Agilent 34401a noise on measuring resistors

[d55d11]

Hello Forum,

I have an Agilent 34401A produced around 2001~2002. Never (re)calibrated. From Ebay.

When I measure resistor in 2W or 4W mode the number on the display varies and I don't know if this is normal or my unit is having a problem.
I'm using "6 digits" in FAST mode and also tried in SLOW mode. The unit is probably de-calibrated, but before I sent it for calibration I was wondering if there is anything else wrong.
for a 383K 0.1% 15ppm 250mW I get reading of  0.383760 .. 0.383840 Mohm that is a variation of 80counts
for a 750K 0.1% 15ppm 250mW I get readign of  0.750900 .. 0.751011 Mohm  - variation is about 110counts

it does the same in 2W mode and 4W mode. the values are the same.

if I select "6 digits SLOW" the display updates less often (as expected) but the variation is still there. One value might read 0.750980 and the next will read 40 counts more or 100 counts more or 40 less. Overall it does "stay" in a range but I was expecting less variation.

the above examples are just some examples. I left it running for hours and check the value from time to time. Sometimes like magic the display gets stable with only the last digit changing like 2-4 counts up or down. And then a minute later it gets back to +/- 20 from one reading to the next. or more.

when reading miliohms there is the same variation. a 5mOhm will read between 4 and 6 mOhm.

when reading DCV from a battery I get a stable value. only the last digit will change sometime +/-1 count but otherways the number is stable.

is the variation on measured resistance normal for such a device ? Please share your experience. Thank you.

[Dr. Frank]

Hello,
The 1MOhm range is relatively sensitive, as the measuring current is low.
If you have switch mode power supplies or cell phones near your DMM, such fluctuations might easily happen. So search for such external noise sources first. Use short cables.

Please try lower ranges, like 10k, that should be rock stable.

Only if these possibilities can be excluded, an internal problem should be tracked.

PS, the calibration status has nothing at all to do with such problems.

Frank

[d55d11]

Actually I did tried to put my cell phone next to the probes and DMM while in a call to see if there is a difference. I did not notice any change (in good or bad).

readings on VDC are steady. I'll try to setup something to measure currents and see how it looks. Also I ordered some better test cables, but will take a while to arrive.

I'm not in a nosy industrial environment, I'm at home, I have a laptop on the next desk like 1m away. It does have a power brick under the desk.

Trying now with a 10k ordinary resistor from the parts drawer. I'm reading 9.870,22 Kohm and the last 2 digits fluctuates like this 22 24 26 22 20. so this is MUCH better than on 1Mohm range. Also the reading slowly changes. now last 2 digits read 41 42 43 42 43 43 43 44 45 44 45 46. The unit was cold and there are less than 5min since I turned it on, so it probably warms up. this was done in 2W mode 6 digits FAST. also the resistor might warm up a little and this migh show since it is not precision/high stability.

[Dr. Frank]

readings on VDC are steady. I'll try to setup something to measure currents and see how it looks. Also I ordered some better test cables, but will take a while to arrive.

That means, that the reference and the DCV path is fine.
DCI mode will not change anything, as it does not add any active components .. you can skip that.

Focus on Ohm mode, as this adds several active circuit elements to the DCV functionality.
If you can really exclude external noise sources, the problem will be buried inside the Ohm circuitry ONLY, it's no use to search somewhere else!

For 100k and higher Ohm ranges, the test cables should be short like 10cm or so, and they should have good surface metallization, like gold  plating, at least it should not be oxydized.

I'm not in a nosy industrial environment, I'm at home, I have a laptop on the next desk like 1m away. It does have a power brick under the desk.

"nosy .. environment" - that's great , if you live in a whisky distillery  (sorry for that joke..)

Sometimes, you can't imagine where switch mode PSUs are located.. I had a new LED lamp, and the mains adapter produced a lot of disturbance, so I had to throw that lamp out of my lab again.. So look for further possible sources .. or simply change the location of the instrument.

Maybe you repeat this experiment w/o the laptop PSU plugged in..

Trying now with a 10k ordinary resistor from the parts drawer. I'm reading 9.870,22 Kohm and the last 2 digits fluctuates like this 22 24 26 22 20. so this is MUCH better than on 1Mohm range. Also the reading slowly changes. now last 2 digits read 41 42 43 42 43 43 43 44 45 44 45 46. The unit was cold and there are less than 5min since I turned it on, so it probably warms up. this was done in 2W mode 6 digits FAST. also the resistor might warm up a little and this migh show since it is not precision/high stability.

The 34401A needs half an hour at least to stabilize.. please don't make any observations before that time, that makes no sense.
The resistors self heating can be neglected! That's about 100µW, you will hardly see any effect, if you use thin film resistors (50ppm/K), and wait a minute for stabilization, at least.

The 10k range should be rock solid, (using appropriate cables), that means, the last of 6 digits (fast or slow) should only change by +/- 1 count.

 
What happens if you toggle the front/rear switch, several times?
Does that change the reading afterwards?
(Some units suffer from a defect / dirty switch, especially in 2W mode).

Frank

[d55d11]

IT WORKS !

I moved it to another room. unplugged tv&stb. the only thing with power is the DMM. I twisted the cables together (they're 1m long and don't have any short ones) and then coiled them.

now the reading is STABLE. i'm using 2W mode. tested with 383K and 750K. it varies slowly +/-1 max 2-3.  If I am near it it varies more like +/- 10-15. When I step back 1m, it gets stable again.

I also tested 4W mode with 383K & 750K and also with 5mOhm and 50mOhm. the value is stable, it varies a little +/-1 in a range like +/-5 counts (=0.5mOhm) but that is understandable as this is the bottom of the range it can measure.

I will let it warm up and run for few hours but I think it will be ok.

Thank you Dr. Frank.

[Vgkid]

Glad you figured it out.

[cncjerry]

Common problem.  In order to get rid of the noise you need to move away as you noticed from computers and other sources of RF.  I use shields leads and added ferrite traps as well.  You could have PS noise that is getting in but there you will see it with shorted leads as well. 

For testing resistors I made a set of plugs that attach directly to the unit and they have aligator clips soldered as closely as possible between the source and sense plugs. I'll post a picture.

[HighVoltage]

Recently I was surprised that even an iPhone with a distance of 1 m had an effect on a voltage measurement with a 34401A and a 34470A.
That is why I am setting up a room right now, far away from RF sources, just to be on the safe side.

[ConKbot]

Phone transmitters, especially GSM ones put out watts of power peak, and have been a prime source of RF interference for years. Every hear a phone interfere with speakers before it rings?