[volt-nuts] users of Fluke 750A's ...

[frank.stellmach at freenet.de]

Hi  Stephen,

The problem you describe is related to the relatively low  impedance of such divider.This has always to be taken into account, like the voltage  drop over connection leads.

This special effect (inside the instrument) is not explicitely described in  the 750A, 720A, 752A manuals, but indirectly in the Fluke book, Calibration -  Philosophy in Practice, 1st edition, p. 52, description of 7105 system, page 63,where the lead wire resistance error is  calculated. (This can be downloaded via google  books.)

In your example, you find a 20uV error.
The 750A draws 1mA  from each input voltage applied, so that equals an internal lead resistance of  about 20 milli -Ohm, which is quite reasonable. another internal wiring would not change the game.In this sense, I do not agree, that the 750A is wired in a wrong manner, this divider techniques generall has its accuracy limitations.External lead resistances might  be 10 times as high, causing 10 times higher errors.

So this internal error depends on the input voltage,  or equivalently on the divider resistance in use.

A 20ppm error occurs  only at 1.1V input, 10V create 2ppm, and so forth.

This may be  compensated either by using Kelvin sensing of the voltage source, or in some  cases by the 721a, lead compensator.
These methods are also described in detail in  the Fluke Calibration handbook.

Frank

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