[Fwd: Re: [SI-LIST] : The correct way to short tow nodes in SPICE is?]

About this list Date view Thread view Subject view Author view

From: Kim Helliwell (khelliwe@acuson.com)
Date: Fri Oct 20 2000 - 08:39:10 PDT


Bo wrote:
>
> Hi
> 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?
>
> Regards,
> Bo
>
> __________________________________________________
> Do You Yahoo!?
> Yahoo! Messenger - Talk while you surf! It's FREE.
> http://im.yahoo.com/
>
> **** To unsubscribe from si-list or si-list-digest: send e-mail to
> majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
> si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP.
> si-list archives are accessible at http://www.qsl.net/wb6tpu
> ****

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

**** To unsubscribe from si-list or si-list-digest: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu ****


About this list Date view Thread view Subject view Author view

This archive was generated by hypermail 2b29 : Tue May 08 2001 - 14:29:48 PDT