Drifting 3458A -- fixed? Or coincidence? - Page 1
Forum section: electronics / general

author: Oldtestgear
author_rank: Regular Contributor
timestamp: September 09, 2025, 09:13:25 am
content: Service Note 3458A-18A refers to an acceptable maximum drift of 0.43ppm/day. This is higher than most users experience but it is the stated limit.I have one 3458A left from my experiments with the spare boards I purchased last year. It is complete but had been used to test all the boards so had had a fairly hard time in the last year. I re-assembled it to test for drift & ran the test from 1st to 18th July. Changes in the ambient & internal temperatures were logged with the Cal 72 results on a daily check. The average drift was 0.7 ppm/day over 17 days. Also noted is the drift was always in the same direction. An obvious fail.I then tried re-capping both power supplies at the end of August after it had been left switched off for the 6 weeks or so between the test & re-cap. Some caps were direct replacements with a few increased where it was possible to fit them. I then performed the test again from 29th August to 8th September. From initial switch on it was left 6 hours before the test was started. Total drift is 0.32 ppm/day from the start of the test. Ignoring the first 48 hours the drift is 0.14 ppm/day.So my faulty 3458A is now within specification after a re-cap.Has anyone else tried this & got a similar result? Any thoughts?To be clear this 3458A had been stored in a barn when I bought it but has been in my workshop for more than 12 months when the experiment was started so had ample time to adjust to the new surroundings. I am continuing to monitor drift in case there is a sudden change in performance in a few days time.Any ideas welcome.Phil

author: lichen8867
author_rank: Newbie
timestamp: September 09, 2025, 11:55:41 am
content: in our lab, we have 4 hp3458a and 2 fluke5700. half of them 10 years old and the others 5 yeas old. most of them keep powered on in half a year in one year. and we calibrate them every 2 years. one of the hp3458a, which is the worst one, read out error 6~7 ppm, comparing with others, even if it was calibrated yestday. the other 3458a read data match well with fluke 5700(<1~2ppm).in my idea, keep the 3458a powered on will help it settling for long time drift. after it powered for several years, the residue longtime drift is small.but stll case by case, some 3458a(like the worst one in our lab) need 1 week to settle. calibration is always done after it is powered up for 1~2 hours, so such this 3458a usually has a error of 6~7 ppm.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 09, 2025, 03:14:56 pm
content: The long time drift that one sees relative to a calibrator is a different point from the SN18 service note looking at the CAL72 data.The normal long term drift is to a large part drift of the LTZ1000 ref. module. If not too large, ACAL can compensate for small drift of the ADC gain that is reflected in the CAL72 values.The problem is the drifty U180 case: large drift with the CAL72 values that would need rather frequent ACAL and is thus hard to work with. In addion to the ADC gain drift there can also be other faults (increasing DNL) that may happen with bad resistors in U180 - these are however hard to measure. The CAL72 part in contrast is easy to check and thus used as a test for a drifty U180. Some initial (e.g. 2 weeks) drift after a long time of non usage seems to be normal. So far in most cases a bad U180 seem to drift in mainly one direction and things tend to get worse over time. It is still possible that both sides of a resistor ratio are effected and the drift could even change sign. There is still the (more theoretical) possibilty that U180 goes bad and the effect is hardly visible in CAL72.The now observed 0.14 ppm/day or 0.32 ppm/day are still not really good. An intact U180 usually tends to be well below 1 ppm/week. So I would consider the meter still suspect and worth still logging the CAL72 values and checking from time to time, so see if the drift gets worse or continus over a long time.I see relatively few chances how the supply capacitors would effect the ADC gain. Borderline capacitors (and thus more ripple) are more causing extra noise.The can be a slight chance that poor contacts (especially for grounds) in the A1 to A3 link could cause extra drift. Reseating the cable connectors may be worth a try and could happen during capacitor change.

author: aronake
author_rank: Frequent Contributor
timestamp: September 10, 2025, 05:08:50 am
content: Just keeping the 3458a running may reduce CAL72 drift over time. I have one that was drifting at around 0.4ppm per day for a couple of month to later settle down to less than 0.05 ppm/day. There are many other similar stories for other 3458a. So it may be that you have had it running rather than changing caps that have made CAL72 stabilize. Keep it running 24/7 for a couple of month and see how CAL72 drift develop. It may continue to stabilize.

author: Oldtestgear
author_rank: Regular Contributor
timestamp: September 10, 2025, 09:52:28 am
content: QuoteI agree that this drift is very far from ideal but it is within the limit set by HPAK. Time may have allowed the 3458a to settle although it was switched off between tests to minimise the possible changes during operation. I do not know what the cause of the improvement is, only that it occurred. I am hoping that other enthusiasts may have done the same tests as me & are willing to share their experiences on the forum. More data has to be useful even if it contradicts my tentative findings. The test continues........Phil

author: aronake
author_rank: Frequent Contributor
timestamp: September 10, 2025, 11:56:38 am
content: another trick some people have seen success with is to disconnect the internal fan while letting it run 24/7 for some week. typically this accelerates the CAL72 drift, but seems to, in some cases, quicker bring it to a more stable state.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 10, 2025, 12:07:00 pm
content: There are some parts that ran rather hot. One example are the fst comparators that are no longer available.So I would not run without the fan. At best reduce the air flow a little.

author: Oldtestgear
author_rank: Regular Contributor
timestamp: September 23, 2025, 11:54:50 am
content: I promised data for the tests I have performed on the 3458a, so as promised:July tests:1st July 987.88645518th July 987.873753Drift average (17 days) 0.76 ppm/dayAugust/September tests:29th August 987.87591215th September 987.872398Drift average (17 days) 0.21 ppm/dayFurther tests31st August 987.87374117th September 987.872398Drift average (17 days) 0.08 ppm/dayThe July drift was always in one direction, irrespective of temperature changes, & fairly consistent day to day.The later tests showed drift that was affected by temperature & moved in both directions as the temperature changed.I have now calibrated this 3458a using a Datron 4910 plus a 10 KOhm standard resistor. AC was compared with my Datron 4920 plus a calibrated Keysight 34470a. The results were consistent, so I did not try the full calibration procedure for AC. The Datron 4910 was checked using my old (Keysight calibrated) 3458a in August. This has now been sold.I plan to run the same drift check now it has been adjusted in case of any sudden changes in drift.All thoughts & comments are welcome.Phil

author: MiDi
author_rank: Frequent Contributor
timestamp: September 23, 2025, 04:17:02 pm
content: Would be nice to have a drift graph.If the drift rate is constant, it is possible to separate drift from TC.My unit shows an input dependent drift, @10V drift is near 0, below 10V drift rate increases.Did you measure drift with constant input e.g. short?

author: aronake
author_rank: Frequent Contributor
timestamp: September 24, 2025, 05:11:30 am
content: Quote from: Kleinstein on September 10, 2025, 12:07:00 pmFair point. I ought to at least have included some warning and do at own risk about this.The specified operating temperature for the 3458a is 0 to 55 degree C. Disconnecting the fan raises the temperature around 10 degrees. So from specification perspective, there is much headroom. But if one of these Elantec comparator is on the verge of breaking down, some fanless running may be the last nail in their coffin...The 3458a also have a specified storage temperature of - 40 to + 75 degrees. That is very "generous"...... I could see a need for very long stabilization time after a 3458a has been stored at minus 40 degrees for some weeks....

author: Kleinstein
author_rank: Super Contributor
timestamp: September 24, 2025, 07:04:12 am
content: The additional 10 K may be the increase in the average temperature or at the temperature sensor. The high power areas/parts would see more increase in the temperature when the fan is not working. Especially the parts that already run hot and are not blocked from air flow get helped a lot from the air flow. For the comparators I would more expect some additional 20 to 30 K.

author: aronake
author_rank: Frequent Contributor
timestamp: September 24, 2025, 09:10:47 am
content: Quote from: Kleinstein on September 24, 2025, 07:04:12 amgood point. so disconnecting the fan may be a bad idea...

author: Oldtestgear
author_rank: Regular Contributor
timestamp: October 06, 2025, 11:14:03 am
content: I was asked for the data behind my observations. This is the raw data for each day's test. I have included the drift calculations for day 7 & the last day in the run. Although the last two data sets are from a continuous run, the results after a re-calibration did show a large change in ACAL / CAL72 value, hence results are split from the date (20 Sept).What do the result prove? I think it shows the 3458a is now working as it should which is good news but I have no idea why the change occurred. As always, I am open to any comments or suggestions.The latest test is ongoing.CheersPhil

author: aronake
author_rank: Frequent Contributor
timestamp: October 06, 2025, 01:06:56 pm
content: Quote from: Oldtestgear on October 06, 2025, 11:14:03 amJust keep it running is likely what has made the difference. Allow things to settle in and dry up. Its not too uncommon this fix not too drifty A3 boards.