[SI-LIST] Re: Common-mode return path for differential signals.

Doug,

I've seen a lot of debate on this lately.  From what I can tell, there
appears to be some misunderstanding about what the meaning of
"differential" is in the context of system design.

For most of the microstrip PC boards I've seen, the spacing between the
nets of differential pairs never gets close enough in relation to the
height above the dielectric to result in more than 5-10% EM coupling.
The traces are effectively single-ended, as you noted for your
situation.  Routing these signals as a "differential pair" essentially
means, in this context, routing the traces to (a) minimize the length
mismatches and therefore timing skews, (b) equalize the effects of noise
from surrounding signals and (c) maintaining the same reference plane
for both.  It does *not* appear to mean actually having strong EM fields
between the traces.  To gain the benefits of truly "differential" routes
in an EM coupling sense, you would have to have something like a twisted
pair, where one line really is the reference and return path for the
other line and no other return path is nearby.  Most high-volume PC
board designs can't ever approach that kind of strong coupling.=20

In general, the benefits for "differential" system design appear to be
more oriented toward the receiver than the board itself (again, for the
microstrip PC designs I commonly see).  For a single-ended system, the
receiver itself is likely a diff amp with its reference tied to a local
voltage divider or other reference source connected to local
power/ground supplies.  Noise on the reference that doesn't occur on the
incoming signal line will result in erroneous data going into the
receiver logic core.  For a "differential" system, the receiver diff amp
uses, as its reference, the signal on the inverted line of the diff
pair.  If this line was routed in nearly the same fashion as the line
carrying the data signal, common-mode rejection will take care of the
noise that affects both lines, resulting in cleaner data into the
receiver.

Thus you can have a very good "differential" (noise immune) system
operating using "differential" (highly parallel with clean referencing)
routes on the board, while still having *effectively no coupling*
between the trace pairs.  The meaning of "the d word" changes depending
on the context, making it very much misused in the industry today.

For an example of such a system in action, see USB 1.1.

My two cents; I hope it helps!

- Michael Mirmak

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Doug Hopperstad
Sent: Tuesday, October 25, 2005 8:23 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Common-mode return path for differential signals.


When routing 100-ohm differential nets is there advantage to keeping the
=3D
two nets closely coupled other than for crosstalk issues? For current =
=3D
designs the space between the two nets is set for 1.5 times the width of
=3D
the net. For a 8mil trace, the gap is 12mils (20mil pitch). If the =3D
differential pair needs to become uncoupled, the trace geometry is =3D
adjusted to maintain the proper impedance. Length matching maintained =
=3D
for the route to reduce unwanted common-mode currents due to length =3D
skew. All routing is done on balanced, striplines using ground reference
=3D
planes only.=3D20

Some of the information I have read recently states to keep the =3D
differential pair tightly coupled while others state that there is no =
=3D
major advantage other than crosstalk. With the 12mil gap design, the =3D
nets are loosely coupled (8mil dielectric thickness) and has shown good
=3D
results.=3D20

Any comments or feedback would be great,
Thanks,
Doug.
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