Author Topic: Keithley 197 - issue with uV DC/AC  (Read 3018 times)

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Offline Telstar

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Keithley 197 - issue with uV DC/AC
« on: February 22, 2013, 02:54:18 pm »
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.
 

Offline c4757p

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Re: Keithley 197 - issue with uV DC/AC
« Reply #1 on: February 22, 2013, 03:01:17 pm »
So the voltage climbs high, but zeros when you short the probes? That is perfectly normal.

I dont believe I have halv volt of DC floating in the air...

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.
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Offline Huluvu

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Re: Keithley 197 - issue with uV DC/AC
« Reply #2 on: February 22, 2013, 03:51:27 pm »
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
« Last Edit: February 22, 2013, 04:12:09 pm by Huluvu »
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Offline Telstar

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Re: Keithley 197 - issue with uV DC/AC
« Reply #3 on: February 22, 2013, 06:09:04 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.

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.

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.
 

Offline c4757p

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Re: Keithley 197 - issue with uV DC/AC
« Reply #4 on: February 22, 2013, 06:45:56 pm »
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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Offline Telstar

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Re: Keithley 197 - issue with uV DC/AC
« Reply #5 on: February 22, 2013, 09:04:48 pm »
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.

But it was :( the reading didn't change -it kept drifting- couple days ago when I was trying to measure the DC offset of an headphone amplifier i'm working with (it should have <1 Ohm output impedance).
 

Offline Telstar

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Re: Keithley 197 - issue with uV DC/AC
« Reply #6 on: February 24, 2013, 01:01:13 pm »
Good news: last night I rechecked all measurements and I found out I had a loose plug, so the readings actually DO WORK when connecting to a normal load on the two most sensitive ranges.

Despite my noobness, I consider now the issue solved :)
 


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