[volt-nuts] Precision Current Source

[Andrea Baldoni]

On Mon, Aug 16, 2010 at 10:54:12PM +0200, Dr. Frank Stellmach wrote:

> Resistance of the 34401A drifts 100ppm in one year, even cheap wirewound  
> resistors may drift 20ppm / yr. only.
> This bad drift behaviour is due to the complicated current source  
> design, with the nested reference resistors. 3 resistors in summary  
> determine the currents precision: R201/ R202, the 28k57 and the 5k ...  
> 1M in the fineline array. And those are drifty thin film or thick film  
> resistors only.

I had today the opportunity to test the 1mA current source of two 34401A.
They looks not very stable at first sight (I have not yet a better reference
to give sensible numbers), not accurate to 1mA, and also very (1%) different
each other.
Adding the fact you must mathematically invert the calibration of each
multimeter (and each different ohm range), thus you don't get direct readings,
or manually trim the multiplication factor of the external circuit and then
limit the use to a single multimeter and a single range, it seems that using
the internal current source to null the internal voltage source drift (at
least with the 34401A) is not a viable solution.

You are right: it is definitely better to create an independent instrument and
use the 34401A only as a voltmeter.

Best regards,
Andrea Baldoni