**From:** J. Eric Bracken (*bracken@ansoft.com*)

**Date:** Fri Oct 20 2000 - 11:35:34 PDT

**Next message:**Ingraham, Andrew: "RE: [SI-LIST] : The correct way to short tow (two) nodes in SPIC E is?"**Previous message:**John Fisher: "RE: [SI-LIST] : Differential TDR probes"**In reply to:**Bo: "[SI-LIST] : The correct way to short tow nodes in SPICE is?"

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
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*> 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.
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*>
*

Method 2--using a zero-valued voltage source--is the best way.

The reason has to do with the way Spice handles resistors. It writes a KCL

equation at every circuit node. So if you have a resistor R from node i to node

j,

the contribution of the resistor to KCL at node i is something like this:

vi / R - vj / R

For a zero-valued resistor, 1/R is infinity. The software can't permit this

infinite value to enter into its matrix of equations, so it forces R to some

small minimum value. This minimum value varies from one Spice

implementation to the next.

For a voltage source between nodes i and j, Spice actually writes a

separate KVL equation:

vi - vj = 0

and this is enforced exactly. (At least to the numerical precision of the

matrix solver, which is pretty darn good.)

Only reason I can think of for preferring a small resistor is to avoid

problems caused by loops of voltage sources--which you shouldn't

be creating anyway.

--Eric

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**Next message:**Ingraham, Andrew: "RE: [SI-LIST] : The correct way to short tow (two) nodes in SPIC E is?"**Previous message:**John Fisher: "RE: [SI-LIST] : Differential TDR probes"**In reply to:**Bo: "[SI-LIST] : The correct way to short tow nodes in SPICE is?"

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