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).

Have you asked the manufacturer?

No. It's long out of warranty.
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? Does it
change if you use shorter leads? Coil the leads?