Keysight 34470A, Calibration and ACAL problems

[HighVoltage]

I have two Keysight 34470A and one 34465A.

The 34465A works perfectly and is very stable over a long time with no noticable drift and the ACAL function works perfectly as well.

But the two 34470A have shown huge drifts over the years and I am wondering, if this is caused by using the ACAL function.
Because of these drifts, Keysight exchanged both instruments and delivered two brand new instruments to me with new calibration certificates about 18 month ago.

At present time, I have a very stable and verified 10.000,040 V reference as a loaner and use it to calibrate all my 6 1/2 digit meters. All my 6 1/2 digit meters were within +/- 40 uV of 10.000,040 V before calibration, except the two 34470A. Even most of my 34401A were between +/- 20 uV!

The two 34470A showed this:
34470A_#1 = 10.000,135 V (+95 uV)
34470A_#2 = 10.000,178 V (+138 uV)

After calibration against the reference, the 34465A and the two 34470A showed a perfect value 0f 10.000,04x V
34465A       = 10.000,04 V
34470A_#1 = 10.000,040 V
34470A_#2 = 10.000,042 V
And within 1 hour after the calibration there was no significant change.

Then about 1 h later I pressed ACAL on all three DMM and got the following results:
34465A       = 10.000,04 V (no change)
34470A_#1 =   9.999,965 V (-75 uV)
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?
I know of at least one eevblog member who has the same problem on his 34470A and would rather return it to Keysight for a refund.

I have followed the calibration procedure of the service manual and it passes the calibration perfectly.
May be one should NOT use the ACAL function of the DMM?

Any ideas?

[Magnificent Bastard]

Wow!  There appears to be something really wrong with their ACAL design and/or firmware technique.  The LTZ000A references in these meters appear to be a design where less care was taken (as opposed to the reference in a 3458A) to minimize thermal issues.  I particularly dislike the fact that they placed it right in the air stream of the fan; and also their choice of using a cheap header for connection to the main board; these are sources of drift against temperature; and having a "breeze" flowing across this cheap connector (under the reference board) cannot be helping matters.

In a DMM that relies on ACAL to stay accurate, it is imperative that the voltage and resistance (artifact) standards be rock-solid over time and temperature.  This does not appear to be the case at least with the voltage artifact reference.  You did not make any tests of the resistance readings through ACAL cycles; but I suspect that they are not having the same issue as the voltage readings.

It would be an interesting experiment to attach a cable to the 7V reference on the main board.  Then put the meter in a chamber, and plot what happens to this voltage as the ambient temperature is varied.  This voltage must be rock stable, no matter what.  Perhaps a second cable can be attached directly to the 7V reference module; this would tell you if the voltage variance is due to the reference itself, or the cheap 0.100-inch header used to connect the 7V reference output to the main board.

[Kleinstein]

Even a rather poor thermal design can hardly explain some 100 µV of drift in the 10 V range, thus some 70 µV at the 7 V reference.

It is odd to come back to nearly the same + 138 µV on the meter number 2
This looks like something is really wrong in the ACAL procedure.

I simple test without opening the case would be a repeated (e.g. 5 times) ACAL call and see how much the voltage reading or maybe cal constants read back from the meter develop with repeated calls. Ideally there should be just a very small amount of noise (e.g. less than 0.1 ppm).

[iMo]

https://community.keysight.com/thread/23709
It could be with the new FW they introduced a bug into the 70A..

[HighVoltage]

https://community.keysight.com/thread/23709
It could be with the new FW they introduced a bug into the 70A..

Yes, this was a known problem on earlier FW but solved since FW v2.14
FW v2.14 and higher solved the problem completely on the 34465A as far as I know.

I also believe that by following the Service-Manual calibration procedure, I am doing it all correctly.
And the 34465A works perfectly and therefore shows that the CAL procedure is correct.

Also, both 34470A were factory calibrated in 2017 and the calibration was not touched until today.

Measuring the 7V of the LTZ1000 output is a good idea and see the relationship after an ACAL.
I will try that.

[Magnificent Bastard]

Even a rather poor thermal design can hardly explain some 100 µV of drift in the 10 V range, thus some 70 µV at the 7 V reference.

It is odd to come back to nearly the same + 138 µV on the meter number 2
This looks like something is really wrong in the ACAL procedure.

I simple test without opening the case would be a repeated (e.g. 5 times) ACAL call and see how much the voltage reading or maybe cal constants read back from the meter develop with repeated calls. Ideally there should be just a very small amount of noise (e.g. less than 0.1 ppm).

I totally agree with you on all of your points.  I would also like to see the results of the test I outlined.  What we need are more clues, so we can narrow down what might be going wrong.  It could be firmware, or hardware, so some nasty combination of the two that is causing this.  Keysight will be better able to address the issue if they know exactly what it is; (although I have to believe at this point, they "have their best guy on it right now").

[e61_phil]

I did a short test with a 34470A here. I run ACAL and the 34470A showed +4µV after 55min ist showed -5µV. That isn't as much as you see, but significant. I love ACAL meters

[Dr. Frank]

Hello HighVoltage and e61_phil,

I remember to have characterized the first '470A of HV: https://www.eevblog.com/forum/testgear/keysight_s-new-34465a-(6-5-digit)-and-34470a-(7-5-digit)-bench-multimeters/msg889215/#msg889215
Back then, I did not see that drift at all, even after ACAL. Only the LTZ had drifted -5ppm after 10 months of use, but that's to be expected and fully inside specification.
As the 465A and the '470A share exactly the same motherboard and firmware, I really can't imagine, how ACAL should influence the LTZ1000A reference, especially this extreme drifting.
There's a known common behavior of both versions, that after an ACAL, a slight decrease of about 1..2ppm of DCV reading over 1h can be observed...somebody wanted to clarify that with KS, but I did not yet observe any further feedback.

As this 10.000 040 V reference is probably that special reference loaned from another volt-nut from Bremen, who also REALLY dislikes all ACAL featured instruments   , I suppose that today, on 2nd of April, your '470A might just behave much better? 
Anyhow, how would one 'calibrate' a '365A or '470A from just one 10V reference? These are no Artifact Calibration instruments, like the 3458A.. Right, Phil?

Anyhow, if that's NOT been an April fool's, I'm really curious, what KS might say about that design flaw.. and we still have not completely reverse engineered the '470As LTZ1000A circuit and its connection to the main PCB.. for an upgrade solution...

April greetings to both of you!
Frank

[HighVoltage]

Hello Frank

LOL!
No, may be this was bad timing to post it on April 1st.
But this is a serious matter with both of my 34470A and it has not vanished over midnight....

To calibrate the 10V range of the 34470A I first calibrate the zero reference with a short block and store the data.
Then calibrate +10V with the +10.000,040 V
Then calibrate -10V with the -10.000,040 V (reversed polarity)
Then store the data.

After the ACAL yesterday, the instrument stabilized within a few hours and I made another 10V calibration.
This is now running almost 20h and seems to be very stable.
I will do another ACAL shortly to document the difference.

Keep in mind, both instruments had a factory calibration from 2017 and were not touched in the calibration
since, except ACAL.
And in these 18 month, they drifted (+95 uV) and (+138 uV)
For me that is too much for an instrument used in metrology.

In comparison, my Keithley DMM7510 read from the beginning + 3ppm high (probably because of sloppy calibration) but has not moved at all in several years.

[TiN]

While I do agree that 34470A shouldn't change THAT much, it is not really metrology targeted instrument. 

Also there is bit of terminology conflict in your post, HV.

Then calibrate +10V with the +10.000,040 V
Then calibrate -10V with the -10.000,040 V (reversed polarity)
Then store the data.

That is not calibration, but adjustment, and it contradicts

had a factory calibration from 2017 and were not touched in the calibration
since, except ACAL.

Unless you had reverted original calibration ROM to prior state after adjustment? 
Calibration is measurement comparison only, not the adjustment. Isn't ACAL on 34470A only internal correction mechanism, that does not need external reference?

On a side note, since your reference source type is guessed by Dr.Frank (correctly?) I observed some wierd behaviour output with Wavetek/Fluke 7000. Reliable results were obtained only when I had this ref on highly-isolated power source/battery. Usual brick that comes together with F7000 isn't a good source for ppm-accurate comparisons. Other voltnuts had questionable results due to noisy DC/DC in W7000 as well. So ideally such a test should be performed with multiple stable sources for sanity check. I'm still working on schematics of my W7000 

Keep in mind, I don't have/never touched 34461/65/70A before, so I might as well be wrong here.
 
Dr.Frank  For whatever Keysight do on 3459A, I hope they don't break 3458A's ACAL  .

[Dr. Frank]

Hi HighVoltage,
ok, I assume that you still let your '470As run 24/7.
Therefore, they are inside specification, for 10V dc, i.e. 16ppm / 1yr., 20ppm / 2yrs., and you've seen 10 and 14ppm after 1 1/2 years.
If you subtract these 8ppm of 'basic accuracy' for 24h, you get about these 8ppm/yr. and 14ppm/2yr. specs of the 3458A, caused by the same violated LTZ1000A @ 95°C (+).
Metrology grade instrument? Nope, neither is the 470A, nor the 3458A, only because of that LTZ reference.

And because the 470A  has the same 6 1/2 hardware (these 8ppm..)  on the main board, and no real artifact / ACAL calibration to compensate for that, it's a good and bit more stable 34401A replacement, but nothing more.

If you have real 'guts', (.. Olli Kahn ...), then you may pimp your LTZ references to 12.5k/1k or 65°C, and send them off for re-calibration and adjustment, and live happily with < 2ppm/sqrt(yr) ever after.


Hi TiN,

I doubt that KS will ever design a successor.. the 470A is good enough for that industrial market, at least on paper.

Also, all of the old eggheads left the company, or pitiful passed away..

But if they do, they really should keep the existing ACAL procedure, but add Vishay BMF resistors (or something like FLUKEs hermetic TF arrays) all over the place, like inside the Fluke 8508/58/88, AND should consult us for a much better LTZ reference. (Daniel B., did you hear the prophecy?)

Such a model update would also silence this certain ACAL denier 

Frank

[HighVoltage]

Dr. Frank
You guessed correctly on the 7000 and it seems to be very stable.

Good points on the yearly drifts of the Keysight  instruments.
May be I am just surprised that some of my 34401A seem to be more stable.
And yes, my two 34470A and the 34465A are running 24/7

Well, I have a very good 10V baseline now on all my DMMs.
It looks impressive, when all of them show the same number on the display to the last digit.

Yes, point taken on not being metrology gear.

[HighVoltage]

Ok, next test.
Both Instruments were running very stable almost a full day.

Reference voltage: 10.000,040 V

And the instruments show:
34465A       = 10.000,04 V
34470A_#1 = 10.000,036 V (-4 uV)
34470A_#2 = 10.000,035 V (-5 uV)

Then next, I performed ACAL 5 times with 5 min resting in between.

And the instruments show:
34465A       = 10.000,05 V
34470A_#1 =  9.999,969 V  (- 67 uV)
34470A_#2 = 10.000,178 V (+ 143 uV)
 
In comparison to the 34470A, the 34465A is rock solid.

 

[Magnificent Bastard]

So, this is a REAL PROBLEM--- and not some imaginary thing nor is it an April Fool's joke.  Furthermore, this is NOT FUNNY Keysight !!!!

@Keysight DanielBogdanoff:

   We, the "early adopters" of the (rather expensive) 34470A deserve some answers.

[Keysight DanielBogdanoff]

Hey you Magnificent Bastard (I've never called anyone that until now!), I'll dig into it on my end.

Jokes aside, have you contacted your local support center? For this type of thing, they have access to a couple issue escalation avenues that I don't get to use. Most of my support here is unfortunately ad hoc.

I will dig in, but I highly recommend you also give our support folks a call. They're very good.

[HighVoltage]

Hello Daniel,

Thanks for the response, I have sent you a PM.

[HighVoltage]

Just to show, how stable the Keysight 34461A is.
I had them turned off over night and turned them back on this morning.
And they move very beautifully towards 10V. Interestingly, one does it differently than the other 3.

This new reference source is a Fluke 731B at 10.000,01 V

[HighVoltage]

Last night, before I went home, I had the two 34470A adjusted again on the 10V range

This new reference source is a Fluke 731B at 10.000,01 V
And both instruments stayed stable over night.
(NO ACAL !)

[Andreas]

The two 34470A showed this:
34470A_#1 = 10.000,135 V (+95 uV)
34470A_#2 = 10.000,178 V (+138 uV)

After calibration against the reference, the 34465A and the two 34470A showed a perfect value 0f 10.000,04x V
34465A       = 10.000,04 V
34470A_#1 = 10.000,040 V
34470A_#2 = 10.000,042 V
And within 1 hour after the calibration there was no significant change.

Then about 1 h later I pressed ACAL on all three DMM and got the following results:
34465A       = 10.000,04 V (no change)
34470A_#1 =   9.999,965 V (-75 uV)
34470A_#2 = 10.000,178 V (+138 uV)
And immediately after that, I can see a drift in in trend chart.

Any ideas?

Hmm,

the behaviour of #2 reminds me of some roumor regarding personal weight scales:
to hide the drift/instability of the scale they remember the last showed value(s).

If a value of similar weight (with small difference) as one of the last values is measured the display is (more or less) the according last stored value.

with best regards

Andreas

[Towger]

Any ideas?

If a value of similar weight (with small difference) as one of the last values is measured the display is (more or less) the according last stored value.

If you can't meet the specs/requirements, fix the firmware.  A common enough occurrence these days. Often back fires in big ways as VW and Boeing have discovered.

[Keysight DanielBogdanoff]

Hi Everyone,

I met with the tech support manager today to get an update on where they are with this. The division support folks are working on verifying/replicating this on their 34470As and will proceed from there.

One thing that would be helpful is, if you are experiencing this, please send me your serial number! DM is fine, or you can email me directly at daniel.bogdanoff@keysight.com.

Thank you!

[eplpwr]

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.

[Magnificent Bastard]

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 behavior is 100% reproducible.

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?

[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.

[eplpwr]

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.