[SI-LIST] Re: AC Coupling and Differential Pair's Termination
- From: Paul Levin <levinpa@xxxxxxxxxxxxx>
- To: jeff.loyer@xxxxxxxxx
- Date: Mon, 21 Oct 2002 16:19:11 -0700
Folks,
It is always advisable to terminate both modes. In general, you need the
three resistor T termination to terminate the two modes. If, however, the
lines are so loosely coupled that Zodd ~ Zeven, then you can use two
separate R's to ground, each equal to Zodd or Zeven. Regardless of the
degree of coupling, if you use a single line-to-line resistor, you may
be able to terminate the odd mode correctly, but you ignore the even mode
by failing to terminate it.
Regards,
Paul
____________________
Loyer, Jeff wrote:
> 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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--
Paul Levin
Senior Principal Engineer
Logic Innovations, a Xyratex company
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