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Keithley 197 - issue with uV DC/AC

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Telstar:
Hello,

I have this bench DMM, bought 2 years ago. Since last week the highest sensitivity ranges readings are out of whack with values that start from 0.2V and go to 1V with or without probes attached. If i set the range immediately greater (200m), everything is fine. Therefore I cannot do measurements smaller than 0.1 mV, which is OK for DC offsets, but not for measuring the ripple of a PSU.
This is pissing me off very much.

I checked the manual, which says only to zero (with probes shorted I read all 0 or almost that, so it is NOT a DC offset issue), and be careful of EMC. I dont believe I have halv volt of DC floating in the air... but If i get closer or farther from the multimeter (or the probes, if attached) the voltage fluctuates a good .3-.4V (!)

The DMM is mains-powered, and I don't have the battery-pack installed (but I plan to get one if i find reasonably cheap).

I googled but didnt find any help. So thanks in advance to anyone giving me some clue.

c4757p:
So the voltage climbs high, but zeros when you short the probes? That is perfectly normal.


--- Quote from: Telstar on February 22, 2013, 02:54:18 pm ---I dont believe I have halv volt of DC floating in the air...

--- End quote ---

Sure you do! The Keithley 197 has a very high input impedance on the lowest two ranges (greater than 1 G?), so the charge does not drain off the way it does with a lower-impedance meter. My HP 3468A will happily climb above 3V with the probes attached and floating!

Edit: This is just since last week? I may have misunderstood you, then, because as I understand it, the meter should have been doing this all along.

Huluvu:
Hi Telstar,

As already mentioned the low ranges are very sensitive and you would easily catch up some magnetic field (even from your Body).

You can try also to put a high resistor (a couple of meg ohms)  in parallel close to the input jacks and check the readings again.
If this doesn't help I would suggest to check the internal shielding connection point.

Rgds

Telstar:

--- Quote from: c4757p on February 22, 2013, 03:01:17 pm ---So the voltage climbs high, but zeros when you short the probes? That is perfectly normal.
...
Edit: This is just since last week? I may have misunderstood you, then, because as I understand it, the meter should have been doing this all along.

--- End quote ---

I have mostly measured higher voltage and resistance, so it is possible that I never noticed this behaviour and it could indeed be normal. I dont do measurements everyday and I cannot recall exactly whether it was always like this or not.
It also does the same with the auto range.


--- Quote from: Huluvu ---You can try also to put a high resistor (a couple of meg ohms)  in parallel close to the input jacks and check the readings again.
--- End quote ---

AFAIK I need to build some adapter/cable, maybe something like the probes used in this nanovolt meter?
I'll do as suggested and check again.

c4757p:
It should not be a problem when you're actually taking a measurement, as the circuit you are measuring will have a much lower impedance than the meter.

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