Electronics > Metrology

3458A's Current shunts/resistance current source tempco study

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ap:
Replacing the 40k by a VHP 101 and others by better resistors will certainly help. However, improving the 3458A data sheet values (, accuracy over time/aging, drifts) formally so that it can be used in traceable (not even ISO certified) cal docs will be another story. It seems quite impossible to do that, not knowing the exact propagation and thus impact of any such design change.

zlymex:
I do have some red HP wire-wound resistors, and I've seen one photo inside a HP standard resistor showing two red resistor with PFR*** marking, this again mislead me.

Dr. Frank:

--- Quote from: ap on June 01, 2016, 12:05:58 pm ---Replacing the 40k by a VHP 101 and others by better resistors will certainly help. However, improving the 3458A data sheet values (, accuracy over time/aging, drifts) formally so that it can be used in traceable (not even ISO certified) cal docs will be another story. It seems quite impossible to do that, not knowing the exact propagation and thus impact of any such design change.

--- End quote ---

That's an interesting question.

In fact, HP already improved the 3458A over the years, at least by assembling the VHP101 from about 1995 onwards, giving probably 0.3ppm/K and < 1ppm/yr. stabilities.
But they never updated the datasheet, and also the Transfer Accuracy for Ohm mode is still missing. (Can be found for Fluke 3458HFL)

If you list the DCI datasheet parameters and the T.C.s of the different resistors side by side, you easily see, that both were correlated.
The 40k resistor is related to "T.C. With ACAL", the other resistors relate to "T.C. Without ACAL".
Bias current and thermal offset voltages mainly determine the "ppm of range" parameter.

See table below, as-is, and possible improvement.


So any improvement in the resistors specification will directly improve the datasheet.

To get a reliable, metrological clean characterisation, one would simply apply the GUM:
http://www.nist.gov/itl/sed/gsg/uncertainty.cfm

You may determine A-type uncertainties, by measuring the STD of the different DCI ranges.
For the systematic  B-type errors one would measure or estimate the range-to-range transfer uncertainty of the 3458A, and the real T.C. by changing its inner temperature.

An external precision current source would be required.

For ACI, a comparison of the frequency behaviour of the old and the new resistors could be done, and then the A-type and B-type uncertainties, of course. That's much more sophisticated, than DCI.

Frank

TiN:
ap
My plan currently stands as testing all A1's resistors from bad board, evaluation their impact on different functions, and then repairing board and using as test vehicle in my 3458A.
By the time it's ready, I'll have suitable 10V and 10K source, and both of my 2002's calibrated (planned in August already), so I can open up 3458 and mess with it's guts without worrying about calibration.

Dr. Frank as always on top of it.

I do agree that HP back then was remarking everything, even jelly-bean LM339 and 74HC00 logic chips. Parts which the selected/aged specifically inhouse - they assigned HP logo/branding.
I saw such "HP" resistors in HP 11177B calibration module for 3455A together with ovenized 5.1V zener reference.



I have done little article about it on my site about year ago.

And yes, my A1 PCB in 3458 is equipped with VHP101 (Board is Rev.D, manufactured in 1996, by datecodes on ICs).

quarks:
thanks a lot for sharing
looking forward to see your results

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