[SI-LIST] Re: AC Coupling and Differential Pair's Termination
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: "'ggafni@xxxxxxxxxxxxxxxxxxxxx'" <ggafni@xxxxxxxxxxxxxxxxxxxxx>,Si-List <si-list@xxxxxxxxxxxxx>
- Date: Mon, 21 Oct 2002 15:28:13 -0700
My take on the 2nd question:
A completely "bullet-proof" termination of a differential pair accounts for
termination of both the even and odd mode components of the signal. A
possible scheme for realizing this is given on page 66 of the Hall, Hall,
McCall book (and others). Call this scheme "A", and it takes 3 resistors
per differential pair.
For a truly IDEAL differential pair, termination can be simplified. The
even mode termination can be eliminated. Assuming the signals transition at
exactly the same time and travel identically for the entire length of the
interconnect, the signals see only the odd-mode impedance, and each leg can
be terminated to ground for its odd-mode impedance (perhaps about 47 ohms)
only. This scheme "B" takes 2 resistors per differential pair.
If you're doing this, simple superposition of the currents shows that a
single resistor (of twice the odd-mode impedance) between the two legs will
serve the exact same purpose. This scheme "C" takes only 1 resistor for
each differential pair.
In non-ideal systems, there's skew between the 2 signals of the differential
pair. This can be a result of driver skew, or skew introduced by the
interconnect. In this case, there will be some reflections induced by
either scheme "B" or "C" (simulations I've done have shown scheme "B" to
induce less reflections than scheme "C"). Scheme "A" will have no
reflections.
Picking the best scheme is (surprise) a compromise between the number of
resistors required and acceptable signal integrity degradation. Many
systems in the past have been able to use scheme "C", and it's the cheapest.
Scheme "B" seems to be better, signal-integrity-wise, than scheme "C", but
it's more expensive (parts and real-estate-wise). Of course, scheme "A" is
best for signal-integrity, but is pricey.
The simulations are fairly straightforward, and should be done if you're not
dealing with a mature bus or have doubts about recommended terminations.
Jeff Loyer
-----Original Message-----
From: ggafni [mailto:ggafni@xxxxxxxxxxxxxxxxxxxxx]
Sent: Monday, October 21, 2002 12:04 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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