[SI-LIST] Re: diff pair termination

Gopalakrishnan,

For each family you may find a preferred way, but in general,
I dont think there is a standard way to terminate differential signals.

Similar to single-ended signals, you can terminate the source, the
far end, both, or you can play with 'distributed' terminations at
several locations (in case of multipoint connections).  In the selection
among these, the same principles apply as in single ended signaling.
Some basic guidelines: source termination is useful in point-to-point
one-directional connections, far-end termination is useful in multi-point
connections, and distributed termination can be helpful if you have a
plug-in system with variable configuration.

Two things will further add to the above options in differential signaling.
First, unless you use loosely coupled traces for differential signaling,
you now have two impedances to terminate instead of one: differential
mode and common mode.  You can elect not to terminate for common
mode, as long as you ensure that common-mode noise and reflection
will not bother your differential signaling.  Second, several differential
drivers require a DC bias path, and will not work without it.  Most
implementations of GTL, BTL, and several implementations of PECL
and some LVDS drivers require a separate DC current to be provided
to the output.  This is usually done by an appropriate bias resistor, but
it may or may not serve as termination at the same time.  For instance,
DC bias resistors for an emitter or source-follower PECL output will
not provide source termination at the same time; bias resistors for
open-collector devices, such as GTL, BTL or in some LVDS drivers,
can do both biasing and source termination.

Regards,

Istvan Novak
SUN Microsystems

----- Original Message -----
From: "Gopalakrishnan Sethuraj" <gopalak@xxxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Sunday, June 20, 2004 11:45 PM
Subject: [SI-LIST] diff pair termination


> Hello Gurus,
> I would like to know the information about the standard differential
> pair termination used in present high speed design world
>
> I had gone through some of the article/doc from standard vendor like TI,
> Onsemi . But every vendor defining their own termination scheme for diff
> pair for PECL, GTL, BTL interfaces.
>
> Is there any standard termination available for diff pair?
>
> Could anyone pls provide the standard diff pair for PECL, LVPECL, GTL,
> BTL ... interfaces?
>
>
> Thanks&Regards
> Gopalakrishnan
>
>
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