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

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From: D. C. Sessions (dc.sessions@vlsi.com)
Date: Tue Dec 21 1999 - 11:14:21 PST


Charles Bishop wrote:
>
> You say "for a reflection free system...". Who wants a reflection free system? As long as there
> are no differential reflections, and as long as all receivers on the line reject common-mode,
> what little common-mode signal there is can bounce back-and-forth all day for all I care. Of
> course for low EMI and cross-talk we'd like no common-mode at all, but that's a different issue.

Keep in mind that common-mode potential almost^H^H^H^H^H^H always
affects input delay, so those reflections WILL show up as receiver
jitter. If your application is completely insensitive to jitter,
well and good. Offhand I can't think of any like that.

> > I strongly disagree. The differential impedance for the line is definitely needed. Otherwise,
> there will be reflections at the terminator/transmission line interface for the differential mode of
> the signal. The common mode will not be affected because the approach you suggest will handle the
> common mode. For a reflection free system, BOTH 50 ohm transmission lines and 100 ohm differential
> impedance for the transmission line is required along with a 100 ohm tremination across the
> differential pair.

-- 
D. C. Sessions
dc.sessions@vlsi.com

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