On 2008-10-01, greenpjs wrote:
On Wed, 1 Oct 2008 12:24:49 +0000 (UTC), Andre Majorel
wrote:
On 2008-10-01, William Sommerwerck wrote:
On 2008-10-01, Andre Majorel wrote:

I've got a Chauvin Arnoux C.A 5220 DMM that bottoms out at 9 mV.
Even with the inputs shorted and in "Velec" mode (270 k input
impedance), it never goes below 9 mV.

The problem is only for AC. In DC mode, it *does* go down to 0.

Obviously, "something" is noisy. (Duh...)

Looks like it's 50 Hz because when trying to measure a 50 Hz
sine from a sound card, the voltage slowly oscillates by about
+/-10 mV around a mean value (fraction of Hz beating against the
sound card).

It is possible that the shorted leads are acting as a loop
antenna and the 9 mv is legitimate. Does it change if you go
outside?

It keeps changing until you hold everything perfectly still. But
even outside, it never goes below 9 mV.

Does it change if you use shorter leads?
Coil the leads?

Not appreciably.

Just tried putting everything in a tin biscuit box and still get
the parasitic reading. Obviously, I couldn't close the box
completely.

--
André Majorel URL:http://www.teaser.fr/~amajorel/
"Je regrette le Concorde. Au moins il vous amenait directement à
l'hôtel." -- Cyrano