[SI-LIST] Re: Choosing W and S for Differential signal?
- From: "Kai Keskinen" <kalevi@xxxxxxxxxx>
- To: <ched_chang@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 18 Mar 2004 17:39:45 -0500
Chai:
Several of the gurus will tell you that what you actually want are loosely
coupled pairs. Howard Johnson discusses this in some of his presentations. A
trace width that is a little less than you need to get single ended 50 Ohms
with the pair spacing such that you achieve 100 Ohms will achieve this.
There are several benefits to this approach including less impedance
discontinuities when you have to separate the pair when going through a pin
field.
Regards,
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Ched-Chang Chai
Sent: Thursday, March 18, 2004 6:07 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Choosing W and S for Differential signal?
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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