From: Jim Freeman ([email protected])
Date: Tue Dec 21 1999 - 14:42:30 PST
The issue with the capacitor would be assymetrical CMV that would cause an offset of the levels that are seen by the receiver. As you said, the CMV
can come from several sources as well as to either polarity of the differential signal. The capacitor would charge to the average value of current
recieved from these sources and in an assymetrical situation would cause the differential mode ground at the receiver to be different than the
differential mode ground in the transmission line.
[email protected] wrote:
> 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
> Michael L. Conn
> Owner/Principal Consultant
> Mikon Consulting
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