RE: [SI-LIST] : Comments from your SI seminar (SendII)`

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From: Doug Brooks ([email protected])
Date: Thu Oct 28 1999 - 13:23:29 PDT

At 03:30 PM 10/28/99 -0400, you wrote:
> I have read an article recently that states that the use of
>specifying the differential impedance of two traces on a circuit board is
>unnecessary. The only thing you need to worry about is the individual trace
>impedance. If you need a differential impedance for two lines to be 100
>ohms, just use two 50 ohm lines rather than using two signals whose
>differential impedance is 100 ohms. Also when connecting a 110 ohm twisted
>pair to PCB you should just connect it to two 55 ohm traces to achieve the
>110 ohm differential impedance. I have done enough research to draw my own
>conclusions, but I would like to get the reaction from people in this forum
>regarding this issue.
>Thank you,
>Paul Denomme
>Viasystems Inc.

You might be interested in my column "Impedance Terminatios" in the June 99
issue of the magazine, reprinted on our web site. It shows that we ALWAYS
must consider adjacent trace coupling (the bad news) but that most of the
time the degreee of coupling turns out to be negligible (the good news) ---
unless there is close differential or common-mode coupling.

Doug Brooks
Doug Brooks, President [email protected]
UltraCAD Design, Inc.

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