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Author Topic: Induced noise from DMM?  (Read 1912 times)

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Offline zed65Topic starter

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Induced noise from DMM?
« on: February 10, 2019, 08:12:31 pm »
Hi, I measure a standard LT1021 5 volt reference and tested it against my different DMM:s (Agilent 34401A and HP 3457A). Below is screen cap from my Rigol DS1104.
Is this noise "normal" to differ from those 2 meters? Are one meter more "noisy" than the other?
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Offline Andreas

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Re: Induced noise from DMM?
« Reply #1 on: February 10, 2019, 08:34:51 pm »
Hello,

did you use fully shielded cable or a probe with a pig tail?
Is your reference battery supplied, and kept in a metal (cookies) box.
Are there any switchmode supplies near the measurement setup? (or LED lamps)
How does the FFT spectrum look like. (I see the local FM stations in the spectrum when using non shielded cable).

with best regards

Andreas
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Offline zed65Topic starter

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Re: Induced noise from DMM?
« Reply #2 on: February 10, 2019, 08:59:03 pm »
Hi, I use "standard" BNC cables but my concern is why is the 2 meters show different noise levels (nothing else is changed) when i just unplug the 3457 and put the cables into the 34401.
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Online Kleinstein

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Re: Induced noise from DMM?
« Reply #3 on: February 10, 2019, 09:16:32 pm »
It may only take very minor changes to influence the RF reception. So it may be just a different load capacitance to cause a resonance. The first curve looks a lot like there are 2 dominant frequencies that are very close.

So the FFT might tell a lot. Another point would be reading a really quite source like a 1.5 V battery.
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Offline zed65Topic starter

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Re: Induced noise from DMM?
« Reply #4 on: February 10, 2019, 09:55:00 pm »
Hmm the FFT function on the Rigol isn't that great (or i may do something wrong). :palm:

I will try with a 1.5v battery later.

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Offline zed65Topic starter

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Re: Induced noise from DMM?
« Reply #5 on: February 10, 2019, 10:11:50 pm »
Here is when I measure a 1.5v AA battery.
The meters are ontop of each other so I just swap the cable between the meters.

Not that this is a problem, i'm just curious why this happen :)
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Offline David Hess

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Re: Induced noise from DMM?
« Reply #6 on: February 11, 2019, 12:29:30 am »
Depending on the design, digital meters generate various levels of charge injection at their inputs but measurement is tricky because of common mode coupling from ground to the meter's low side input.  If your meters support it, try making the same measurement with automatic zero disabled.
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Offline SilverSolder

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Re: Induced noise from DMM?
« Reply #7 on: February 11, 2019, 02:21:35 am »

I have seen similar effects with my DMMs here (various older Flukes). 

Have you tried putting something like a 1K resistor in series with one of the test leads to see if that reduces the interference?
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