[SI-LIST] Re: About common-mode
- From: Ross_Amans@xxxxxxxx
- To: a.ingraham@xxxxxxxx, Fu.Gerg@xxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Tue, 4 Nov 2003 12:29:36 -0600
For a differential pair, the only return currents would be common
mode return currents, from common mode issues.
The differential pair currents should cancel out.
Ross Amans
Dell Inc
-----Original Message-----
From: Ingraham, Andrew [mailto:a.ingraham@xxxxxxxx]
Sent: Tuesday, November 04, 2003 10:03 AM
To: Fu.Gerg@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: About common-mode
> I see a document to explain the common-mode as follows:
> common-mode refers to signals that occur on both traces of a
> differential signal pair or on both the single-ended trace and ground.
> I have a question, where is the return path? Confused...
Probably on what we call "ground".
If we're talking about differential traces on a circuit board, the return
path is mostly the "ground" and power planes that surround the traces.
For common-mode currents on something like a coax cable, the return path
might be a chassis, or any other conductors that go between the source and
the load.
"Ground" is never ground; or stated another way, no two grounds are the
same. You can have several "ground" paths in a circuit between point A and
point B, and they are not all carrying the same currents. You can have
currents that go out one "ground" path and back (in the opposite direction)
on another "ground" path.
Regards,
Andy
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