RE: [SI-LIST] : via effect

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Fri Jun 01 2001 - 11:23:03 PDT


> i have a question .is the presense of a via along a trace inductive or
> capacitive?
 
Yes. Both.

Seriously, though, at a high speed design seminar I attended several years
ago, the instructor stated that the answer depends on where you live. Most
engineers on the USA west coast treated vias as inductance, whereas those on
the east coast treated vias as excess capacitance. But this was some time
ago.

If you treat the via as a "thing" between the ends of two traces, then
surely it has both. It has inductance to get from one layer to the other,
and capacitance of the pads and the barrel to the planes.

Then there is the big picture, which includes the return path (actually
required in order to think about the inductance) ... which is probably not
continuous from one layer to the other. That might add inductance that you
wouldn't otherwise consider if you were looking at only the via barrel
itself.

Andy

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