RE: [SI-LIST] : LVDS signal observation

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From: Degerstrom, Michael J. ([email protected])
Date: Wed Feb 02 2000 - 11:55:45 PST


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 Degerstrom Email: [email protected]
Mayo Clinic
200 1st Street SW
Gugg. Bldg. RM 1042A Phone: (507) 284-3292
Rochester, MN 55905 FAX: (507) 284-9171

> -----Original Message-----
> From: Tom Dagostino [mailto:[email protected]]
> Sent: Wednesday, February 02, 2000 12:39 PM
> To: [email protected]
> 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
> [email protected]
> 503-685-1613

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