New User with 3458A - Page 1
Forum section: electronics / general

author: AndyTT
author_rank: Newbie
timestamp: April 15, 2020, 04:51:18 pm
content: Hi All,I stumbled upon this forum while looking for answers to problems I am experiencing with my 3458A DMM. So far I have spent a good few hours reading all the related posts on 3458A and its known problems. My particular problem with my instrument is I suspect it is suffering from short term drift.Before I attempt an SN18 procedure could you kind people answer the following:If my instrument fails the short term drift test, could this still be a long term drift issue with the onboard reference?I don't have a stable 10V reference to check this out myself. Is it even worth doing SN18 if I cant ascertain if the long term drift is good?SN18 refers to CAL? 72 but I have seen others look at CAL? 1,1 and CAL? 1,2 etc. Are these also good to plot alongside CAL? 72 values? What do they tell me?I am only interested in DCV stability / drift at the moment so have only been using DCV ACAL. I cant think how it would impact the DCV but should I be doing an ACAL ALL?The room which my meter is kept can fluctuate in temperature +/- 3C. Will the SN18 procedure still be of benefit or will it prevent me from finding the cause of the suspect drift. (Meter is kept in instrument rack with other instruments)Thank you for your helpAndy

author: maat
author_rank: Regular Contributor
timestamp: April 15, 2020, 10:13:54 pm
content: Hi Andy,welcome to the forum. Failing the SN18 test is (in my opinion) a meter that is so sick, it is bleeding out of every orifice. The test itself depends on the assumption, that the reference itself is OK. There is no way of telling which one is drifting - the ADC or the reference. History has shown, that most likely it is the ADC. If you see random sudden jumps, then you should have a look at the reference.Regarding the test procedure, to get any meaningful result, the temperature should be stable within at least 1K, but I do recommend a slightly alternative approach:- Continuously run ACAL DCV and log the "CAL? 72,1", "CAL? 71,1", "CAL? 175".- At the same time log the internal temperature of the DMM using a PT100 or 10k thermistor with decent resolution (<100 mK).- The temperature sensor should either be placed within the inguard chasis, or attached to it. This can even be done without opening the DMM.I also chose CAL 71 (1V DCV range), because it is the next less stable range of the HP3458A. Use the "CAL? 175" value to check, that the temperature sensor is following the internal temperature (ignoring a constant offset).That way you can plot the cal values against time and temperature and get a feeling for its behaviour. You need a decent temperature sensor to actually see the temperature drift of the device cleanly above the noise floor to later easily distinguish temporal drift and temperature drift.Depending on you Python foo, I do have scripts, that collect data from various instruments using Serial, LAN and GPIB via Pyhton asyncio. I do have a script specifically for testing SN18. It collects the data from the HP3458A using an NI GPIB adapter and the temperature from a Fluke 1524 using serial tty. I can post the script if it is of any help. Depending on your Python skills you can of course add any sensor instead of the 1524 logger. Unfortunately asyncio has a rather steep learning curveTo answer your last question, if you are only interested in DCV, then ACAL DCV is fine.

author: TiN
author_rank: Super Contributor
timestamp: April 15, 2020, 10:25:00 pm
content: QuoteI'm yet to see a meter with drifty A9 reference (and I've seen/have data from many of them, 15+ units. So typically it's pricey ADC pill.QuoteThey will tell you nothing, because CAL? 1 and CAL? 2 values change only after external calibration. They'll never change from running ACAL.QuoteTemperature does not matter, as long you store TEMP? reading together with data. You may have to run longer time and more points to collect reliable stability trend.Also don't go by SN18 criteria 0.43 ppm/day. It's not a good measure to test ADC stability. Some forum members will surely disagree on this, but I'd say anything with drift over 0.1 ppm/day is a bad/drifty ADC.

author: maat
author_rank: Regular Contributor
timestamp: April 15, 2020, 10:30:27 pm
content: Quote from: TiN on April 15, 2020, 10:25:00 pmI second that.The tempco of a non-faulty ADC is also well below 1ppm/K.

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 16, 2020, 12:31:19 pm
content: Quote from: maat on April 15, 2020, 10:13:54 pmThe stability of the reference doesn't matter for the ADC test, because it measures a ratio only. You can't see a reference drift with this test. Therefore, you can tell if it is an ADC problem, but you can't figure out if the reference has also a problem. That is only possible with an external reference.

author: maat
author_rank: Regular Contributor
timestamp: April 16, 2020, 12:59:14 pm
content: Quote from: e61_phil on April 16, 2020, 12:31:19 pmI was thinking along the way of popcorn noise. Therefore I was refering to jumps in the readings. I should have pointed that out more clearly. You are absolutely right, if the reference is steady for the ACAL duration, then you can isolate the ADC.

author: AndyTT
author_rank: Newbie
timestamp: April 16, 2020, 01:06:49 pm
content: Thank you all for your answers. Looking forward to trying out the SN18 procedure but not feeling confident about the result :-(As a general rule, can I start using the meter after DCV ACAL or should I wait a certain amount of time to let the meter settle?I will update you all with my results as soon as I have some.ThanksAndy

author: notfaded1
author_rank: Frequent Contributor
timestamp: April 16, 2020, 03:26:15 pm
content: Welcome to the club Andy!

author: Kleinstein
author_rank: Super Contributor
timestamp: April 16, 2020, 04:00:41 pm
content: Quote from: AndyTT on April 16, 2020, 01:06:49 pmUsually the meter should be warmed up before doing ACAL. After ACAL one can directly use it - it is not an 34470 after all (these seem to have an odd settling problem after ACAL).

author: maat
author_rank: Regular Contributor
timestamp: April 16, 2020, 10:01:23 pm
content: Quote from: Kleinstein on April 16, 2020, 04:00:41 pmI am not 100% convinced. On my meter I see a very short hiccup (~0.05 ppm) after an ACAL DCV. It then needs about 10 minutes to return to the previous valueThe attached picture shows the ratio measurement of 17k/20k divider before and after ACAL. The divider is sitting in an isothermal chamber and fairly stable. Typically my ADC does not drift at all if the ambient temperature is stable.Edit: Fixed the attachment

author: Kleinstein
author_rank: Super Contributor
timestamp: April 17, 2020, 12:13:52 pm
content: That is an interesting effect of ACAL. My best guess is that there is some thermal effect, e.g. from one of relays at the input. During ACAL the input is disconnected and thus a different power level.

author: SilverSolder
author_rank: Super Contributor
timestamp: April 17, 2020, 01:14:28 pm
content: Quote from: TiN on April 15, 2020, 10:25:00 pmThat sounds serious... How do you measure drift to that level of accuracy? Surely an ant peeing in the corner of the garden outside would influence a measurement at that level?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 17, 2020, 01:31:18 pm
content: If one waits long enough (e.g. 20 days) there is no problem measuring 0.1 ppm/day . It is the internal ADC gain test, so no external reference needed, just a temperature that is about the same as at the start.

author: SilverSolder
author_rank: Super Contributor
timestamp: April 17, 2020, 06:05:06 pm
content: Aaah, I see, it is "referenced to the reference", so to speak.I feel elated when my collection of old gear can stay within +/- 1ppm over 12h... getting it 10x better seems so hopelessly far away!

author: Kleinstein
author_rank: Super Contributor
timestamp: April 17, 2020, 06:45:56 pm
content: It is not about 0.1 ppm over one day, more like 2.1 ppm over 3 weeks. This does not sound as bad and reduces the requirements on the the temperature. It still needs good resistors, but what else to expect in a high end meter.