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

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From: Joel Kolstad (jkolstad@electroglas.com)
Date: Mon Jan 10 2000 - 10:51:39 PST


> In general, doesn't one need to terminate both the
> differential and the
> common modes? The 100-ohm-only termination terminates the
> differential
> mode but not the common mode.

You'd argue that two 50 ohm resistors to ground would terminate both
common
mode and differential mode?

It seems to me there's little need to terminate common mode -- a
differential receiver will see the same common mode reflections on both
of
its inputs, thereby nullifying any common mode mis-termination
(reflections)
so long as the common mode input voltage maximum of the receiver isn't
exceeded.

BTW, WRT the guy who said to route two 50 ohm lines instead of a 100 ohm
differential pair... two 50 ohm lines as close together as differential
pairs usually are is going to produce something more along the lines of
90
ohms of differential impedance, I believe.

---Joel Kolstad

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