Re: [SI-LIST] : Physcially-small far-end LVDS terminations?

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From: MikonCons@aol.com
Date: Tue Dec 21 1999 - 14:03:14 PST


Although many of the comments I have read in this thread have merit, a few of
the participants in the subject discussion either have lost sight of other
(perhaps "unknown unknowns") factors that need consideration for a more
robust system design, or perhaps they have only designed in an electrically
"clean" environment. With a purely modeling environment, this is a
particularly easy trap to fall into by overlooking additional noise sources
and not including them in the model.

The use of two series 50 ohm resistors between the two differential receiver
inputs, center-tapped with a capacitor to the local (receiver) ground is far
more tolerant of common-mode noise (CMV) than a simple 100 ohm differential
load. The arriving CMV is bypassed to the local ground, effectively reducing
the CMV seen by the receiver by at least 6 dB (based on individual 50 ohm
lines, loaded by 50 ohm resistors, ac-grounded via the capacitor). Note that
the CMV signal may come from an externally impinging source as well as a
poorly designed transmitter that is directly connected. If no ac coupling to
the local reference is provided, the CMV magnitude could easily exceed the
tolerance range of the receiver input because the CMV load impedance is
extremely high (not existent).

The variation in jitter (from imposed CMV) astutely noted by DC Sessions
would also be reduced (though not eliminated) by the center-tapped
configuration.

Respectfully,

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

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