# Re: [SI-LIST] : Via Capacitances ...

Gregg Fokken ([email protected])
Mon, 12 Jul 1999 17:06:39 -0500

Mike (Jenkins),

Doesn't Zo*C/2 really only apply if the capacitor is truly a lumped
element?
I still thought of a via as a distributed LC circuit which often "appeared"
more capacitive than inductive, however, if I happened to build a via with
an impedance which matched my line, then I would get no reflection and
according to your formula, no capacitance, when in fact the via still has
capacitance associated with it (only the "ratio" of L to C matches that of

On Jul 12, 2:01pm, Mike Jenkins wrote:
> Subject: Re: [SI-LIST] : Via Capacitances ...
> [email protected] wrote:
> >
> > I agree with Ron's email on via being nothing but a capacitive stub.
> >
> > One can verify that by measuring a typical via with a TDR.
> > Run a length of trace with an open via in the middle and launch a TDR
signal
> > (~35pS) from one end. Make sure the line is terminated in its
characteristic
> > impedance. You will discover a negative reflection (capacitive
discontinuity).
> >
> > The equivalent capacitance can be derived by utilizing the reflected
voltage,
> > risetime, incident voltage and Zo.
> >
> > Michael Cheong
>
> Michael,
>
> Using risetime to calculate capacitance from a TDR trace is
> problematic since it continually degrades. For a capacitance,
> C, along a transmission line of impedance Zo, the AREA of the
> reflection is the time constant Zo*C/2, exactly. (This can be
> shown from the LaPlace final value theorem.)
>
> Regards,
> Mike
>
> _____________. . . . . . . . ._____________
> / \ _/
> / \ Zo*C/2 __/
> / \________/
> /
> ____/
> --
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>-- End of excerpt from Mike Jenkins

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