RE: [SI-LIST] : Combined single-ended/differential termination pa ckage?

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From: Larry Miller (ldmiller@rhapsodynetworks.com)
Date: Wed Feb 28 2001 - 11:48:14 PST


---And your recommendation for termination is ....??

Thanks,

Larry Miller
P.S. I like your codger jokes. I am heading into that zone myself.

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of D. C. Sessions
Sent: Tuesday, February 27, 2001 8:35 PM
To: Ron Miller; 'Larry Miller'; 'Eric Goodill';
si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Combined single-ended/differential termination
pa ckage?

On Tuesday 27 February 2001 12:09, Ron Miller wrote:

# Considering single-ended(unbalanced signals) all the ground current
# for each circuit must flow back to the source through that ground. Now
# visulaize a PCB with many signals and power returns flowing from one part
# of that ground to another, flowing across different parts of the board and
# in different directions someplaces orthogonal and some places in the same
# direction coupling strongly. Well, ground is not ground but a matrix of
# tranmission lines all connected together and mixing the low level
sensitive
# signals with higher current power currents. It is a mess, too complex
# to analyze thoroughly on a practical basis. So, differential signals with
# care to keep common mode currents from flowing by differentially
terminating
# is the best we can do to keep the noise/coupling effects down.

If driver devices were perfectly linear this would perhaps be ideal.
Unfortunately, drivers are quite nonlinear. As a result, common-mode
reflections mix in interesting ways with the incident signals at the driver.

In addition, of course, the receiving comparator generally has some
interesting interactions between the common-mode potential and the
receiver delay, resulting in jitter. This is particularly messy in LVDS
due to the requirement for wide receiver common-mode range, which
demands that the receiver use two differential amplifiers and mix the
results. Doing so consistently across PVT and CM is, shall we say,
Not Exactly Easy.

--
| I'm old enough that I don't have to pretend to be grown up.|
+----------- D. C. Sessions <dcs@lumbercartel.com> ----------+

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