RE: [SI-LIST] : LVDS signal observation

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From: Degerstrom, Michael J. (degerstrom.michael@mayo.edu)
Date: Wed Feb 02 2000 - 11:55:45 PST


Tom,

I also like to us a good high bandwidth 10X probe instead of a FET probe.
However, the user must be very careful as a 10X probe looks like
500 ohms to ground to the LVDS output. When you only have 3-4ma
of current swings you do not want 1.2/500 = 2.4ma pulled to ground.
Depending on the LVDS design you won't just get reduced swings you
may get nothing if the buffer is pulled out of its bias range. Luckily
I have always have control of the bit pattern coming out of
the LVDS buffer. Therefore I simply apply a well balanced
2^N-1 PRBS code so I can use AC coupling from my 10X probe into
my 50 ohm oscilloscope to do my evaluations.

Mike

_______________________________________________________________
Mike Degerstrom Email: degerstrom.michael@mayo.edu
Mayo Clinic
200 1st Street SW
Gugg. Bldg. RM 1042A Phone: (507) 284-3292
Rochester, MN 55905 FAX: (507) 284-9171
WWW: http://www.mayo.edu/sppdg/sppdg_home_page.html
_______________________________________________________________

> -----Original Message-----
> From: Tom Dagostino [mailto:tom_dagostino@mentorg.com]
> Sent: Wednesday, February 02, 2000 12:39 PM
> To: si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : LVDS signal observation
>
>
> Be more concerned about the C loading of the probe than the
> input R. I'd
> much rather use 150 fF of input C from a good 10X 500 ohm
> passive probe than
> 400 fF 100K Ohms from and active probe. 1 pF at 1 GHz is
> about 160 Ohms.
>
> Tom Dagostino
> ICX Modeling Group
> tom_dagostino@mentor.com
> 503-685-1613
>
>

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