[SI-LIST] Choosing W and S for Differential signal?
- From: "Ched-Chang Chai" <ched_chang@xxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 18 Mar 2004 19:06:36 +0800
Dear all,
I have doubt in choosing trace width (W) and separation (S) for differential
signals.
(1) According to some articles/books, we should use the smallest S that can
be made by the PCB maker for better coupling and EMC control. However for
certain differential signal, the smallest S can not satisfy the required
Zdiff. Since PCB stack-up has been fixed earlier in some cases, we cannot
change the stack-up to get the required Zdiff. In such case, which is more
important: to satisfy the required Zdiff by increasing S or still use the
smallest S?
(2) For certain differential signals, we need to satisfy both Zdiff for the
pair and Zo of a single line in the pair. For example, DVI requires
Zdiff=100+/-15% and Zo=50+/-15%. As such, it is very difficult to fulfill
both at the same time. Again which is more important: Zdiff or Zo?
Please advise.
Thank you very much in advance.
--
Regards,
Chai
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