RE: [SI-LIST] : AC Coupling vs DC Coupling

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From: Farrokh Mottahedin (Farrokh.Mottahedin@quantum.com)
Date: Mon Apr 17 2000 - 12:36:29 PDT


I was under the impression that AC coupling is mainly used in cabled
environments where we are trying to avoid common mode offsets and noise
terms and there is concern for ground loops. Is this correct? If so, and
if your signalling is contained to the backplane, DC coupling is a good
choice. Also, assuming that you are using a start/stop protocol (and not
using a continuous PHY like Fibre Channel), getting stable voltage levels
and signals through the capacitor could take many microseconds. For these
and other reasons (galvanic isolation), the 1394 bus standard defined AC
coupling with 8b/10b for their S800 cabled topology while retaining the DC
coupled backplane profile.

Regards,

Farrokh Mottahedin

Quantum Corp.
500 McCarthy Blvd.
Milpitas, CA 95035
(408)324-7934
farrokh.mottahedin@quantum.com

-----Original Message-----
From: Bryan Robb [mailto:brobb@lsil.com]
Sent: Monday, April 17, 2000 9:45 AM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : AC Coupling vs DC Coupling

Hi,

I am trying to determine the pros and cons of using
AC vs DC coupling in a backplane application. From looking
at several different types of systems, it appears as if
both are used in some form or another. I am thinking
about DC coupling the system, but I don't want to overlook
any compelling reason to AC couple.

Furthermore, I am trying to maximize my data transfer and
do not want to incur the penalty of an encoding scheme like
8b/10b. So if I don't encode the data stream in some manner,
the data will not be DC balanced which would automatically rule
out AC coupling. Would non-DC balanced data cause a problem in a
system that is DC Coupled? I am speaking about data transfer
rates in the 622 to 800 MHz range.

Thanks in advance,

Bryan

-- 
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*
   Bryan Robb                           LSI Logic Corporation
   Engineering Manager                  200 West Street
                                        Waltham, MA 02154
   Phone  (781) 290-0238               
   Fax    (781) 890-6158
   Pager  (888) 524-9468

email: brobb@lsil.com **************************************************************************** *

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