Keysight 34470A, Calibration and ACAL problems

[Grandchuck]

Just wanted to chime in. I'm seeing a jump (positive offset) after ACAL, then slowly returning to "baseline" after 5-10 minutes.

Normal monitoring of 732A voltage reference is essentially a straight line, after ACAL it will jump and then over time return to previous value. The behaviour is 100% reproducible.

See a similar behavior with a 34465A.

[eplpwr]

The time constant looks a little like a thermal effect. It is rather slow for a real RC and I won't expect that much dielectric absorption. With a Keithley meter I would suspect something like averaging on zero readings - but this a Keysight and should not have that bug included.

Just for a test, how does it look if one has the stable reference connected, change the range to 100 V or 1000 V for some 5 minutes and than back to 10 V. I would not be so much surprised to see a similar settling. Still it would be a bit disappointing to see so much thermal effect.

@Kleinstein

I have tested to change to 100V range and let the meter sit there for >10 minutes, then changed back to 10V range. I can not see any "jumping" in this case. Since the resolution is 1/10th in the 100V range, and the input Z is changed to 10M, the measurements are obviously a bit off. I think it's quite visible when I change to the 10V range in the picture below.

[HighVoltage]

Kleinstein

Here I switched the 34465A and the 34470A to manual 100V range, waited 5 min and switched back to 10V.
The 34465A seems to be more stable
The 34470A shows a small jump.

In comparison I also added the Keithley DMM7510 with am much larger jump.

Test's done on a stable 10.000,040 source.


 

[HighVoltage]

I have an 34465A with s/w v3.0 as well, and if I measure the same stable voltage source the '465 is ruler flat.

After pressing "Autoscale Once" the best resolution, "span", the 34465A does is 100 uV but the 34470A does 10 uV.
So, to really compare correctly, one must set the vertical scale manually also to 10 uV span.

But I agree, my 34465A is also "ruler-flat" even when the vertical scale is manually adjusted.

The '465, however, actually needs ACAL if the temp has changed, whereas the '470 seems to make such minor adjustments that not even the least significant digit changes.

Interestingly, my 34465A almost is not changing after an ACAL and stays rock solid.
And it has been like this since I have it.

[HighVoltage]

In comparison, the Keithley DMM7510 is also rock solid after an ACAL
This test was done on a 10.000,010 V source.
My DMM7510 has a +30uV offset since I have it (Sloppy Keithley Calibration, I guess)
But otherwise it is not drifting at all, that I have noticed.

[Kleinstein]

The time constant looks a little like a thermal effect. It is rather slow for a real RC and I won't expect that much dielectric absorption. With a Keithley meter I would suspect something like averaging on zero readings - but this a Keysight and should not have that bug included.

Just for a test, how does it look if one has the stable reference connected, change the range to 100 V or 1000 V for some 5 minutes and than back to 10 V. I would not be so much surprised to see a similar settling. Still it would be a bit disappointing to see so much thermal effect.

@Kleinstein

I have tested to change to 100V range and let the meter sit there for >10 minutes, then changed back to 10V range. I can not see any "jumping" in this case. Since the resolution is 1/10th in the 100V range, and the input Z is changed to 10M, the measurements are obviously a bit off. I think it's quite visible when I change to the 10V range in the picture below.

It's obvious the 100 V range readings are not good. The interesting part is the recovery in the 10 V range. It looks like there is essentially no extra recovery here. This indicates no much thermal effect with the ADC just from a different reading.  So more like an ACAL specific effect.

A possible reason I could think about of could be a numerical temperature correction that used temperature reading from the last 5 minutes or so. During ACAL those temperature readings may be missing or off for some reason. There is a good indication the Keithlay 7510 uses such temperature corrections - this helps to get good readings even shortly  after power on. So maybe the 34470 uses some similar corrections too to get better stability than the 34465.

If the effect is recovering after something like 5 minutes I would not worry so much about this. However it would be nice to know for sure.

[eplpwr]

Interestingly, my 34465A almost is not changing after an ACAL and stays rock solid.
And it has been like this since I have it.

Well, it's a matter of both selection and also statistics/distribution - you obviously got the "golden" LM399AH of the lot! 

[kj7e]

Interestingly, my 34465A almost is not changing after an ACAL and stays rock solid.
And it has been like this since I have it.

Well, it's a matter of both selection and also statistics/distribution - you obviously got the "golden" LM399AH of the lot! 

My 34465A is also surprisingly stable.  I do see a slight (like 2-3 ppm) downward drift over about 15 min after ACAL, but it seems to land on the right readings after the drift.  If the room temp changes more than a few deg, the ACAL does make an improvement. 

[Magnificent Bastard]

I have this too.  I always wondered why that was happening-- I assumed it is something heating up (or a capacitor charging) somehow during the auto-cal process, and then settling down.

What firmware version is your DMM on?

I'm on the latest version - 3.0. I didn't see this "jump" happening when the meter was on the previous 2.x version, at least it wasn't as pronounced. Since the measured value gets back on track rather quickly I can live with this. I only seen this "bump" on the '470; I have an 34465A with s/w v3.0 as well, and if I measure the same stable voltage source the '465 is ruler flat. The '465, however, actually needs ACAL if the temp has changed, whereas the '470 seems to make such minor adjustments that not even the least significant digit changes.

Interesting!  I wonder if it's possible to "downgrade" the firmware to the last 2.x version, and see if it still happens.  (I would ask Keysight before you do this-- it might "brick" the meter to go backwards in firmware versions).

If the problem goes away at 2.x and then reappears at 3.x, then that would be a huge clue for the Keysight people!  I'm certain that they have source code control, and they will be able to see exactly what was changed in the ACAL from 2.x to 3.x (and who did it too)...

[Keysight DanielBogdanoff]

Quick update, the product support teams have been characterizing a couple units since late last week when this popped up. So far we do see some offset induced by ACAL, but not as big as described here and not anything out of spec.

We have been and will continue to working with the individuals seeing this over e-mail, we may reach out to swap/borrow some to attempt to recreate the problem.

[maxwell3e10]

Here are some very quick data not from a calibration lab! All meters have not been calibrated since manufacture. Voltage source is a REF102.

KS34470:   9.996340 before autocal (previous autocal maybe performed 1 year ago)
                 9.997021 after autocal
KS34465:   9.99689 before autocal  (previous autocal maybe performed 1 year ago)
                 9.99685 after autocal
DMM7510: 9.996920 (didn't do autocal, it takes too long)
HP3457:    9.99690

So, Keysight 34470 does move a lot with autocal, but maybe closer to the right value in this case. But the bottom line, why bother with all these new instruments if a 30 years old uncalibrated HP3457 is just as accurate? 

[iMo]

Quick update, the product support teams have been characterizing a couple units since late last week when this popped up. So far we do see some offset induced by ACAL, but not as big as described here and not anything out of spec.

We have been and will continue to working with the individuals seeing this over e-mail, we may reach out to swap/borrow some to attempt to recreate the problem.

Has the fix been found?

[HighVoltage]

Keysight is working on this problem now and really have put their attention to it.
Thank you Keysight !!!

[iMo]

And the result? What was the issue?

[HighVoltage]

Keysight is still working on this and it might take a little longer.
So it seems we have to have some patience.

[jaromir]

Right now I'm going to setup new electronics laboratory, deciding what TE to buy and I'm expected to submit final list shortly. I thought of buying a few lower grade (lower than 34470A) multimeters and one 34470A to rule them all; but reading this thread made left me a bit unsure about it.

I wonder whether this is problem systematic to all 34470A units or is it just a few isolated cases?
In any case, does any of the affected users have (at least preliminary) fix to the problem?

[eplpwr]

In any case, does any of the affected users have (at least preliminary) fix to the problem?

My fix is to wait 3-5 minutes after ACAL before re-trusting the 34470A readings. Mine is well within spec BTW, so I trust it - checking it against two 3458A at least weekly. Also, I have DMM7510 and Fluke 732A for intercomparisons.

[e61_phil]

Our 34470As drift less than 1ppm after ACAL. That is only around 10% of the 24h spec.

[GEOelectronics]

34470A_#2 = 10.000,178 V (+138 uV)
And immediately after that, I can see a drift in in trend chart.

Has anyone here experience the same with their 34470A?
Any ideas?

I'm beginning to think so yes. I've had mine for a year or two now, I picked it because of the autocal feature and the LTZ-1000, and bought it expressly for a few specific measurements:

1)1000 V with 1G input impedance- Well it doesn't and it's my fault, only up to 10V can have 1G Ohm input, above that, only 10M.

2) Measure 1G Ohm resistors with high precision. It should but it doesn't really. Up to 500M it seems fine, at 1G it takes forever to decide, basically .

For example, I've been working with 99M 0.1% resistors. What I need is 990M Ohms with precision and stability for a HV divider. The 34470A reads 99M swiftly and with stability, all are within tolerance./ Any two read 2X 99M, any 3 read 3X 99M and any 5 read 495M just as they should. Try to read all 10 resistors it gets muddy, same with a single 2W 1G 1% resistor

I trust the resistors, they show good on every instrument I own. I trust the series stack of resistors, they read fine on an ESI Guarded Wheatstone bridge and on an HP 4329A.

Some of the difficulty is in my inexperience with the modern high end, high tech instruments.

So far the ESI has proven the best instrument for me, it gives enough digits to be meaningful, and is rock steady, is fast and accurate. The 4329A is good and can read at 1000V which is what I need, but is analog, no resolution and rated rather poorly, although it does work very well.

I'm not disappointed yet but am confused and confounded, that something that should be easy is so hard to master.

George Dowell

[TiN]

You'd have much better results if you try use external stable 1kV source applied to your 990M/10M divider and measure output voltage by 34470 on 10VDC range instead.
Megaohms+ is where leakage/cabling/shielding start to matter a lot.

[GEOelectronics]

Understood. Thanks. Using a NIM HV PSU (Nuclear Instrumentation Module), pretty stable. Made to drive large sensitive photomultiplier tubes (can run 100M Ohm dynode string, down to <100k Ohm)  where any HV change will ruin measurements.

1)Measure resistors with '4470A = 990M
2)Measure NIM HV PSU with '4470A = 1000.0VDC
3)Measure NIM with 990M resistor in series with '34470A on 10V = 10.07

It's as if the 10 M input is not exact, could this be? Will try it with 34470A set to 10V but 1G input and an external precision 10M resistor tomorrow.

It has to be well below 1% accuracy to be of any use to me.

George Dowell

[maxwell3e10]

Yes, the spec on 10M input impedance is +/- 1%

[GEOelectronics]

Yes, the spec on 10M input impedance is +/- 1%

OK. Well it's within specs then. Thanks.

George Dowell

[e61_phil]

I wouldn't use the internal 10Meg as part of the divider. I made bad experiencies with such a setup.
It is much better to switch the meter to high Z and have the whole divider external.

[Dr. Frank]

If you need really high impedance @ 1kV, best way would be a differential method, i.e. creating 1kV with a stable/precise DC source / calibrator, and then sensing the difference at high level, maybe also with the 34470A. That way, you'll also get high precision.
One these old Fluke 88x Differential Voltmeter really had a 1kV reference inside.

The other way is to use a dedicated HV divider, which usually is base on the 10M input impeidance inside the DVM.
You'll have to calibrate the divider ratio first at a lower DC level, and then maybe set the '470A for a math multiplier.
I wonder if there are still 990MOhm divider available..

Frank