Hello, Doug!
It still amazes me how intelligent engineers and techs can make really simple,
stupid mistakes. (Then I make one myself.) However, knowing the limits of the
equipment in use goes a long way towards avoiding error.
OT: Recently, I spoke with an engineer who wondered about the loading effect
of a "normal" 150 MHz probe on a logic level digital waveform. We have been
down that road more than once, eh? For what it's worth, he switched to a
FET probe. I still recommend your resistor passives.
Cortland RIchmond
----- Original Message -----
From: Douglas Smith <mailto:[email protected]>
...
Subject: Re: scope probe gnd
Hi Everyone,
Differential probes are one of the most misused pieces of equipment and
their
output should always be verified to be true. It is very easy for a
differential measurement to result in garbage output. From the discussion, the
posters so far seem to be referring to very low frequency probes, below one
megahertz. Using two probes and channel subtraction does not work above 20-30
MHz, or at least the common mode rejection can actually be gain instead of
rejection. I present data on this subject in my classes. If there are two
probe cables used, they should be in close contact for most of their length.
Very bad things can happen if this is not done, including becoming sensitive
to every noise source in the room.
The best way to know if your differential probes are working is to
short the
two tips together and touch them to each node, one at a time, to be measured.
Both results, I call it a "null experiment," should be small co! mpared to
what you are trying to measure. An alternate method is to reverse the probe
tips. The waveform on the scope should be a mirror image. This alternate
method only works on a scope.
Except at low frequencies, 20 dB of common mode rejection can be hard
to get.
Doug
Cortland Richmond wrote:
Hi, Derek.
I know how it is. I also know there was no way was I going to
take a chance
on a reversed ground to a floating 'scope when I could get rid
of the
problem with one extra probe. FWIW, at 60 Hz it's easy to
match probe and
channel gain, and since we were only looking at decay time
there was no
need to buy a diff. probe. Where I work now we have some. HV
ones, too!
On a recent assignment, a place I was helping was cited by
their State OSHA
equivalent for not having the power terminals on LISN's covered
to prevent
contact. There's a lot of stuff in a lab that can bite. Hard.
Cheers,
Cortland Richmond
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