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March 1999

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Subject:
From:
"Peterson, Gary D" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 18 Mar 1999 10:26:20 -0700
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Joe,

The first question is whether you are actually driving the traces with a
differential signal.  If so, the single-ended impedance of the traces
doesn't matter.  You really need to specify the differential impedance of
the pair(s).  Hyperlynx Signal Integrity software does a great job of
describing the physical constraints required to get a true differential
impedance from a signal pair in a pwb.  It also gives the single-ended
impedance of the traces should you wish to terminate for both single-ended
AND differential signaling.

Summary:  It is quite possible to specify a single-ended impedance for a
differential pair and end up nowhere close to the desired differential
impedance.  Many PWB fabricators can do the job for you if you just identify
the differential pairs and the desired differential impedance.

Gary D. Peterson
Sandia National Labs

-----Original Message-----
From: Hurst, Joe [mailto:[log in to unmask]]
Sent: Thursday, March 18, 1999 6:21 AM
To: [log in to unmask]
Subject: [TN] Specifying Differential Impedance


Hi

        I have some very important differential pairs on a PWB that must
have a particular differential impedance.

        The problem is:  how can I specify this on the drawing sent to the
fab shop?

        Our usual method would be to call for a particular single-ended
impedance, which seems to be beating around the bush.     It work to a
point; but I think it leaves too many variables that could change the
differential impedance.

        Any thoughts on this?


Joe Hurst
PWB Designer
Computing Devices Canada
A General Dynamics Company
[log in to unmask]
613-596-7816

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