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Bug report: error in CAL / ACAL routine for 34465A / 34470A DMMs

Question asked by Dr_Frank on Jun 30, 2015
Latest reply on Oct 13, 2015 by Dr_Frank
I got my brand new 34465A DMM last week, it had been calibrated by Keysight (initial/ production calibration) June 1st.

Out of the box, it had a disturbingly high calibration count of 222.

Another owner of a new 34470A found 76 cal counts, which seems also too high, maybe up to 20 or 40 seems to be reasonable, as the count seems to increase by 1 per calibration group only. (calibraton of 100V and 500V DCV increases by 1 only)

I assume, Keysight currently has big problems to finally calibrate these instruments in their production run.

I found out, that the DCV ranges 100mV, 1V, and +/- 10V were spot on, after using ACAL, always < 1ppm accurate compared to my Fluke 5442 and HP3458A.
The 100V and 1000V ranges were both off by +14 and +17ppm (this can be checked precisely relative to the lower ranges by the 3458A, at least).
I could not understand why the lower ranges were in accordance to the test report (also indicating <1ppm deviation) , whereas 100V and 1000V should have drifted 14 and 17ppm in these 3 weeks, as the instrument was no used in that period of time.

So I recalibrated  100V and 1kV, and it turned out, that the following readings (verification) were indeed spot on (< 0.3ppm deviation) , as expected, even after cycling power.

As soon as I engaged ACAL, the +14ppm and +17ppm deviation was introduced again.
This is very probably a SW bug, which does not implement T.C. correction for the 100:1 divider in the correct manner.
Maybe the routine misses the 100:1 attenuation..

If I recalibrate the 1kV range only (500V applied), this will already correct 1kV and 100V range, i.e. it's not necessary to recalibrate 100V.

There is a second (software) bug:

In 100V and 1kV range, the reading is correct, when the input impedance is set to AUTO (i.e. > 10GOhm).
If I switch to 10MOhm, the reading is +3ppm higher in both ranges, although there is no hardware switching involved, as the HV ranges are already in this 10MOhm mode!

If I calibrate with 10MOhm setting, the verification also fails by +3ppm, switching to AUTO removes this error.

Both errors sum up to +17 and +20ppm, which already is the 90 day drift.

During Keysights calibration process, as ACAL probably is engaged before verification, that will probably cause a calibration error, explaining the high 222 counts of my instrument.

Secondly, this instrument will fail the 90 days or 1 year specification, if it already starts at 20ppm and the normal drifts will add to that.

So I need support from Keysight engineering, to get a confirmation of that bug, and a fix in the firmware, or in hardware, if it turns out to be HW related (less probable to my understanding).
I could also supply serial number, test report and further details of my own measurements.


Dr. Frank