[SI-LIST] Re: AC Coupling and Differential Pair's Termination
- From: James_R_Jones@xxxxxxxx
- To: ggafni@xxxxxxxxxxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Mon, 21 Oct 2002 15:58:28 -0500
Gil,
I'll try to take a stab at the Differential Pairs Terminations question. I
beleive that the termination between P and N of a differential pair will
terminate a diff pair differentially. But it may not terminate the pair
properly for common mode. If you have a true differential input, then this
may not be important. I would think that it may be possible for a common
mode signal to resonate back and forth along the T-line for this
configuration. If this common mode signal approaches the rails of the
receiver, you may have a problem.
The two 50 ohm terminations will terminate both the differential and common
mode signals correctly when using a loosely coupled differential pair. For
extremely tightly coupled differential pairs, you will get the correct
differential termination, but it will be slightly off for the common mode
termination. Once again, this may or may not be a problem, depending on the
common mode sesitivity of your receiver, and whether or not your common mode
signal approaches the rails of the receiver.
James R. Jones
Dell
-----Original Message-----
From: ggafni [mailto:ggafni@xxxxxxxxxxxxxxxxxxxxx]
Sent: Monday, October 21, 2002 2:04 PM
To: Si-List
Cc: Simon Assouad
Subject: [SI-LIST] AC Coupling and Differential Pair's Termination
Dear Experts,
I would like to address this forum with two issues:
AC Coupling
I read the recent thread about the AC coupling, and I would like to
raise couple of more generic questions related to that:
1. When do you think that AC coupling is applicable? By saying that I
refer to more than the case where a 5V device is connected to a 2.5V
device.
2. What are the pros and cons for AC coupling a clock (where one do not
have a DC Balancing issues)
Differential Pairs Terminations
Generally, there are two flavors of differential pair's termination -
Parallel 1 - where a 100ohm or 120ohm resistor is applied between the
positive and the negative traces.
Parallel 2 - where two 50ohm resistors are pulled to one of the rails or
to a Vref.
1. When do you think the first is applicable and when is the other?
2. What are the pros and cons for deploying each one?
Regards,
Gil Gafni
Silverback Systems
695 Cambell Technology Pkwy
Suite #150
Campbell, CA, 95008
Phone: (408) 376 1303
Fax: (408) 376 1399
Email: ggafni@xxxxxxxxxxxxxxxxxxxxx
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