have a choice, but the end result is the same.
-- Kim Helliwell Senior CAE Engineer Acuson Corporation Phone: 650 694 5030 FAX: 650 943 7260
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