Keysight 34470A, Calibration and ACAL problems

[djac]

In the meantime I bought a 34470a. This one also has the "ACAL Jump", about 5µV amplitude.

Because I wanted to know if this has other consequences, I compared the 34470a with the 34465a in different aspects.

The report about it as PDF.

Regards Dieter

[djac]

In chapter "IV. Noise measuring 10V" on page 11 I noticed an error: the measurement was done with 100plc. To be on the safe side I repeated the measurement again with a shorter measuring time, so that the room temperature changed only 0.04°C during the 10 minutes. However, in this measurement the distance between the two devices turned out to be much more modest.

Regards Dieter

[splin]

An interesting report - I'm surprised it didn't attract more responses, but these are interesting/difficult times. A few points:

1) The difference in shorted input noise between the 34470A and 34465A at longer integration times is surprising. Have you compared the Allan deviance graphs to those from other posts in the DMM noise measurement thread to see if this might be specific to your samples or systemic?

You hypothesised the differences could be due to differences in autozero behaviour between the two DMMs; is this based on known differences or musings on possible causes?

2) The differences between the various shorts is also surprising - did you cover the terminals to protect them from drafts? Did you do enough repititions of each run to establish the repeatability of your tests to be sure that the results didn't reflect random drifts of the instruments rather than real differences between the difierent types of shorts?

3) On page 9, the noise measurements of an LM399 10V reference: the noise of the reference is likely 4 or 5uVpp; the .165ppm/C looks to be based on peak reference noise voltages at both the upper and lower temperatures - note the 4uV drop at 1250 seconds. Using the mean values could easily halve that TC (which should probably only be shown to one DP given the large uncertainty). Similar for the 34465A. You need a very much quieter external reference to make meaningful measurements, or at least *very* many repetitions of the tests - but you obviously know that.

4) Page 12: the 10V reference noise pp:rms ratios of the 34470A and 34465A are very different at 13.5 compared to 5.3. This surprised me; the noise from the internal and the external references are presumably Guassian, so I would expect their geometric sum to also be Guassian with a pp:rms ratio around 6. Any suggestions why the 34470A ratio is so high? In any case, the LTZ1000 noise should around 1/4 of the LM399 so should not be a material factor in the overall noise measurement.

Why are the pp measurements so similar at approx 1uV for both given that the 34465A has an LM399? I would expect the 34465A result to be almost sqrt(2) higher than the 34470A.

Do these suggest some other non-Guassian or correlated noise source in the 34470A results?

[Kleinstein]

For a noise test with an external reference one should have a really low noise reference.  So either a good LTZ1000 based one or maybe just a stable battery (e.g. in a well insulated box and mechanically protected). Ideally one would run both meters at the same time to have the same reference noise.  It makes sense to check the noise with an external voltage, as there can be other noise sources (e.g. excess noise of not so good resistors and maybe similar noise sources that change with the voltage reading)  than just the reference itself.

With LM399 there can be relatively long time scale popcorn noise. If one only looks at short windows like 100 seconds the noise reading can vary quite a bit, with some quieter and some more noisy phases.

The longer times in the Allan variance plots, where the curve goes more horizontal are likely due to thermal effects at the input / short.

The measurements of temperature effects are over a rather short time and also quite small temperature rise. It may take a little longer to really stabilize.

[djac]

Hi splin,

to 1.
If I didn't miss it, there is no measurement of the 34470a with shorted input (only with 10V input) in the "noise comparision" thread. By the way, the thread is confusing and the evaluation methods are partly different and not always clearly documented, so that a comparison is difficult.

I only tried to show a connection between the autozero mechanism and the behaviour of the 34470a, I can't say anything about the exact causes and connections.

to 2.
You are right, I did not systematically investigate the behaviour of the individual short circuit plugs, this would have gone beyond the scope and objectives of this documentation. I have only investigated which of these plugs gives a measurement that is as temperature independent as possible.
But it remains to be said that thermal voltages must be involved, because as far as I know the sockets of the 465/470 are made of copper, but the plugs used are gold plated (300nV/°C).

But the important thing is that over many measurements and all kinds of short-circuit plugs the difference between 365 and 470 remains. And this suggests that the autozero mechanism of the 470 is not as precise as the 365. It is the rapid temperature turbulence of the surrounding air which, via the very small thermoelectric voltages, causes the change from measurement to measurement in the middle range of the Allan Deviation of the 470.

to 3.
I have chosen the difference of the peaks deliberately, this results in a worst case temperature coefficient. I am aware that the absolute numbers are only correct in order of magnitude. Above all, however, it is a matter of comparing the two instruments which carried out the measurement in parallel at the same time.

to 4.
I have no better reference than the LM399 at hand. But again, it was only a matter of determining whether the better reference of the 470 is noticeable in a practical measurement. An exact measurement of the absolute value of the 470's inherent noise requires completely different measurement possibilities which are not available to me. For the rest, this point is subordinate to the objective of the documentation.

I am also aware that there are differences between the individual specimens. Therefore I would appreciate it very much if some 34470a owners could be found here, who would also do the measurements for the Allan Deviation. This could help a lot to find out the reasons for the difference to the 465. If necessary, I would be happy to provide my algorithm for calculating the Allan Deviation, so that the exact same evaluation can be done.

Regards Dieter

[Dr. Frank]

In the meantime I bought a 34470a. This one also has the "ACAL Jump", about 5µV amplitude.

Because I wanted to know if this has other consequences, I compared the 34470a with the 34465a in different aspects.

The report about it as PDF.

Regards Dieter

Hello Dieter,
thank you for your report.
It confirms my findings from 2016: https://www.eevblog.com/forum/testgear/keysight_s-new-34465a-(6-5-digit)-and-34470a-(7-5-digit)-bench-multimeters/msg889215/#msg889215
The similarities between both instruments even go further, like the same resolution of the A/D, which is fully transferred over the bus.

My conclusion at this point was, that the 34465A can be used exactly like the  34470A with 7 digit resolution, either by using the statistics function, or using scaling function, e.g. expressing the result in ppm deviation, or using readings over connectivity.

The difference in T.C. is also barely to be distinguished, as the LTZ1000A module is (like in the 3458A) probably not trimmed for minimum value, therefore could be as high as 0.3ppm/°C, which the LM399 circuit might also deliver.
Timely drift is also comparable, as the LTZ1000A circuit is operated (misused) at 95°C, like the LM399.
I received the 34470A from HighVoltage which was running continuously for nearly 1 year already, and it showed 5ppm drift in 10VDC already.
My own 34465A shows a drift of about 2 ppm after 5 years only, due to the different use case, i.e. it's only powered on when used, therefore the LM399 did not drift as much as specified (that would always imply continuous use)

Frank.

PS: This 34465A in the photograph is not yet warmed up, usually it displays exactly 10.00002V, after all these years.
Please take note of the 7 digits and of the enhanced resolution of the statistics data, simply by using scaling with M = 1 and B= 0.

[djac]

Please take note of the 7 digits and of the enhanced resolution of the statistics data, simply by using scaling with M = 1 and B= 0.

Thanks, that was new for me.

Regards Dieter

[djac]

I have revised the report once again due to the objections of splin (point 4.) In particular I increased the number of bins in the histograms to 50. Then you can see the reason why the ratio stddev/Vpp does not correspond to 6, because with the 34470a there is no normal distribution, but the drift distorts the histogram.

Regards Dieter

[HighVoltage]

Hello Dieter

Thanks for your nice reports

Very interesting to see the drift in your new instrument already now.
It will be more interesting, when the 34470A is 6 month or a year old, that is when my two instruments started to show a much larger drift from ACAL.

BTW, I have not received a final update from Keysight.

[Keysight DanielBogdanoff]

Hi all,

The R&D team and support team have been digging into this since thread popped up and have been able to pick out what's happening.

Here's the guidance from them:
"
The ACAL offset (jump) issue observed on DMM after performing ACAL can be resolved by performing full range calibration on the DC Voltage function.

The full range calibration will correct the gain ratio based on the DMM recent condition.

During ACAL, the ratios of the measurements made at a complete DMM calibration and recent measurements are used to correct the internal drift of major measurement blocks.
"

They are also digging into some other things brought up in this thread, thanks everyone for their patience!

[branadic]

Seriously? Calibrating the whole unit once ACAL was encountered is the solution provided? Wow. 

-branadic-

[Dr. Frank]

Seriously? Calibrating the whole unit once ACAL was encountered is the solution provided? Wow. 

-branadic-

Hello branadic,

no it's meant differently.
ACAL only works correctly, if a proper DCV calibration (CAL) was done before.
Once working correctly, ACAL will always work correctly, w/o another CAL.
Some units failed, for unknown reasons, maybe CAL was not done right. In these cases ACAL caused an erroneous jump up to 20ppm.
Therefore, a new CAL does cure this problem on these units.

The ACAL procedure seems to work like this:
During CAL of the 5 DCV ranges, the nominal gain factors of the ADC / REF circuit, of the 100:1 divider and of the x10, x 100 amplifier are determined very precisely .
These gain factors will drift over temperature and over time.
ACAL then precisely re-measures these actual gains and corrects to the nominal ones, by using the internal voltage reference as a stimulus.

Such 10:1 ratio re-measurements are normally not possible to do precisely enough, when using an ADC which is linear to 6 digits only.
So they probably have invented something to make this transfer 10 times more precise, at these special calibration points... maybe by additionally determining the non linearity at these fixed ADC points during the calibration procedure.

This technique is also used in a similar manner inside the 5440B calibrator, which also requires to once calibrate the nominal ratios for the different ranges.

Frank

[View[+]Finder]

I particularly dislike the fact that they placed it right in the air stream of the fan

I believe the fan exhausts, with cool air coming in the vents on either side of the case. I'm no expert on airflow in electronics cases, but logic says "take the shortest path" and there is no other "force" in the case to steer the air currents otherwise. Thus, the airstream would most likely be the result of the slight negative pressure gradient from the exhaust and the positive pressure on each of the vent holes.

[Dr. Frank]

I particularly dislike the fact that they placed it right in the air stream of the fan

I believe the fan exhausts, with cool air coming in the vents on either side of the case. I'm no expert on airflow in electronics cases, but logic says "take the shortest path" and there is no other "force" in the case to steer the air currents otherwise. Thus, the airstream would most likely be the result of the slight negative pressure gradient from the exhaust and the positive pressure on each of the vent holes.

If you have a closer look at these vent holes, the right ones give an unhindered air flow, whereas the ones on the left are mostly blocked by the inner case and the transformer.
Therefore the air goes mainly from the right holes, over the reference board area, and out to the fan, therefore nearly identical if the flow direction would be reversed. 

[HighVoltage]

Hello branadic,

no it's meant differently.
ACAL only works correctly, if a proper DCV calibration (CAL) was done before.
Once working correctly, ACAL will always work correctly, w/o another CAL.
Some units failed, for unknown reasons, maybe CAL was not done right. In these cases ACAL caused an erroneous jump up to 20ppm.
Therefore, a new CAL does cure this problem on these units.

In my experience with the two 34470A instruments, this is partially true.

Over time the ACAL drift got larger and larger.
If at that time a correct full DC calibration procedure is applied, then the ACAL function will be relative stable again.

However, after a few month time, ACAL will start drifting again.
To stabilize ACAL again, it requires another full calibration.

It seems that some instruments show a larger drift than others and some may not have this ACAL drift problem.

Full DC calibration means:

ZERO
+10 V
-10 V
+100 mV
-100 mV
+1 V

[Blue]

I'm not quite sure what equipment I should have to calibrate it fully.

Does it mean that I have to have a proper voltage reference of
ZERO
+10 V
-10 V
+100 mV
-100 mV
+1 V
?

Unfortunately I do not have proper voltage reference equipment. What to do? Send it to Keysight 4 times a year?

Any one any idea?

[View[+]Finder]

I particularly dislike the fact that they placed it right in the air stream of the fan

I believe the fan exhausts, with cool air coming in the vents on either side of the case. I'm no expert on airflow in electronics cases, but logic says "take the shortest path" and there is no other "force" in the case to steer the air currents otherwise. Thus, the airstream would most likely be the result of the slight negative pressure gradient from the exhaust and the positive pressure on each of the vent holes.

If you have a closer look at these vent holes, the right ones give an unhindered air flow, whereas the ones on the left are mostly blocked by the inner case and the transformer.
Therefore the air goes mainly from the right holes, over the reference board area, and out to the fan, therefore nearly identical if the flow direction would be reversed.

On a closer look, the holes on the left are not "blocked," there is a just redirection of the airflow to openings more to the rear of the case. There are fewer openings on the left side than on the right, true, however the airflow on the left will be warmed by the heat of the transformer as conducted through the metal case and the flow from the right will pass over warm components before reaching the reference. The key detail is that the reference is not "in the air stream of the fan," it is in the general airspace of the case some of which will be exhausted by the fan.

My experience in hot climates may be clouding my reasoning, but I recall the difference between a fan blowing directly and one exhausting air with replacement through open windows. If this bears on the success of my reference transplant, it would be a fun project to put some sensors in the case and run some tests. Let me know . . .

[ScoobyDoo]

These (high) drift figures for 34470A were recently reported by maxwell3e10 in post:

https://www.eevblog.com/forum/metrology/usa-cal-club-round-2/msg3552046/#msg3552046

Best regards
ScoobyDoo

[Kleinstein]

The ACAL change shown in the graphics is different from the 34470 specific ACAL problem, that is more transient: so after ACAL there is additional settling for some time (~ 1 Hour ?). The reason behind this still seem a bit unclear and the fix from KS looks a bit odd, likely not really solving the root cause, more like a work around. Different meters seem to be effected to a different degree.

The graph shows how much change from calling ACAL after a very long time of not doing so. This is more like long time drift of resistors that are not made to be long time stable. In addition the times are different and the degree of what is corrected by ACAL can also vary. So the meters are only partionally comparable.

[HighVoltage]

For me, the comparison reference is the Keithley DMM7510. It is always spot on stable, even if the environment temperature is changing and it does NOT need the ACAL to be pressed at all, although it has an ACAL function.

The 34470A is most stable, if the ACAL is not pressed at all.
However, it will drift over time.

Where the 34470A is really shining bright is the low current range.
And here it is better than the DMM7510.
So, at the end of the day we need both instruments.
I still like the 34470A but one needs to be aware of its limitations.

[Kleinstein]

The 34470 may very well also be quite good with AC - though probably not much difference to the 34465 in this respect.

The Keithley meter seem to do some real time measurment and correction of the ADC gain, so kind of doing a partial ACAL in the background all the time. However chances are this is relatged to the odd extra noise one gets with most Keithley meters at some 100 PLC or 10 s time scale. So the low drift comes at a price - though this is likely more like a software problem, not an absolute technical requirement. So it looks like the KS34470 is not alone with a slow to fix software problems effecting the performance.

The real time correction likely does not include the amplifier gain, so one may still want an occasional ACAL for the 100 mV and 1 V ranges with the DMM7510. The 10 V range likely would not need it at all.

With the partial "fix" from KS, not sure if they will find the true reason behind the settling after ACAL and fix it.

[HighVoltage]

The ACAL function of the Keithley DMM7510 is hiding in the menus.
But each time I have used it, it had almost no effect at all on both of my units.
So, from my experience, one does not need it at all.

Actually, you are right, the 34470A is very good in the AC ranges but I usually have no need for that and therefore I have no deep experience with it.

Most impressive is the low range DC current. Here I am measuring a 1n source with the 34470A. Even over longer time, this is very stable.

[HighVoltage]

Keysight released a new Firmware version 3.03 on 2021-08-25

Today I installed this version 3.03 on my 34470A but unfortunately the ACAL problem was not addressed.
It seems like other important issues have been addressed.