[volt-nuts] Semi-precision high resistance measurement

ed breya eb at telight.com
Fri Jun 21 10:42:04 EDT 2013

This is now the third try at sending a message to the group. Here's 
what I wrote before:

I sent this on the 18th, but it didn't show up, so here goes another 
try. I don't know if something's wrong with my email. Please excuse 
if the redundant original shows up too.

Original message:

Yes, it is common-mode noise, and maybe some leakage paths - if you 
measure in-circuit, you are also seeing the effects of everything 
else those parts are attached to, interacting with the different 
impedances of the ohmmeter inputs. You'll get identical swapped 
readings only on parts that are in perfectly symmetric circuits - not 
often encountered. You may get some improvement by connecting the DMM 
guard terminal only to the DUT chassis, and of course don't have the 
DUT plugged in. The best accuracy should be with the "common" or 
"low" DMM input tied to the cold side of the component - the end with 
the most circuitry attached, or lowest impedance to the rest.

I'd recommend not worrying about it - if the values are within a 
percent or so in one direction or the other, then close enough. Any 
that aren't you can remove and test out of circuit to make sure. I 
doubt that you'd even be able to notice the difference on an old 
analog meter - the meter movement's retrace/repeatability probably 
wouldn't be good enough to see it anyway, from one test to the next.

More information about the volt-nuts mailing list