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

From: Scott McMorrow ([email protected])
Date: Thu Mar 15 2001 - 10:15:36 PST

All,

Chris is correct, but to better clarify his statement can
be restated as follows:

AC current flows along the path of least impedance at
any particular frequency.

In some cases that path is resistive (DC).
In some cases that path is inductive. (AC)
In some cases that path is capacitive. (AC)

regards,

scott

```--
Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
Tualatin, OR  97062-3090
(503) 885-1231
http://www.siqual.com
> Dear Chris,
>
> "Reid, Chris" wrote:
>
> > 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.
>
> Are you sure? I thought Howard might have some explanation/interpretation.
>
> > 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.
>
>    ^^^^^^^^^^^^^^^^^^^
> Are you sure? You are not relating inductance to ohm's law, I suppose.
>
> Just want to make sure.
>
> Thanks,
> Sainath
>
> >
> > Chris
> >
> > -----Original Message-----
> > From: Sainath Nimmagadda [mailto:[email protected]]
> > Sent: Wednesday, March 14, 2001 6:19 PM
> > To: Howard Johnson; [email protected]
> > Subject: Re: [SI-LIST] : Re: approximations for partial self inductance
> >
> > Dear Howard,
> >
> >
> > 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
> > > 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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```

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