[SI-LIST] Re: Decoupling of diff pair devices

Steeve, 

If you have a perfect odd mode on your differential pairs, then yes,
theoretically all currents will flow in the pairs and your PDS would be
perfectly clean.

However, you will likely have some even mode on your pairs as well. Even
mode return currents cause EMI and problems in the PDS like ringing and
spikes... and they eventually couple into the odd mode. My suggestion would
be to treat odd mode and even mode separately. For the even mode, the peak
current can be calculated as if your differential pair is a single ended
signal.  I would take this as a basis for the PDS study on the differential
board.

Also, in my experience a design well decoupled for the even mode will be a
good design for the odd mode as well.

Mike Khusid
Ansoft Corporation

-----Original Message-----
From: Steeve Gaudreault
To: si-list@xxxxxxxxxxxxx
Sent: 6/25/02 12:02 PM
Subject: [SI-LIST] Decoupling of diff pair devices


Hi SI gurus,

I have seen a lot of thread about decoupling capacitor, and proper =
approach that should be taken for a good PDS design. One thing that is =
still not clear to me is how do we calculate the peek current for =
differential-pair I/O. If they are routed as differential lines, would =
that number be close to zero??

Thanks,
Steeve.
Meriton Networks



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