Here is a graph with a 2 hour run using my LTC450C 10K "standard" (measured by HP3458 about a week ago and by Keysight 34465A today) . Overnight variations were even larger - about 25ppm p-p. Is this a typical behaviour for the 34465A ?
Cheers
Alex
This size of fluctuation, about 1ppm, up to 25ppm, has nothing to do with "noise". If your instrument would be the root cause, this would really be a defect of its Ohm circuitry.
But I doubt that. At least, I can't imagine the electrical failure mode, which would create such a failure picture. The Ohm circuit is assumed to be the very same as the one inside the 34410/411, and we've never seen a similar failure. It's also quite improbable, that the 34465A as a new instrument would fail, or you know already, that you somehow damaged the Ohm circuit by external overvoltage; which is also lesser probable to cause a failure, by the instruments protection circuitry.
Your resistor assembly, anyhow looks as to tend more probably to fail:
It has no 4W connection, so you will have to make 4W measurements artificially on these two jacks.
I don't know the quality of your cables, but broken cables or oxidized connectors might also create such failures.
You have used 3x 100M as trimming resistors. This gives about 33ppm change of 10kOhm, and that's in the exactly same ballpark as the ~25ppm you observe as fluctuations. That alone makes me suspicious.
Resistors in the high MOhm values are usually unstable, especially these thin film or coal film types, and tend to fail fatally. (which type are these, actually??)
Maybe there's a cold solder junction, or a crack inside these resistors, which create these fluctuations.
Then you mentioned, that you drilled the cables. What kind of isolation do these have? Which cables are paired?
If you drilled Sense+ and Sense-, and your cables have a poor isolator like PVC, this will create big errors by leakage, on the order of many ppm, and also depending on the moving of these cables.
If these are isloated by TF, it's better, but otherwise, you have to separate cables, which are on positive and negative potential.
Then you said, that you measured this resistor OK with the 3458A, but one week BEFORE this measurement.
Therefore you can't be sure, that your resistor assembly did NOT go defect in between.
If you re-measure with the 3458A NOW, maybe it also shows this fault.
I only can hint to my own experience, being tricked by the resistor DUT itself.
Comparing Ohm measurements (on 100k) of 34465A vs. 3458A, which differed by about 30ppm also, I thought, that my 3458A would be defect!
In the end, it turned out that the DUT showed a dielectric relaxation effect, which I discovered only by the different OCOMP timings of both instruments.
So, purely from a logical aspect, I propose that you look for the fault at the more probable resistor, than at the less probable 34465A.
Maybe you have another stable 10k resistor, which you can test. 1k and 100k resistors also might do the job, as it's only another range. A simple thin film 10k resistor would show a big, but smooth temperature drift, but no fluctuations.
Or you simplify the DUT by measuring the precision resistor 'naked', as I have done; and by simplification, I finally found the root cause.
Frank
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