[volt-nuts] Update on 720A

[Chuck Harris]

Hi Dave,

Fundamentally, any input impedance from your 3458 is going
to throw your 720A out of wack.  the 720A is not supposed to
have even picoamps of loading.... nothing, nada, zip.

I would suggest that before you damage your 720A further,
you read the manual for the 720A and follow the instructions
to the letter.  An HP3458A is not a substitute for a null
detector and a voltage calibrator.

The calibration of a 720A requires that no current, none, zip,
be drawn from the 720A on any port.  It can only be done by
using a voltage calibrator, and a null detector, as a
differential voltmeter.

Remember, and heed my words, *no* current may be drawn from the
720A.

-Chuck Harris

David C. Partridge wrote:
> Update:  After changing the resistor I added in series with R1008 (A decade
> position 1.0) from 204.8 to 202.4 Ohms (I wonder if this means that R302 is slowly
> increasing in value back towards nominal) and re-calibrating again, the A decade
> is now within 1ppm of linear (using the 10V range and nominally 10V input).
> 
> I note that when I'd done this, with the A decade set to 0.9 (reading 9.000000V on
> the 3458A), the actual input voltage needed was 10.000040 or so.  Is that to be
> expected?
> 
> Two of the positions on the B decade wouldn't quite adjust for a null when
> calibrating it.   In this situation I think I have two options:
> 
> 1) Add series resistors to compensate for slightly low value 9.898k resistors in
> the relevant two positions of the decade and recalibrate.
> 
> 2) Adjust R203 to change the bridge balance slightly to the 10K ohm decades and
> recalibrate.
> 
> Your thoughts on which approach to take is much appreciated.
> 
> Thanks Dave
> 
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