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

Mike Jenkins (jenkins@lsil.com)
Mon, 12 Jul 1999 14:01:48 -0700

mcheong@molex.com 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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