Running-in phase behaviour of Keysight 34401A - Page 1
Forum section: electronics / general

author: Verticon
author_rank: Contributor
timestamp: August 15, 2023, 09:27:29 pm
content: I wonder if my Keysight 34401A DMM that I have bought a year ago and that was professionally calibrated afterwards shows abnormal behaviour. Problem 1 is the long running-in phase after switching it on. It needs almost 2 hours until it is stable within a 1ppm window. The 1st picture shows a typical behaviour after the instrument is switched on. The measured voltage was supplied by a ADR1399 reference that had enough time to stabilize on a ppm scale.The second problem is a small drift of the displayed value on a time scale of approximate 5 minutes after a voltage step has applied. The drift is in the order of one or two ppm. This happens in both directions of a voltage change, from smaller to higher and vice versa. A typical baviour is shown in the 2nd picture where the input voltage was abruptly changed from roughly 7V to 10V.I hope somebody who also owns a 34401A or a similar instrument can tell me if this is normal or perhaps signs of dielectric absorption.

author: Andreas
author_rank: Super Contributor
timestamp: August 16, 2023, 06:02:47 am
content: Hello,a warmup drift of 2-4 hours is completely normal.If the instrument was off for a longer time it may take weeks until the instrument stabilizes.Attached a picture with 1 minute averaged values during warmup.with best regardsAndreas

author: iMo
author_rank: Super Contributor
timestamp: August 16, 2023, 07:11:59 am
content: The longer warmup is typical, afaik, it may take even longer to fully stabilize, like 5-12hours (your graph shows the temperature is still not stabilized). 10ppm during the warm up is a good result for this meter (the diff between ambient and internal temperature is around 18.5C from my measurements). Mind the meter has its own TC you have to know as the meter simply follows the ambient temperature after it stabilizes. I mounted a temperature sensorinside the unit(and with thermal isolation beneath the cover to slow down the temperature changes) and I make a correction based on that.Random walk within 1ppm is typical as well.

author: Verticon
author_rank: Contributor
timestamp: August 16, 2023, 08:12:44 pm
content: Many thanks, Andreas and iMo,for your quick answers. So I don't need to buy a new one.

author: Andreas
author_rank: Super Contributor
timestamp: August 16, 2023, 08:20:12 pm
content: Hmm,I cannot confirm the long settling time.On my side after a change from 6,6V to 10V it needs 1-2 readings 4-8 sec @100NPLC until I am within noise level.So I would repeat the experiment after 2 weeks 24/7 operating time.Assuming that you have proper connections between reference and DMM. (low thermal EMF).with best regardsAndreas

author: Hydron
author_rank: Super Contributor
timestamp: August 17, 2023, 11:40:39 pm
content: Yep, as per the other replies an annoyingly long warm-up is to be expected. It's quite a dramatic comparison to my DMM6500 which is about an order of magnitude quicker to warm up (but on the other hand takes much longer to boot, so the 34401A is what gets used for any quick measurements).Can't comment on the other issue sorry - haven't really tested that sort of behaviour.

author: iMo
author_rank: Super Contributor
timestamp: August 18, 2023, 09:18:51 pm
content: It could be the slow warm-up is caused by the heavy transformer mounted in the opposite corner and isolated from the ADC by a rather large air gaps. There is none fan (compared to the 6500 and others). The transformer slowly warms up the "U" shape like frame where it is mounted and it takes hours till the thermal field spreads via the frame to the other side where the ADC is located. Also the modern meters have got switching supplies with much lower thermal mass and with the fan.

author: IanJ
author_rank: Unknown
timestamp: August 25, 2023, 04:09:35 am
content: Vertigon,Of the 34401A units I have/had I never noticed such long warm up etc.I'd suggest run it 24/7 for a week and re-test.Also, what is the environment like, I.e. any temp swings, the humidity and also its physical location next/against other units etc.Ian.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 25, 2023, 05:23:32 am
content: Not havinng a fan causes a weak coupling to the envirinment and thus the long time constant. So not surprize that it takes a long time to warm up. A fan can speed up the warm-up quite a bit, but it comes with noise.The other point with the warm up is how significant the temperature is, so how much temperature rise and how much TC of the meter. The TC can vary between units, so some may be better than others by quite a bit.Humidity, including the indirect effect of getting a lower RH when warm can cause another drift after turn on. However this time-constant is usually quite long (weeks) and it is thus less practical to wait for it to settle and one may not easily see the effect.

author: alligatorblues
author_rank: Regular Contributor
timestamp: September 13, 2023, 10:56:38 pm
content: I presently have 3 34401As, and I've had about 10 in the past. If you read the instructions, it tells you 2-hour warm-up before making measurements. It's also a drifty meter, about 1-2ppm/mo.

author: bdunham7
author_rank: Super Contributor
timestamp: September 13, 2023, 11:18:53 pm
content: Mine takes 3 hours to stabilize as well as it is going to, and then it may move around 1-2 counts (1-2ppm) which I mostly attribute to changing temperatures. If the temperature holds reasonably constant in my room, the 34401A may show the exact same reading for hours on end. If I cold-start it, it reads 15-20ppm low right away but most of that is gone within the first 30 minutes and then it takes the rest of the time to sort out that last 5 counts. If it has been sitting for over a month, I give it overnight to stabilize. I don't see any long term drift, certainly not 1-2ppm/month. Clearly this is all well within the specified accuracy, in fact after the specified 2-hour warmup it seems to be within the 24-hour spec--and the meter is 20+ years old and is on the original calibration settings. When thoroughly warmed up, it appears to be accurate (at 10 volts) to within a few ppm. I'll let you know how few when I get the USA Cal Club kit.

author: J-R
author_rank: Super Contributor
timestamp: September 14, 2023, 12:43:42 am
content: Seems like it would be better for these graphs to have at least two other 6.5 or 7.5 digit DMMs along with the 34401A to compare against?My black terminal, HP34401A also matches bdunham7's observations and I have not seen much drift over time at all. I can say 100% for sure less than 4ppm over the last 3.5 years since it was calibrated at time of purchase.

author: iMo
author_rank: Super Contributor
timestamp: October 07, 2023, 07:00:11 am
content: Quote from: Andreas on August 16, 2023, 06:02:47 amWhile having better external reference I've been observing this (not well understood by myself) behavior in my 34401A (+10V range, the reference is 24x7 on and wired to the dmm):1. after powering on (after a couple of days or weeks being powered off) the "initial voltage" after the initial warm-up (say after 6 hours from the power-on) is by couple of ppm higher than expected, and then slowly drops down towards the "expected" value (ambient temp compensated)2. with say 1-2 days powered off the initial voltage is approx 1ppm higher and it takes ~2 days till it drops down3. with say 2-3weeks powered off the initial voltage is approx 2-3.5ppm higher and it takes ~10 days till it drops downSure, it is not much for such a meter (moreover with up to 1ppm p-p noise) but still I would be interested to know how to track down a specific part inside the meter causing this perhaps by the humidity induced behavior?Or is it a capacitor's leakage or DA?Or is it a leakage caused by the 25y old dirt on the pcb?..

author: Kleinstein
author_rank: Super Contributor
timestamp: October 07, 2023, 09:04:26 am
content: The main culprit for the drift after a long time on not using the meter is humidity: in the times of no using it and thus keeping it relatively cool humidty gets absorbed be the PCB and some plastic parts. This can cause mechanical stress and this way effect the readings a little. How senstive the meter is to humidity depends on the details - a hard to predict part in the design. THT parts may be a bot less sensitive and so are ceramic / hermetic cases (they still see the PCB stress).When using the meter it gets warm and this drives out some humidity. Some 10 K wrmer inside corresponds to half the RH. The time constant can be on the order of days to weeks (a little faster when warm).

author: iMo
author_rank: Super Contributor
timestamp: October 07, 2023, 09:49:38 am
content: The difference T_dmm (measured on the aluminum shield above the ADC after the warm-up) vs T_ambient is aprox 17.5 degreesC in my case, btw.

author: guenthert
author_rank: Frequent Contributor
timestamp: October 19, 2023, 10:15:15 am
content: Quote from: iMo on October 07, 2023, 07:00:11 amKeep in mind that the meter's input isnotof high impedance when it's off.

author: guenthert
author_rank: Frequent Contributor
timestamp: October 19, 2023, 10:25:45 am
content: Quote from: Verticon on August 15, 2023, 09:27:29 pmAside that the 34401A is EOL'ed now for seven years and your unit long, long time out of warranty, afaik, short time stability was never specified for those. The closest I find is the 24h 'accuracy' given as 15ppm of reading + 4ppm of range in the 10VDC function. So I don't think the claimed 'problem' is one. It's after all a (fine) bench meter, never meant for metrology use (the input connectors alone prohibit such).Quote from: Verticon on August 15, 2023, 09:27:29 pmHow is the source switched? Is this a mechanical switch? If so, I'd guess thermal EMF from the switch contacts.

author: iMo
author_rank: Super Contributor
timestamp: October 19, 2023, 10:41:20 am
content: Quote from: guenthert on October 19, 2023, 10:15:15 amMy voltage reference can source ~20mA (as well as a short).

author: Kleinstein
author_rank: Super Contributor
timestamp: October 19, 2023, 11:56:55 am
content: Quote from: guenthert on October 19, 2023, 10:15:15 amForm the circuit diagram the HI input therminal fist goes to a reed realy (K101) and withput power this relay would be off.So in the normal off state the input is actually high impedance. There may just be a short phase on power off that may no longer be very high impedance.

author: guenthert
author_rank: Frequent Contributor
timestamp: October 19, 2023, 01:23:07 pm
content: Quote from: Kleinstein on October 19, 2023, 11:56:55 amRight, I stand corrected. Now how did I get the idea that it wouldn't be? I just checked my meters (except the 3457A, which is on) -- only my Datron 1271 isn't high impedance when off.