For an old instrument even slow aging should have come to an end, except if it depends on operating temperature and the instrument was essentially off all those years. Did you think about trying a TEC pile on that ADC hybrid U180? I mean if the firmware of the 3458A does not support temperature and drift corrections, maybe you can do it by chip temperature control. If aging depends on chip temperature, one could try running that chip a little cooler.
Before trying mods on an expensive instrument i would decide upon a certain goal in terms of ppm/year or so.
Regards, Dieter
Yes, i understood that the 3458A has a firmware procedure "ACAL" that takes care of ADC temperature and time drift. Anyway, this thread is about the risk of loosing the ADC and apparently an observed drift of 0,03 ppm/day (about 10 ppm/year) of the ADC alone makes people nervous. That's why i am asking: At which temperature is that ADC hybrid running? Is the temperature sensor of the instrument inside that hybrid?
..
Regards, Dieter
I don't understand why they consider the LTZ1000 drift. A drift of the LTZ1000 shouldn't influence CAL72.
Whatever Agilent/Keysight write, Midi's records demonstrate the ADC aging isn't a random process but something else and very regular. And it does reach 10 ppm/year at a daily drift rate of 0.03 ppm in his case. Hard to believe nobody tried to find out what that process is (and how to stop it).
Regards, Dieter
My 3458A came back from Keysight. They replaced A1 DC board, A3 ADC board, and A7 display board, the EMI filter, fan, and mechanical switches. This is very thoughtful actually, so I have a new display too This was all included in the initial repair service contract, amazing experience. I've sent it in a pretty shitty box, and it came back in this amazing box They tested and calibrated, and declared it "like new" factory state. This is great The cal data looks amazing. Now we'll see how it drifts. I'll take it apart at some point to see the replacement boards, but hesitant given the very nice cal sticker on the screws.
Whatever Agilent/Keysight write, Midi's records demonstrate the ADC aging isn't a random process but something else and very regular. And it does reach 10 ppm/year at a daily drift rate of 0.03 ppm in his case. Hard to believe nobody tried to find out what that process is (and how to stop it).
Regards, Dieter
Why would anyone make the effort when the 3458A is not a primary standard anyway (and not intended to be).
If i want more accuracy for DC i hook up a Fluke 732 and do a differential measurement using the transfer accuracy of the 3458A. Thats a sub-ppm measurement right there, no need to go through the trouble trying to get the 3458A in that region by itself.
edit: Or don't use a 3458A at all, only 732, dividers and null detector.
Which Keysight Lab did you send your 3458A to?
...
Whatever Agilent/Keysight write, Midi's records demonstrate the ADC aging isn't a random process but something else and very regular. And it does reach 10 ppm/year at a daily drift rate of 0.03 ppm in his case. Hard to believe nobody tried to find out what that process is (and how to stop it).
....
To protect his investment. If that non-stochastic drift process is the one that finally destroys the ADC, as was argued before. It's nice that you can get a repair, but it's bad if the instrument has a built-in failure mechanism like those Tektronix 2465 scopes i mentioned before.
Regards, Dieter
PS: For example before i buy a 3458A i would ask: Is there a numerical limit in the ACAL procedure? Maybe ADC failure just means that ACAL arrived at a 100 ppm limit or so. Then you can predict how long it will take.
PS: I guess Midi posted his findings in this thread because he was afraid of loosing his ADC. It's not about me.
@ Frank: 0.03 ppm/day times 365 days/year gives 11 ppm/year. That is an observation of Midi, please look at the diagram posted above. Let's stick to facts.
Regards, Dieter
PS: I guess Midi posted his findings in this thread because he was afraid of loosing his ADC. It's not about me.
Good ones (still "cheap") are less than 1 ppm when new.
No, of course it matters for a buying decision if there are reports about drifting or failing ADCs in 3458A voltmeters, especially since the ADC seems to be the better part of that instrument. You don't have to repeat Agilent/Keysight advertising.
The question for the limits of the ACAL procedure is completely valid and i haven't seen a useful answer. As the existence of this thread shows, there may to be a gray zone, where the ACAL procedure is hiding real problems until it is to late.
As far as i remember 1 ppm is the tracking limit of LT5400 resistor arrays, which have been used by others to make high resolution ADCs. Those are specified with tracking of 0.2 ppm typical. My own experiments with Nomca resistor arrays gave an upper limit on tracking stability of about 0.03 ppm over twelve weeks. That would be 0.12 ppm after a year, if it were a continuous linear drift like your 3458A ADC.
Regards, Dieter