From: D. C. Sessions (email@example.com)
Date: Tue Feb 27 2001 - 20:35:23 PST
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 <firstname.lastname@example.org> ----------+
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