RE: [SI-LIST] : Differential End Termination

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From: Stockalis, Anthony ([email protected])
Date: Fri Feb 23 2001 - 14:35:43 PST


The suggested termination scheme is a .01uF series capacitor on each line
into the load along with a 50ohm .01uF AC terminator to gnd. The other
termination scheme is to have .01uF series capacitors into the load and a
100ohm resistor between the two pairs to match with the 50 ohm impedence of
the trace. I have contacted the manufacturer, but I am waiting to get a
response. I wanted to see if anyone had tried using either of these
termination schemes.

Thanks,

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

-----Original Message-----
From: Fred Balistreri [mailto:[email protected]]
Sent: Friday, February 23, 2001 4:12 PM
To: Stockalis, Anthony
Subject: Re: [SI-LIST] : Differential End Termination

Hello Anthony, the two resistors with a common capacitor method will
work. However I would get better clarification with what the IC chip
manufacturer told you. The 50 ohms with a .01uf capacitor acting as AC load
will not
work. The .01uf they are talking about may be in series with the driving
device to block any DC currents. This is common on some Transeiver chips
which
don't like DC currents on the input. In this case the terminating resistor
is
across the differential and should be choosen for the differential impedance
of the
connecting transmission lines not necessarily 50 ohms. I don't believe that
the IC
manufacturer would make such a mistake. The problem may be in communication.
Get a
sketch/schematic of what they are saying for clarification.

Best Regards,

"Stockalis, Anthony" wrote:
>
> 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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-- 
Fred Balistreri
[email protected]

http://www.apsimtech.com

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