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[SI-LIST] Differential signals - even and odd modes
- From: Chris Mesibov <Chris.Mesibov@xxxxxxxxxxxxxxx>
- To: SI list <si-list@xxxxxxxxxxxxx>
- Date: Tue, 12 Jun 2001 08:56:50 -0400
Has anyone used/interfaced with ASICs designed with the TI OZ macros?
It seems that the I/O macros offer source integrated 50 Ohm pull-up
termination for their drivers and 50 Ohm pull-up termination for their
receivers. Does this bother anyone else but me? Of course I need to
connect these ASICs together. The same configuration applies to the
differential driver receiver macros.
The main thrust here is that when using the differential macros in
driving/receiving on a back plane, I am concerned about terminating
both even and odd modes of propagation. If the scenario was just a
single diff pair with no skew, I would not be two concerned. However
this is never the case. I have a Ga-Zillion signals, so I am worried
and even mode excitation from cross talk and signal skew. Hall, Hall
and McCall discuss terminating both modes in delta or T networks,
although this is brought up in the crosstalk chapter. It would be
impossible for me to add a common mode (odd mode) termination at the
receiver inputs (to many). One thing I considered to do in an effort to
match both even and odd modes was to find a characteristic impedance
that would result in an even mode impedance equal to 50 Ohms, given the
fact that the odd mode impedance is arbitrarily high (>10000 Ohms).
This resulted in a board impedance of ~47 OHms.
Any body have any thoughts about this?
--
Chris Mesibov
Sr. Hardware Engineer Fujitsu Network Communications, Inc.
Phone:(845)731-2037 2 Blue Hill Plaza, 6th Floor
Fax: (845)731-2011 Pearl River, NY 10965
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