From: Ron Miller (firstname.lastname@example.org)
Date: Tue Feb 27 2001 - 11:09:26 PST
If differential termination is used(with a high Z comparator)
1. common mode currents do not flow from the receive lines
back to the source through the ground.
Remember that though the current of a comparator is very
low it does exist and is not perfectly balanced as in a
differential amplifier input-offset voltage and current.
This input-offset voltage/current(with respect to ground)
can only flow (causing noise in the output)if the signals
are referenced to ground near the comparator. Floating the
input does not allow this current to flow.
2. Common mode voltage will develop between the input of the
comparator and the driving source if the termination
resistors are not referencded to ground. However, from
the point of view of the comparator it cannot be detected.
So, it does not matter.
From a practical point of view I have always got my best, lowest
differential test results when floating the termination.
A FURTHER NOTE ON GROUND COUPLING:
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.
From: Larry Miller [mailto:email@example.com]
Sent: Monday, February 26, 2001 8:11 PM
To: 'Eric Goodill'; firstname.lastname@example.org
Subject: RE: [SI-LIST] : Combined single-ended/differential termination
---which brings up something that has been bothering me:
the LVDS standard goes with the single 100-ohm line-to-line resistor (which
is REQUIRED for DC biasing) but does not seem to provide any specific line
to ground termination. I know that LVDS was originally intended for cables,
but it is showing up in a lot of 2-3 Gb signals, including serial
It looks to me as though you end up with unterminated (to ground) wires in
What do you do with this?
[mailto:email@example.com]On Behalf Of Eric Goodill
Sent: Monday, February 26, 2001 6:09 PM
Subject: Re: [SI-LIST] : Combined single-ended/differential termination
Our reason for both resistors is to terminate any common-mode signal
that might be on the lines either from crosstalk or from an unbalanced
Ron Miller wrote:
> Hi Eric
> Why not just use a single resistor to terminate the differential mode,
> which is the signal you are intrested in.
> Tieing the center-tap to ground allows common mode currents to flow
> which usually end up allowing power supply ground currents to get into
> the sensitive signal paths.
> -----Original Message-----
> From: Eric Goodill [mailto:firstname.lastname@example.org]
> Sent: Monday, February 26, 2001 3:47 PM
> Subject: [SI-LIST] : Combined single-ended/differential termination
> We often use this termination scheme:
> SIG+ -------+
> | |
> R V
> | GND
> SIG- -------+
> That is, two resistors in series between the differential signals with a
> center-tapped capacitor to ground. Is there a combined device to sucks
> all these parts into a three-pin package?
> Regards, Eric
-- Eric Goodill Redback Networks email@example.com 300 Holger Way voice: (408) 571-5319 San Jose, CA 95134-1362
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