**From:** Reid, Chris (*chris_reid@mentorg.com*)

**Date:** Thu Mar 15 2001 - 09:00:44 PST

**Next message:**Sainath Nimmagadda: "Re: [SI-LIST] : Re: approximations for partial self inductance"**Previous message:**Doug Hopperstad: "[SI-LIST] : RE: Online HSPICE Training or Tutorials"**Maybe in reply to:**Sainath Nimmagadda: "[SI-LIST] : Re: approximations for partial self inductance"**Next in thread:**Sainath Nimmagadda: "Re: [SI-LIST] : Re: approximations for partial self inductance"**Reply:**Sainath Nimmagadda: "Re: [SI-LIST] : Re: approximations for partial self inductance"**Reply:**Vinu Arumugham: "Re: [SI-LIST] : Re: approximations for partial self inductance"

Hello,

Actually, the statement:

*> It is a principle
*

*> of Maxwell's equations that high-speed returning signal
*

*> current will flow in whatever path produces the
*

*> least overall inductance.
*

is not really correct. Instead of inductance, think of

resistance for a moment. If you put two resistors in parallel,

one 100 Ohms and the other 1 Ohm the effective resistance is

1/(1/100 + 1/1) =~ 0.99 Ohm. Clearly most of the current

flows through the 1 Ohm resistor, but some also flows through

the 100 Ohm resistor.

Inductance is similar, only now the result depends on frequency.

Chris

-----Original Message-----

From: Sainath Nimmagadda [mailto:sainath@lsil.com]

Sent: Wednesday, March 14, 2001 6:19 PM

To: Howard Johnson; si-list@silab.eng.sun.com

Subject: Re: [SI-LIST] : Re: approximations for partial self inductance

Dear Howard,

Please see below:

Howard Johnson wrote:

*> Dear Itzhak Hirshtal and Brian Young,
*

*>
*

*> The difficulties with approximating the inductance
*

*> of a via are even worse than you
*

*> may have suspected. Both approximations are flawed whether
*

*> you use +1 or -3/4, (or, as I have also seen, -1).
*

*>
*

*> The issue of the exact constant (1, -3/4, or something
*

*> else) depends critically on your assumption about
*

*> the path of returning signal current. (Current always
*

*> makes a loop; when signal current traverses the via,
*

*> a returning signal current flows SOMEWHERE in
*

*> the opposite direction.). It is a principle
*

*> of Maxwell's equations that high-speed returning signal
*

*> current will flow in whatever path produces the
*

*> least overall inductance.
*

My question is on this last statement. I like to understand which

Maxwell's equation suggests this and how? Thanks.

Sainath

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**Next message:**Sainath Nimmagadda: "Re: [SI-LIST] : Re: approximations for partial self inductance"**Previous message:**Doug Hopperstad: "[SI-LIST] : RE: Online HSPICE Training or Tutorials"**Maybe in reply to:**Sainath Nimmagadda: "[SI-LIST] : Re: approximations for partial self inductance"**Next in thread:**Sainath Nimmagadda: "Re: [SI-LIST] : Re: approximations for partial self inductance"**Reply:**Sainath Nimmagadda: "Re: [SI-LIST] : Re: approximations for partial self inductance"**Reply:**Vinu Arumugham: "Re: [SI-LIST] : Re: approximations for partial self inductance"

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