[volt-nuts] High precision calibration for the poor man

[Bruce Griffiths]

Samuel DEMEULEMEESTER wrote:
> Hi guys,
>
> I love your level of knowledge and I would like to get your thought about a
> way to achieve a calibration of a high precision meter without a high
> precision calibrator. Let me explain myself. I have many 6.5&  7.5 digits
> meter and only two calibrated and certified tools: a Fluke 5500A calibrator
> and an HP 3458A meter. The 5500A is very stable but can only achieve 5.5
> digits precision. The idea is to source a known stable voltage with the
> calibrator, then use the 3458A to correct (lower) the offset in order to
> feed a 6.5 or 7.5 digits meter in calibration mode.
>
> Example : I need 10.00000V +/-  25µV. I set the 5500A (or any other very
> stable voltage source) to 10.0000V. I get 10.000073V, according to the
> known-good 3458A. I’m now looking for a very precise way to step-down the
> voltage (-70 µV) in order to achieve the rated accuracy needed to calibrate
> the meter. That accuracy doesn’t need to be maintained for more than one or
> two minutes.
>
> Is it a stupid idea? What can I use to add a very small and very stable,
> variable negative offset to a known voltage? A variable resistor should not
> be able to meet the required accuracy without a drift. So, perhaps a high
> precision aop ?
>
> Any idea greatly appreciated :-)
>
> ---------------------
> Samuel DEMEULEMEESTER
> Presse Non Stop - Canard PC
> http://www.canardpc.com
> Tel : +33.6.13.73.4003
> MSN : sam at x86.fr
>
>
>
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>    
You are only making an adjustment of around 7.3ppm an offset stability 
of 1% will contribute an instability of 0.073ppm.
This is readily achievable with a resistive attenuator without using 
precision components (eg 1 ohm + 140k).
However the thermoemfs in the attenuator will need to be low and the 
added output resistance (1 ohm) of the attenuator may be unacceptable.
One could also use a low drift chopper stabilised amplifier or 
equivalent plus a low resolution DAC and a few 1% or ).1% resistors to 
produce a lower output impedance source.

Bruce

Bruce