RE: [SI-LIST] : Differential End Termination

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From: Boris Yost ([email protected])
Date: Fri Feb 23 2001 - 12:35:55 PST


I actually did this--the two resistors to a common capacitor. It was on a
multidrop LVDS bus on twisted pair ribbon cable. It worked. Furthermore,
now you have a path for the residual common-mode to get to ground, so it
made less EMI.

Boris

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Stockalis, Anthony
Sent: Friday, February 23, 2001 2:21 PM
To: '[email protected]'
Subject: [SI-LIST] : Differential End Termination

I am investigating the ac end termination of a differential data signal. I
have reviewed section 6.4 of Howard Johnson's "Handbook of Black Magic" on
ac end terminators, and it seems that two resistors to a common capacitor to
ground is the way to go. The recommendation of the chip manufacturer is to
have a 50 ohm to .01uF capactor end termination on each line of the
differential pair. The chip is a Gigabit Ethernet Chip running at 1.25Ghz.
This capacitance value seems rather high to me, and I believe that the
common capacitor would be the correct solution. I was wondering if anyone
else has any valuable experience doing ac end terminations on differential
data lines. I have used the Mentor Graphics Interconnectix simulator, but
have not gotten very conclusive results because a random data signal cannot
be used in the tool. Any help would be appreciated.

Thanks,

Anthony Stockalis
Marconi Communications
BXR-48000 Hardware Team
email: [email protected]
Phone 724-742-6124

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