[volt-nuts] 3458A calibration

[Bruce Griffiths]

Randy Scott wrote:
>> The effect of dielectric absorption can be minimised by
>> ensuring that the average voltage across the integrator feedback
>> capacitor remains low over the integration cycle.
>> HP/Agilent have several patents covering this aspect of DVM
>> integrator runup cycles.
>>     
>
> Are you referring to what HP calls their "charge-balancing" A/D converter where during the run-up phase of the conversion they attempt to keep the output of the integrator balanced around 0 volts?
>
> The Agilent 34401A does something like this and then uses a second, fast A/D converter to replace the run-down phase.
>
>   
The 34401 runup algorithm is a little more complex than that, it
actually sums the output of the integrator with the input to the
integrator and uses that as the comparator input.

The term balanced is ambiguous, I mean that the algorithm attempts to
keep the integral of the integrator output constant.

Even lower dielectric absorption effects are achieved if a term
proportional to the integral of the integrator output is included in
effect a second order delta sigma runup cycle.
>> However the runup cycle of the 3458 unlike some other
>> HP/Agilent DVMs doesn't appear to use this technique.
>> Later HP DVMs do use such techniques.
>>     
>
> It would seem that with an A/D, like the 3458A's, that relies on multi-slope run-down, it would be advantageous to keep the integrator output as high as possible at the end of the run-up phase to maximize the resolution achieved during run-down.
> Randy.
>   
Bruce