Re: [SI-LIST] : Physcially-small far-end LVDS terminations?

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From: Lee Ritchey (leeritchey@earthlink.net)
Date: Sat Dec 18 1999 - 08:07:01 PST


If you use R-packs, be careful you don't wind up coupling noise line to line through the common connection of the R-pack.

Lee

D. C. Sessions wrote:

> Eric Goodill wrote:
> > I want to use the typical LVDS termination where you have two 50-ohm resistors in series placed between the two lines of the LVDS diff. pair and also have a cap at the junction of the resistors to ground. I have a jillion LVDS pairs to terminate at a high-pin-count ASIC, and I'm looking to squeeze more terminations per square inch. (There's no hope of putting the terminations inside the ASIC at this point.) I'm hoping some company makes a single three-pin part the has those two resistors and a cap even if not specifically for termination use.
> >
> > How about any other ideas for even higher density?
>
> Well, a newline every 60-70 mcolumns would help :-)
>
> Seriously, you should be able to skip the cap and just use a 100-ohm resistor.
> Especially if you're running both ends of the line on a common ground.
> Alternately, use a pullup pack (several 50-ohm resistors to a common pin)
> and one cap. The application you're describing sounds like all of the pairs
> come over a common path, so they should have pretty well-matched common-mode
> potentials.
>
> --
> D. C. Sessions
> dc.sessions@vlsi.com
>
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