Re: [SI-LIST] : Parallel single-ended traces routed to achieve 35Ohm controlledimpedance

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From: Scott McMorrow (scott@vasthorizons.com)
Date: Tue Oct 31 2000 - 15:13:59 PST


Salvador,

There is nothing wrong with your idea. A driver which drives two 70 ohm
traces will see a 35 ohm impedance. There are some other issues which
you will want to look at carefully.

First, assuming that this is a reflected wave circuit, you will need to
make sure that the trace delay for both traces is close to equal. Otherwise,
you will get a nasty little ledge in the middle of the signal transition, the
duration of which will be equal to the difference in delay between the two
traces.

Second, you need only provide ground stitch vias near the layer
change vias for the traces. This provides a place for the AC fields to
follow as the signal changes layers.

Otherwise, your design should work just fine.

regards,

scott

--
Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
Tualatin, OR  97062-3090
(503) 885-1231
http://www.siqual.com

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