Keysight 34470A, Calibration and ACAL problems

[e61_phil]

The 34470A isn't able to float on 1kV (If I remeber correctly max. 500V). There are nullmeters which are able to float on 1kV. Most of the benchDMMs are not designed for that.

To use the internal 10Meg is also a bad idea for a divider, as I already said. Build a proper divider and measure it with high Z. Or build an active one. I did it for up to 10kV and it works also very well.

[GEOelectronics]

I wonder if there are still 990MOhm divider available..

Frank

Hi Dr. Frank, can you reference some (or one?) 990 Ohm divider even from the past?

I can't even find any more 99M 0.1% HV Hymegs to build them up.

Most of the "HV Probes" for DVMs are much smaller value, no good on a current limited bias supply.

I've had some special made in China for a project but not anything like 0.1% tolerance. Fortunately the project didn't require accuracy, just stability (in that one part).


George Dowell

[GEOelectronics]

The 34470A isn't able to float on 1kV (If I remeber correctly max. 500V). There are nullmeters which are able to float on 1kV. Most of the benchDMMs are not designed for that.

To use the internal 10Meg is also a bad idea for a divider, as I already said. Build a proper divider and measure it with high Z. Or build an active one. I did it for up to 10kV and it works also very well.

Please explain term "float", not understanding the reference, thanks

George Dowell

[e61_phil]

With float I meant you are not allowed to lift the LO input of the 34470A higher than 500V above ground (with a 1000V source for example). On the most bench DMMs only the HI input can go up to 1000V (or 1500V if you add both up). This is normally marked on the unit.


What kind of HV Probes for a DMM do you mean? The ones I know (1:1000) have an input resistance of 1GOhm.

Ohmcraft and Caddock also sell dividers with good TCR.

And again: Don't use the internal 10Meg. There is no specification about TC, VC and so on (for the meter only tracking is important together with the rest of the input circuit). I made a few tests with 90Meg in series to a 34401A to measure up to 10kV in the 1kV range and the readings where very unstable.
If you use high impedance dividers also take care about the input current of your meter.

[e61_phil]

I attached a picture of the divider box. This box was only a quick and dirty build to verify a HV measurement system.

It was build out of Caddock USF 20Meg resistors out of the drawer. Two in parallel and in the beginning nine of such sets in series to build 90Meg. That worked so badly, that I decided to put in another pair of 20Meg resistors to achieve 100Meg (one can see, that the uppermost Caddocks wasn't planned to be there). I added a Vishay 100k Z201 and some wire wound resistors out of the drawer to set the attenuation to 1:1000. The small 25ppm/K metallfilm resistor is just for fine tunning and doesn't affect the TC.
Overall TC of the box is ~1.5ppm/K

[GEOelectronics]

With float I meant you are not allowed to lift the LO input of the 34470A higher than 500V above ground (with a 1000V source for example). On the most bench DMMs only the HI input can go up to 1000V (or 1500V if you add both up). This is normally marked on the unit.


What kind of HV Probes for a DMM do you mean? The ones I know (1:1000) have an input resistance of 1GOhm.

Ohmcraft and Caddock also sell dividers with good TCR.

And again: Don't use the internal 10Meg. There is no specification about TC, VC and so on (for the meter only tracking is important together with the rest of the input circuit). I made a few tests with 90Meg in series to a 34401A to measure up to 10kV in the 1kV range and the readings where very unstable.
If you use high impedance dividers also take care about the input current of your meter.

1)"With float I meant you are not allowed to lift the LO input of the 34470A higher than 500V above ground" OK I understand. OK I understand, thank you. Fortunately all our measurements are referenced to "ground".  Quotes used because much of the generators are battery powered handheld, so no actual earth ground. Some photomultiplier tubes are run with negative HV to reduce noise, but this puts the mu-metal magnetic shield floating at kV levels referenced to ground. I encourage users to convert to +HV the so the mu-metal can be grounded.

2) "What kind of HV Probes for a DMM do you mean? The ones I know (1:1000) have an input resistance of 1GOhm." The ones I have been using for general purpose HV power supply servicing (not current limited bias supplies for nuclear measurements) are of usually add-on to a 3 1/2 or 4 1/2 digit handheld DVM's (or analog VOM or VTVM) regular probe and only one I know of is even close to G Ohm is one Fluke model.

3) "And again: Don't use the internal 10Meg. There is no specification about TC, VC and so on " I agree 100%. Doing so has become commonplace in some circles, worldwide, and I am gathering anecdotal references to refute the practice on several levels.

4) "I attached a picture of the divider box. This box was only a quick and dirty build to verify a HV measurement system." I like that thanks for showing, it looks very "industrial"., which is exactly what I do. The KS 34470A is my first toe dip into precision, after 50 years of being in the electronics service industry. My normal instrumentation is more like what you would see in a combination typical bio-med and nucleonics type service facility.  Our standards are in keV produced by man made radioisotopes.

5) OK it's time for me to get on with this and break out the silicone wire, PTFE wire, Kapton tape and get back to the workbench to refine this project based on all the good advice given by members of this forum.  Thank you all.

George Dowell

[GEOelectronics]

If I may, attached is a typical HV PSU that I make, I call it an HVG for High Voltage Generator.

This one uses 2 of the made-to-specification glaze 800M Ohm resistors, one controls and IC that is part of the HV set reference (200-2500V) while the other feeds the IC that is the onboard HV readout. (works with either analog meter or digital panel meter). Also the small transformer was custom made for me.

In use this is sealed inside a box where a safety resistor in the 1 to 10+ MegOhm range is in series with the output.

Simpler versions of the HVG have non adjustable, fixed HV output and no onboard HV meter driver, but still the safety resistor, so the service/calibration HV measurement must be done through the unknown value M Ohms safety resistor, therefore the need for 1G Ohm HVM (HV Meter)

Thanks for helping. me understand what I was doing wrong.

George Dowell

[GEOelectronics]

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.

A HV Voltage comparator is the very next project- well maybe after the LTZ1000 kit build and the completion of the AD2702UD builds.....

George Dowell

[e61_phil]

A HV Voltage comparator is the very next project- well maybe after the LTZ1000 kit build and the completion of the AD2702UD builds.....

If you already have such supplies you could use a battery powered DMM. Perhaps in combination with your 34470A. A 1kV source with low output impedance will drive the input of the ground referenced 34470A and also the LO from a fully isolated batterie powered (low cost) DMM. With both readings you have a very accurate and high impedand measurement.
I did such crude measurements a few times to measure a couple of mV floating on several kV.

But it sounds you need a robust solution for a service technican.

[iMo]

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

Back to the topic of this thread - has KS fixed the issue with 34470A cal and ACAL already?

[HighVoltage]

No solution so far, Keysight is still working on this issue.

[iMo]

Any feedback from KS?

[HighVoltage]

Only feedback so far is, that Keysight is working on it.
I have no problem for this to take a little longer, as long as all the problems are solved at the end.

[iMo]

If I were KS I would offer you a free replacement for the 3458A/BE as the compensation for the 7 months without the gear.. 

[Andreas]

NO they will not.
Highvoltage could find the same problem in the 30 year old firmware too.   

with best regards

Andreas

[djac]

Any update? I'm thinking about getting a 34470a. But as long as this problem is not solved or cleared up, I will refrain from it.

Regards Dieter

[HighVoltage]

Please keep in mind that not all owners of a 34470A have reported the same ACAL problem as I have.
This should not hold you up from buying one of these great instruments.

[djac]

No, it's not that simple. I have the impression that the problem is being swept under the carpet in silence and not played with open cards.

Surely the jumps to ACAL are extreme with your two devices and a special case. But did Keysight replace the devices without any trouble and how do these new devices behave?

Because after all Keysight has admitted that this problem exists, albeit to a lesser extent. And some forum members have confirmed this in this thread.

But after the announcement here to solve the problem, you just don't hear anything anymore. This makes you feel insecure.

You can probably live with it if you have to wait 5 minutes after an ACAL until you can take measurements on the edge of accuracy, but that this is assured is not confirmed by Keysight.

So one wonders if there is any advantage to paying double the price of a 34465a that doesn't have this problem. Especially since the noise and linearity measurements presented here in the forum do not at all indicate that the 34470a really has a clear advantage here. Thus, it looks more as if the two devices do not differ in linearity and only slightly in noise.

So why a 34470a versus a 34465a?

Regards Dieter

[HighVoltage]

All I can say is that Keysight is working on this and I have confidence that it will be solved.

It highly depends on what you want to do with the instrument.

The 34465A has all features of the 34470A and is ultra stable not just in my lab but also reported by many others.
And if you use software to read out the data, you even get 7 1/2 digits on the 34465A.
And the 34465A is not effected by the ACAL drift at all, as far as I have noticed.

[iMo]

34465A (LM399 ref) cannot be as "ultra stable" as the 34470A (LTZ1000 ref)..
34401A-34461A-34465A are all of the same "stability grade".

[djac]

This is of course correct in principle. But the rest is the practical implementation.

In the dmm noise thread only one 34470a is measured, unfortunately. With the result of 0.01ppm Vrms per range in the 10V range. The 34401a/34461a/34465a have corresponding results between 0.02 and 0.03. Since the noise of the reference is not included in this test, this means that the ADC is less noisy. It must do so in order to meet the linearity specification of the instrument, because the noise of the ADC influences and of course limits the linearity. And indeed, the linearity of the 34470a is specified as 0.6ppm and that of the others as 1.5 to 2 ppm. This is about the same factor of 3 as you find in the noise test.

On the other hand, the results of the linearyity test thread show that the 34470a does not necessarily perform better there. And the noise of the reference is included in these measurements. And the implementation of the reference on the 34470a has also been criticized (Dr. Frank, airflow of the fan over the reference, etc.).

Of course, the measurement of linearity is very demanding and therefore may be slightly inaccurate. On the other hand, even a non-optimal implementation of an LTZ1000 reference can increase the noise. So it may very well be that there is practically no big difference between 34470a and the others. But just more question marks than clarity. And then additional problems like the one of this thread unsettle all the more.

Regards Dieter

[Kleinstein]

ADC linearity and noise are not directly related. There is no real difficulty in getting low noise (about the level of the 34470/34465). However it is quite tricky to get really good linearity.  When choosing the ADCs reference current level there is even some compromising: higher current gives lower noise, but higher INL.

One major contribution to the INL is the TC and self heating of the resistor at the ADC input. This leads to a U³ contribution to the INL.  Even if the ADCs in the 34465 and 34470 are nominally identical, selecting better resistor TC can make a difference. Similar effects may apply to other parts - selected part may improve the INL or in a few cases also the noise.  Another point could be extra production-tests to verify INL for the higher specked units.

The old 34401 uses a different, older ADC type, with considerably more noise. However from the way it is build it can still have good and possibly better INL than the newer meters. The newer ADCs are mainly there for there higher speed to be used in digital RMS, not for improved INL.

Even a relatively poor designed LTZ1000 reference should get a noise level considerably lower than the LM399. Compared to the good noise level of the ADC, the LM399 can be a severe limitation when measuring a not too small voltage. However it also depends on the signal source. Unless one starts with a really low noise signal, noise from the source may still be higher than the reference noise.
The lower noise reference mainly gets important in combination with high end reference source. However the 34470 is still not made for metrology purpose as the INL specs are not that great and in theory ACAL may be limited accuracy because of the INL:

[iMo]

Anyhow, in one month it is 1 year since KS started with solving the issue.. 

[baof]

Version 3.02 still doesn't fix this problem. I think this is a hardware problem, similar to the thermoelectric potential of relay contacts, which can not be solved in software.

[Kleinstein]

Version 3.02 still doesn't fix this problem. I think this is a hardware problem, similar to the thermoelectric potential of relay contacts, which can not be solved in software.

The error during ACAL  is a strange one, so it may need quite some times to find it. My guess would be something like a thermal effect or maybe dielectric absorption, that cause trouble if readings are done with an unusual speed or sequence. So it may be really tricky to track down and maybe also difficult to fix. It looks like different meters are effected to a different extend - so some HW dependence is there.

I would not expect mechanical relays involved in the ACAL procedure (except for the high voltage ranges of cause).