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DMM DC readings inaccurate when leads are reversed

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nukie:
Hey guys,
I am testing a recently acquired 20000 counts DMM based on the Maxim MAX134. I found out that when I reversed the leads the negative reading does not match the actual positive reading. It's off by a little in the mV so its negligible.
 
My question is this normal? My fluke and HPs don't do this...

EDIT: I jumped the gun quite a bit, my bench meters reads identical results both ways but not portable meters.

Kiriakos-GR:
What is the voltage reference source that you use , for your comparison ?

nukie:
Right I did more testing, funny I'm only doing these test after owning the bench meters for so many years just because of a newly acquired DMM. I won't reveal the DUT yet :P until I learn how to calibrate it.

As I edited/mention in my original post, my old and trusty Fluke 8840A, HP 3468A and HP 34401A displays identical DCV results the only difference is the negative symbol when I reverse the leads. I used Fluke leads and a second pair of Chauvin Arnoux leads for confirmation.

For my Fluke 187, 189, Uni-T UT71D all exhibits some sort of difference in the mV range. The voltage source is a Geller SVR with a custom trimmed 5.0000V Analog Devices AD586KQ voltage reference. I don't think precision matters much but the noise does.

These are the results
Fluke 187 - 4.9997 (-4.9995)
Fluke 189 - 5.0000 (-4.9998)
Uni-T UT71D - 4.997 (-5.000)
unknown DMM - 5.000 (-4.983)

The Fluke 189 was recently calibrated.

I couldn't try a battery because the voltage drops as I try to take a reading. I tried a HP E3610A power supply at 0.9xxV. The Flukes and Unknown DMM passed with flying colors, identical results in the positive and negative range. So this means the voltage source is the problem. The Uni-T was reading 0.902(-0.906), the Uni-T never impress me much, it's has very poor repeatability in the low mV range. Just so you know, I was trying to measure current with sense resistor so I need accurate mV readings.

Kiriakos-GR:

--- Quote from: nukie on May 23, 2011, 01:03:13 pm ---I don't think precision matters much but the noise does.
--- End quote ---

Well you had motivated me enough, so to test my Fluke 28II against the old 8050A ,
I  measured  10V , and from my bench power supply , and from my reference ( battery powered ) source.

There was no change about  noise , like comparing an clean DC source  against one PSU ,
that it does have even a minimum level of noise.

Even so, in both cases, I got the opposite numbers from yours.
At minus 10V ,  I got in both the fifth digit, to be plus one digit .. than when both measuring an positive value.
  
I am 42 years old , and every single day I discover just another crazy thing..  :D


Mechatrommer:

--- Quote from: Kiriakos-GR on May 23, 2011, 01:37:24 pm ---I am 42 years old , and every single day I discover just another crazy thing..  :D

--- End quote ---
you only have found 10% of it.

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