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 compared 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
[Original Message]
From: Derek Walton <[email protected]>
<mailto:[email protected]>
Subject: Re: scope probe gnd
Hi Folks,
I was keeping quiet.... but you know sometimes it's just too
much.
Almost all measurements I take in my lab use a differential
scope probe.
The exception is when High voltages are involved. And I dont
mean the
rinky dink two probes/scope math function either....
With diff probes I can connect anywhere to anywhere and measure
it. I
never cringe hooking up the 0 volt reference side.... never
sparks....
Give one a try, you will never go back.
Derek Walton
Cortland Richmond wrote:
In a previous life -- at a company no longer around --
when our Safety
Engineer quit we EMC types had to step in, and one of
the very first
things
I did was get rid of a lethally ungrounded scope setup.
It costs little
to
use probes rated for line voltage or better and dual
channel math
functions
to allow proper safety grounds.
Cortland Richmond
[Original Message]
From: Pettit, Ghery <[email protected]>
<mailto:[email protected]>
To: [email protected] <[email protected]>
<mailto:[email protected]>
Date: 3/10/2009 12:43:37 PM
Subject: RE: scope probe gnd
Or there were the old scopes where you used
patch cords and banana
jacks.
We had what was left of the power cord for one such
scope hanging in the
front of the motors lab in college that showed the
importance of having
your setup checked before applying power. Seems the
hot and ground
leads
to the scope from the motor being measured got reversed
and the 120 VAC
supply was bypassed to ground through the green wire in
the power cord.
For a short period of time. Then the green wire acted
as a fuse. That
cord was NOT repairable. And, NO, I was not the
culprit. Didn't
happen on
my watch.
Ghery Pettit
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Ralph
McDiarmid
Sent: Tuesday, March 10, 2009 9:35 AM
To: [email protected]
Subject: RE: scope probe gnd
Once for me as well. I never forgot that
lesson.
Ralph McDiarmid, AScT
Compliance Engineering Group
Xantrex Technology Inc
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of John
Woodgate
Sent: Tuesday, March 10, 2009 5:47 AM
To: [email protected]
Subject: Re: Surge testing Questions
In message
<9d04b979323dcd428297dda95108893e0120c...@bb-corp-ex2.corp.cubic.cub>
<mailto:9d04b979323dcd428297dda95108893e0120c...@bb-corp-ex2.corp.cubic.cub> ,
dated Tue, 10 Mar 2009, "Price, Edward"
<[email protected]>
<mailto:[email protected]> writes:
What, you never had that happen?!
Only once - so far!
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========= Los Gatos, CA 95031-1457
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