Re: [SI-LIST] : Routing Differential Pairs as 100 differentially Vs individually 50 ohm lines

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From: Ravinder Ajmani (ajmani@us.ibm.com)
Date: Wed Nov 22 2000 - 12:50:05 PST


Following your advice, how can one route 27 pairs of SCSI signals while
keeping spacing of 25 mils or more between differential signals, and how
much spacing needs to be kept between adjacent pairs.

Regards, Ravinder
PCB Development and Design Department
IBM Corporation - Storage Systems Division
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Ritchey Lee <leeritchey@earthlink.net>@silab.eng.sun.com on 11/22/2000
09:38:00 AM

Please respond to Ritchey Lee <leeritchey@earthlink.net>

Sent by: owner-si-list@silab.eng.sun.com

To: "Gaboian, Jerry" <j-gaboian@ti.com>
cc: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Routing Differential Pairs as 100 differentially
      Vs individually 50 ohm lines

Those comments are from my class.  That is exactly how differential signals
should be routed.  Routing them close to each other produces interaction
that reduces the final amplitude.  I route 2.5 GB differential signals as
two 50 ohm lines separated by at least 25 mils.

Lee

Gaboian, Jerry wrote:

I am looking for some feedback on the topic routing LVDS differential pairs
as 100 ohms
with respect to each other Vs. 2 separate 50 ohm lines.

At the PCB East conference several keynote speakers referenced the
following comment:

"No need to specify differential impedance between two lines. Routing
traces side by side
does not guarantee noise immunity from common mode noise. On the contrary,
noise from an
adjacent line will couple as a differential signal"

The length of the pairs should be matched and kept close to a reference
plane (about 5 mils)is the must important criteria.

Comments, suggustions.......

Thanks,
Jerry Gaboian

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