Re: [SI-LIST] : LVDS Skew

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From: Scott McMorrow (scott@vasthorizons.com)
Date: Wed Aug 09 2000 - 15:50:13 PDT


David,

It depends on the edge rate of LVDS signal. Differential skew, besides
causing common mode currents, will cause a translation in time and voltage
of the differential crossing point. This causes the received Eye to close
down and may be perceived as timing jitter. If the skew is greater than
50% of the edge transition time, then the eye becomes totally closed.
(i.e. A 50% edge transition time skew causes the differential crossing to
never be seen at the receiver.)

Some LVDS drivers have edge rates in the 400ps range. 200ps of skew
would totally close the eye. 50ps of skew would reduce the eye opening
by 25%, a significant reduction in noise margin.

With 1ns edge rate drivers, like some of the "nice" LVDS devices, 50ps
of skew is not a big deal. This translates to only a 10% noise margin
reduction.

regards,

scott

--
Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
Tualatin, OR  97062-3090
(503) 885-1231
http://www.siqual.com

David Haedge wrote:

> Dear SIer's: > > I have a person concerned about 55ps of skew between the two traces > on an LVDS differential pair. The LVDS bus is running at 200MHz, a > 5ns period. What negative effect on the system would occur if say, > there was a 100ps or even 200ps mismatch? Timing margins are > still within spec with even a 1ns mismatch. Some common mode > currents may be launched, but I think they would cause minimal > noise and not cause any circuit upsets. Has anybody seen problems > with LVDS signaling with >55ps diff pair line skew? > > David Haedge > Raytheon > > **** To unsubscribe from si-list or si-list-digest: send e-mail to > majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE > si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. > si-list archives are accessible at http://www.qsl.net/wb6tpu > ****

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