[SI-LIST] Re: Bit Error Caused by NOISE

  • From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxxxxxx>
  • To: "Lianghu Xu" <xlh9504@xxxxxxxxx>
  • Date: Wed, 21 Nov 2001 08:57:18 -0500

> I use a oscillscope to observe the VCC and GND respect to the same
> GND.=20

If you are seeing noise on GND with respect to itself (i.e., you short
out the scope probe leads and attach their common point to a GND), then
you have a measurement problem.

Either the probe wires are too long and other noise is being induced
into them (you have a current loop, useful for probing for magnetic
fields), or there is a "common-mode" kind of problem through the scope.
For the former, shorten up your probe wires to the bare minimum, and
keep them close together.  For the latter, try plugging the scope into a
different outlet, preferably the same AC power source as the customer
equipment under test, or try an isolation transformer, or try a
different model scope and/or probes.

Differential probes might help too.

Until you resolve this measurement problem you can't rely on anything
your scope tells you about noise.  You should be measuring zero, but the
scope is showing you 500mV that isn't there.

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