Fluke 5440A repair

[MadTux]

Better approach would be to compare like 3x 5440B with 3x good DVM as null meter in 1-2, 2-3, 3-1 configuration.
Or replace 2x 5440B with good stable 10V reference, like Fluke 732.

Fluke reference inside 5440B should be at least equal or most likely better than the reference inside most good DVMs.
So you basically compare accuracy of DVM and 5440B and 5440B is most likely the winner, here.

[Rax]

Better approach would be to compare like 3x 5440B with 3x good DVM as null meter in 1-2, 2-3, 3-1 configuration.
Or replace 2x 5440B with good stable 10V reference, like Fluke 732.

Fluke reference inside 5440B should be at least equal or most likely better than the reference inside most good DVMs.
So you basically compare accuracy of DVM and 5440B and 5440B is most likely the winner, here.

I think the reference in both the "A" and the "B" 5440 is two stacked SZA263s. The 3458A I'm using is a well aged, but completely rehauled unit (some modules were replaced, but not the critical boards), and then calibrated by Keysight. So there's a well aged LTZ1000A in there.

I find it very challenging to accurately determine where is the tempco I may be observing at a given time (and particularly tricky, why there's "no tempco" for a certain stretch of temperature gradient...  ), but I have a sense my 5440A is not terribly stable across delta T. I'd like to think my sample can reliably calibrate 8.5 digit DMVs, but I'm not quite convinced. This may sound sacrilegious, but I'd trust my 731B (with a deliberate temperature observation) than my 5440A at 10V. This working hypothesis is currently being confirmed by the 3458A.

[Rax]

BTW, as much as those points recommended by the SM look great, at 10V out my 5440A isn't all that great. I am seeing it consistently over 1ppm high. INT CAL doesn't change that much. All this is at 24-25C, which is where my bench is sitting around right now.

[Kleinstein]

It is normal that the internal calibration should not change much with the 10 V. The 11 V range is kind of the base range, where the other ranges are based on with the INT CAL. For the 11 V range the INT cal should only corect the offset, but not change the scale factor - as there is nothing else internal to compare too.
So if the 10 V are consistantly off by 1 ppm, that this a calibration thing of either the DMM or 5440A. The 22 V range should be a little less stable, as it includes a gain stage and an exra scale factor from INT CAL. So if the 10 V values is different and the 20 V not this would be some error of either the meter of calibrator, e.g. ACAL at the 3458, the ADC or INT cal at the calibrator (e.g. DAC linearity).

[Rax]

So if the 10 V are consistently off by 1 ppm, that this a calibration thing of either the DMM or 5440A. [...] ACAL at the 3458, the ADC or INT cal at the calibrator (e.g. DAC linearity).

I think it's probably a combination of ACAL (fresh or due?), INT CAL (fresh or due?) and environmental temperature.

For instance, here's the result of a linearity read of outputs from 1V to 10V after a fresh ACAL on the 3458A. This was collected at conditions encroaching on the high end of the reasonable bench temps (26.9C).

• 1V: .99999954V
• 2V: 2.0000004V
• 3V: 3.0000006V
• 4V: 4.0000014V
• 5V: 5.0000012V
• 6V: 6.0000014V
• 7V: 7.0000014V
• 8V: 8.0000014V
• 9V: 9.0000012V
• 10V: 10.0000008V

I think this would be fine for both linearity (wrong on this?) and accuracy at specifically 10V.

[Kleinstein]

The linearity looks OK (though not perfect) for the combination of the DAC and ADC.

It looks like there is a bit offset on the order of -1 µV or a little less,  likely at the calibrator.


INT CAL should not make much difference for the 10 V range, mainly for the offset.

[Rax]

Further linearity measurements. I'm learning that ACAL on the 3458A is essential. I don't think anyone should do any measurements they want to consider reliable without an ACAL prior to it within less than an hour. I've seen the 3458A stray by over 2ppm within a few hours of bench time (from the last ACAL ALL, or just an ACAL DCV - say these were performed in the morning, and the 2ppm was observed in the mid afternoon).

• 1V: 1.0000010V
• 2V: 2.0000017V
• 3V: 3.0000023V
• 4V: 4.0000028V
• 5V: 5.0000027V
• 6V: 6.0000028V
• 7V: 7.0000028V
• 8V: 8.0000023V
• 9V: 9.0000028V
• 10V: 10.0000021V
• 11V: 11.0000016V

Over 11V, the 3458A switches to the 100V range and I don't think the readings are still useful.

[Kleinstein]

Noamally the 3458 should not be that bad with need for ACAL. There is however a failure mode of the ADC developing a tendency to drift. So maybe the unit start to devolopement that fault (faulty U180 and thus an expensive repair). For testing the drift of the constant CAL72 should be observed over some time (see HP Servis note 18).   Another possibility may be the drift of units that where not used for an extendet time - that can cause additional drift for the first few weeks.

[Rax]

Another possibility may be the drift of units that where not used for an extended time - that can cause additional drift for the first few weeks.

This is probably it - I don't think the lab owning this is giving it much use at all. I expect it's been sitting pretty much since it came back from Keysight.

That said, I'm not sure if this expected drift after storage effects the reference, the additional precision parts (resistors, etc.), or both? I'm thinking in terms of INT CAL on the 5440A, which I think doesn't have much to do with the reference, but associated parts and compensates for their drift.

The reason I'm thinking that it's possible ACAL similarly involves other parts than the internal reference is that the 3458A is in perfect agreement with the US Cal Club traveling FX reference, also a freshly Fluke-calibrated 8846A and other lesser instruments.

[bdunham7]

Noamally the 3458 should not be that bad with need for ACAL. There is however a failure mode of the ADC developing a tendency to drift. So maybe the unit start to devolopement that fault (faulty U180 and thus an expensive repair). For testing the drift of the constant CAL72 should be observed over some time (see HP Servis note 18).   Another possibility may be the drift of units that where not used for an extendet time - that can cause additional drift for the first few weeks.

The reason I'm thinking that it's possible ACAL similarly involves other parts than the internal reference is that the 3458A is in perfect agreement with the US Cal Club traveling FX reference, also a freshly Fluke-calibrated 8846A and other lesser instruments.

I'm not sure how long the 3458A had been running continuously in Rax's garage lab prior to the previous ACAL, but I saw it change ~2ppm over a 4 hour period at a reasonably constant temperature.  But after each ACAL it seems to agree with the FX, the 5440A and other stuff within the limits of reasonability.  That amount of drift seemed a bit surprising.

Just for a frame of reference on the aforementioned 8846A, here's the last overnight log I did with it and the FX reference.  Temperature was 22+/-1C, I went off to bed about at about 1 hour in, got up at about 9 hours and then bumped the HVAC a few degrees, opened some doors, etc.

[Dr. Frank]

Concerning the T. C. of the 3458A, w/ and w/o ACAL, I wrote a comprehensive article here: https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg5230803/#msg5230803

The 3458A will practically not drift, if not powered, therefore, it might still read very accurate.

The 10V cardinal point checks the 11V range of the 5440A most accurately, as this value is used for its basic calibration as well.

As said, the other ranges, 22V, 250V and 1000V, are calibrated once in the factory, and practically never need a new external calibration any more. The internal calibration will always reproduce the original gain factors.
Therefore, to check these 3 ranges of the 5440A, you need to measure 13 and 20V in the 100V range of the 3458A, 100 and 250V in its 1kV range, and about 276V and maybe 1kV for the highest range.
I wrote a longer article about the Fluke 752A precision divider where I also analyzed the different pitfalls of the 3458A, in conjunction with the 5442A:
https://www.eevblog.com/forum/metrology/influence-of-switch-resistance-in-hamon-dividers/msg4221751/#msg4221751

E.g. those 13, 20 and 100V measurements are quite delicate, as ANY DMM has problems with E. M. F. voltages in this 100V range, due to the usual 100:1 divider. You need to make one careful null reading of the 3458A, 100V range, and subtract that value afterwards. Always use appropriate statistics on the 3458A, as described elsewhere.
Similarily, you need this procedure for precise calibration of 200mV and 2V range.
These about 276V will set 5440A to its 1kV range, but can be measured by the 3458A w/o it's mediocre heating /power effect at 1kV, which might be up to 12ppm additional shift.

I recommend again, to not do any external calibration on the 5440A, as it seems to be spot on within 1ppm
Frank

[Rax]

Looping back on this, I think, indeed, the 5440A is well within spec, and quite a bit more. Everything I've seen from the 3458A says 5440a is within 1ppm during two weeks of checks. I'm happy the 3458A was available to decide the argument. To those watching the saga, the Prema 6048 seems to be about 4ppm high before its op amp mods, and its op amp mods seem to have added about 2ppm on top of that, amounting to something like 6.64ppm high.

I feel I can deem the Prema off and the F5440A dead on. Maybe I'll use the latter to calibrate the former. Of course, the factory cal constants of the Prema are restorable by procedure, but given the op amp upgrades, useless in that configuration.

[Rax]

BTW, as much as those points recommended by the SM look great, at 10V out my 5440A isn't all that great. I am seeing it consistently over 1ppm high. INT CAL doesn't change that much. All this is at 24-25C, which is where my bench is sitting around right now.

Just to clarify - this was before I figured how critical fresh ACALs are for the 3458A. Once that became clear and part of my methodology, the 5440A measured well within 1ppm of 10V.

[Rax]

Therefore, to check these 3 ranges of the 5440A, you need to measure 13 and 20V in the 100V range of the 3458A, 100 and 250V in its 1kV range, and about 276V and maybe 1kV for the highest range.
Frank

I realized I forgot to post the results of the checks I did at Frank's recommendation. Please take with a grain of salt, I just did one measurement after another, not undertaking the careful measures he recommended. Partly, by then the time with the 3458A became limited, and unfortunately my time with the 3458A coincided with holidays and guests visiting for the entire period and so on.

Anyway, please see below. The 10V reading was the most recent during that morning, but may have been a couple of hours before the HV checks batch.

• 10V: 9.9999973V
• 13V (100V range): 12.999992V
• 20V (100V): 19.999976V
• 100V (1kV): 99.99872V
• 250V (1kV): 249.99657V
• 276V (1kV): 275.99650V
• 1000V (1kV): 999.98753V

The 10, 13, and 20V readings were with a 1000 NPLC on the 3458A, the rest with 100. In retrospect, I should probably have done the exact opposite, so I'd leave enough time to the 3458A to reach thermal equilibrium.