After about 1/2 year of investigation, I present my Performance Review on the 34465A / 344470A DMMs.
This comprises specification conformance checks, stability and noise figures, A/D linearity and resolution.
The differences between the hardware of the 34465A and '470A were examined.
I also discovered 3 bugs on the 100V/1kV DCV ranges, an unexpected behaviour of the OCOMP function of my 3458A, and how to handle these delicate offsets on the 100mV range.
Each instrument comes with a Test Report. These always appear to be similar, as the instruments are at first calibrated to a Fluke 5720A / 5725A, and then were verified directly afterwards. The acceptance level at the calibrator is a fraction of the 24h specification.
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10V is the most stable DCV range, usually on all bench DMMs. As expected, this range was spot-on for both 34465As on arrival, confirming about 1ppm DCV uncertainty in my lab.
The other ranges were always comprised of resistive amplifiers/dividers, so being more uncertain to a few additional ppm.
The 100mV range gain check requires a very accurate offset correction on both the DUT and the 3458A, as a 1µV thermal offset already gives 10ppm of error. At first, this did not work out well, due to the confusing OFFSET / STAT math function of the 3458A. In the end, I achieved an agreement of the gain constant of the 34465A within a few ppm, also.
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The DCV initial verification table shows the first error on the HV ranges, i.e. a reversible +3ppm shift when setting the input impedance flag to 10M. This software error has been corrected by FW 2.10 / 2.11.
'HighVoltage' kindly lent me his 34470A. This instrument had been running continuously for about 10 months already, so its internal LTZ1000 reference drifted +5 ppm, in accordance to the 8ppm/yr. specification, and the known negative drift of the LTZ1000A. Consequently, all DCV ranges exhibit this same +5ppm drift.
As a second bug, all three instruments feature an identical systematic deviation on the HV ranges of about +10ppm on the 100V range, and a little bit less on the 1kV range.
On the first 34465A, I tried a re-calibration on these 100V and 1kV ranges. Directly after calibration the reading was spot on, but after engaging the ACAL function, the reading immediately jumped back to these +10ppm.
As a consequence, these ranges cannot be calibrated properly, in conflict to the Test Report.
It has to be emphasized, that the instruments are still inside specification limits.
Recently, I found a third error on the self heating compensation of the HV 100:1 divider. Directly after power up, the heater of the 100:1 divider is not operated properly on the HV ranges. Depending on the power on state (Factory Default or Last), either the divider would not be heated at all, or be heated permanently in the 1kV range.
This malfunction vanishes by changing range or input impedance flag, or by triggering ACAL.
This malfunction leads to a -10ppm shift of the divider ratio, very probably being identical on all instruments. Therefore, this error can be assigned to the systematic +10ppm gain shift, as this may occur during HV calibration.
This error has been fixed by FW 2.12.
When I finally re-calibrated the 100V and 1kV of my 2nd 34465A, this +10ppm shift did not show up any more, even after engaging ACAL, so that the HV ranges were now spot-on.
The ACAL function in the end works extremely well at lab temperature, bringing all DCV ranges back to their initial reading, about +/-1ppm for 10V, and +/-2..3ppm for the other ranges.
The uncertainty of the Ohm mode does not depend on the internal voltage reference, but relies on its internal resistors only, especially the 10k VHP101 and a TaN resistor array for the current mirror. See e.g. the schematics for the 34410/411A.
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All ranges were spot on, especially the 10k Ohm range on all 3 instruments deviated less than 1ppm from my own 10k reference resistors. The ACAL function also resets small temperature drifts to about +/- 1ppm deviation.
By comparing a 100k resistor on my first 34465A vs. my 3458A, I discovered an undisclosed failure mode for the OCOMP function.
This mode always requires delays of 1 sec or 5 sec for the 10k or 100k range respectively, to compensate for common relaxation effects on the DUT resistor. The normal delay settings in the msec range on most OCOMP instruments usually create errors on the order of several ten ppm.
The verification of different modes and ranges show that both instruments are still inside the 24h specification, despite the firmware bugs, and the timely drift on the 34470A!
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