Very strange behavior of my Keithley 182 nanovoltmeter - Page 1
Forum section: electronics / general

author: gamalot
author_rank: Super Contributor
timestamp: November 25, 2018, 08:02:45 pm
content: I got a Keithley 182 nanovoltmeter with a display failure a few days ago, which is easy to fix by making a simple 60V boost converter. Then I did some simple tests on this instrument. Unexpectedly, I got a very interesting (strange) result.In short, in the 30V range, near3.25V5.2V, the voltage measurement has a sudden jump about 100uV. I have repeated this many times in the past, and the results are the same, no matter how I change the filter, integration time, or other settings.

author: e61_phil
author_rank: Frequent Contributor
timestamp: November 26, 2018, 12:50:58 am
content: Hi,a couple of month ago I did some linearity tests with my K182. I attached the results. I can do further measurements if that helps (not before december).First row is calibrator output voltage, second row is K182 reading and third row is temperature.best regardsPhilippEdit: The K182 isn't as bad as is looks here. The comparison is not fair in 1/3 of the range. Therefore I did another test in the 3V range with a divider. That showed +/- 0.4ppm. But that is offtopic, I think.

author: gamalot
author_rank: Super Contributor
timestamp: November 26, 2018, 11:51:18 am
content: Quote from: e61_phil on November 26, 2018, 12:50:58 amThank you Philipp!Thank you very much for the reply, it made me find out a mistake in my post: it should be 5.2V not 3.25V. At the same time, your reply also proves that my K182's strange behavior is not a special case.I will do some further testing to try to figure out what is happening near to 5.2V.If you don't mind, can I know the firmware version of your K182?

author: gamalot
author_rank: Super Contributor
timestamp: November 26, 2018, 01:48:05 pm
content: I did a sweep up and then sweep down, I was surprised to find that there is a hysteresis, just feels like auto ranging.

author: e61_phil
author_rank: Frequent Contributor
timestamp: November 27, 2018, 04:59:53 pm
content: I will try this sweep next weekMy K182 shows the following on startup:Keithley 182 SDVA04 B03 A01

author: TiN
author_rank: Super Contributor
timestamp: December 13, 2018, 05:20:12 am
content: I tested my 182-M last night too. It is modified version with 9V battery powered front-end. FW version A04 B03 A01. Calibrated earlier this year using procedure from manual and 5720A/Keithley 262 setup.INL data I got leaves me wondering too:-3/+3V sweep on Keithley 182M 3V range. 100ms integration, Filters off, 6.5 digit mode. Reference meters in 10V rangeCorrect data below:

author: Vgkid
author_rank: Super Contributor
timestamp: December 13, 2018, 12:18:24 pm
content: That is kind of weird.Thanks for posting.

author: TiN
author_rank: Super Contributor
timestamp: December 13, 2018, 12:45:27 pm
content: del some nonsense..Doing test with 30V sweep currently. (other meters on 100V range).

author: David Hess
author_rank: Super Contributor
timestamp: December 13, 2018, 03:03:56 pm
content: What type of ADC does it use? That looks like the glitch commonly found in synchronous voltage-to-frequency ADCs.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 13, 2018, 05:10:08 pm
content: Quote from: TiN on December 13, 2018, 12:45:27 pmThe linear part (like in the 3 V range of the 182) of the "INL" curve kind of indicates a problem in calculating the straight line to subtract. So this is not a property of the meter, but a kind of test problem. So the 3 V and 0.3 V range look good - with possibly some minor problems in the 2.2 V range.

author: gamalot
author_rank: Super Contributor
timestamp: December 13, 2018, 05:29:55 pm
content: Is it possible that the input protection circuit causes this strange result?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 13, 2018, 07:09:50 pm
content: I don't see a problem with the input protection. There may be an issue with the extra 30 V amplifier, if it's only used for more than 5 V instead of for the whole 30 V range.

author: TiN
author_rank: Super Contributor
timestamp: December 14, 2018, 11:31:41 am
content: Kleinstein right on money. Sure, there was a bug in the data plotter. Shame on me for silly mistake. Array with INL points was calculated by using poly from channel 5 (F8508 data) instead of channel 6 (K182M).Pulled wrong pics from post above, will replot the proper ones tonight.

author: e61_phil
author_rank: Frequent Contributor
timestamp: December 16, 2018, 12:40:04 pm
content: Sorry, I forgot my promise to take some measurement.At first here are some old measurements for K182 INL. Measured against my Fluke 5440B. The 5440B was sweeping from 0 to 10V for best linearity. The 0 to 10V was then reduced to 3 to 10 by a SR1010-1k.@TiN: Could you reproduce the hysteresis?Now I will start some up and down sweeps to see if my K182 also has this hysteresis.

author: gamalot
author_rank: Super Contributor
timestamp: December 16, 2018, 01:09:27 pm
content: Thank you guys! I am doing something really boring!

author: e61_phil
author_rank: Frequent Contributor
timestamp: December 16, 2018, 07:20:24 pm
content: I could confirm these hysteresis..

author: TiN
author_rank: Super Contributor
timestamp: December 17, 2018, 04:03:01 am
content: I see the jumps on 30V range too, at about +/-5.2 and +3.5 VDC levels.On lower ranges like 3V and 300mV there are no problems (at least in my unit).

author: e61_phil
author_rank: Frequent Contributor
timestamp: December 17, 2018, 06:36:00 am
content: I also see it only in the 30V range. The measurement above with +/- 0.4ppm was done in the 3V range.I've done some more runs this night. The "switching" points always appear at the same voltages.