Author Topic: Agilent/Keysight Bench Multimeter U3402A residual count when shorted  (Read 4140 times)

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Offline uwerTopic starter

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Hi everyone, I would like to get some input on the following.

At work we use several (5 or 6) Agilent U3402A multimeters. One of them died with an inexplicably defective high current range although the fuse was ok. In another one the low current range died, I am unsure why but that's beside the point anyway. (Both devices were replaced out of warranty at our cost.)
Another one of these meters has developed a high residual resistance display. When shorted I have seen anywhere from a little over 1 to more than 70 Ohms.

Here is my real inquiry:
The replacement devices showed what I consider pretty high residual counts in the AC ranges when shorted, as shown in the attached pics.

Our older meters of the same type do not behave like that and pretty much zero out with somewhere between 0 to 7 digits remaining. We have since exchanged two units only to have the new ones behave in the same (bad) way.

For a supposedly high quality meter, I don't find these residual counts acceptable. What do you guys think?

 

Offline jpb

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Re: Agilent/Keysight Bench Multimeter U3402A residual count when shorted
« Reply #1 on: November 05, 2014, 03:56:00 pm »
See this thread for a discussion of rms converter noise which I think is relevant to what you're seeing:

https://www.eevblog.com/forum/testgear/keithley-2000-trms-converter-noise/
 

Offline macboy

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Re: Agilent/Keysight Bench Multimeter U3402A residual count when shorted
« Reply #2 on: November 05, 2014, 08:05:56 pm »
+1 to what jpb said.

This is RMS converter offset. This exact question pops up every so often here. This offset is normal and always present. Some HP gear will display a false zero count when the count is low enough. This is misleading IMHO, but I understand the RMS converter noise issue and don't expect to see zero.

I have a few Keithley 2001's on my bench, these are $5k meters. They do the same thing.

Very few meters do not have this issue. The Agilent 3458A comes to mind. It can do 100000 samples per second at 4.5 digits, and compute the RMS from them. It can also use an RMS converter in the conventional way, and it will then exhibit this offset issue.
 

Offline uwerTopic starter

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Re: Agilent/Keysight Bench Multimeter U3402A residual count when shorted
« Reply #3 on: November 06, 2014, 04:45:40 pm »
Very enlightening.
Thank you for your comments. You saved me a lot of trouble arguing with our supplier.
 

Offline macboy

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Re: Agilent/Keysight Bench Multimeter U3402A residual count when shorted
« Reply #4 on: November 06, 2014, 06:09:51 pm »
One more comment on this, do not null/zero out this offset. The applied AC voltage does not add algebraically to this offset, it adds as RSS - Root of Sum of Squares. So, hypothecially, if you get 0.200 mV with the inputs shorted, and then apply 10.000 mV, the display will be sqrt(10^2 + 0.2^2) = 10.002 mV, not 10.200 mV. If you had "nulled" out the offset, then applied the 10.000 mV, the display would read 9.800 mV, a large error.
 

Offline lern01

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Re: Agilent/Keysight Bench Multimeter U3402A residual count when shorted
« Reply #5 on: January 18, 2024, 09:45:56 am »
One more comment on this, do not null/zero out this offset. The applied AC voltage does not add algebraically to this offset, it adds as RSS - Root of Sum of Squares. So, hypothecially, if you get 0.200 mV with the inputs shorted, and then apply 10.000 mV, the display will be sqrt(10^2 + 0.2^2) = 10.002 mV, not 10.200 mV. If you had "nulled" out the offset, then applied the 10.000 mV, the display would read 9.800 mV, a large error.

How to enter calibration mode for u3402a. Thanks!
 


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