[volt-nuts] 3458A calibration

[Randy Scott]

In the HP Journal article on the calibration of the 3458A, regarding DC calibration it says:

   The internal 7V Zener reference is measured relative to an externally
   applied traceable standard.  A traceable value for this internal
   reference is stored in secure calibration memory...

Can someone explain this to me?  The 3458A's A/D converter uses +/-12V reference voltages, presumably derived from the internal 7.2V reference.  Are they saying that the external 10V standard is measured using the A/D converter or that the A/D converter's 12V reference is switched to be based on the externally applied 10V and then used to measure the internal 7.2V reference?  The former sounds like there would be too many variables to obtain a useful result, so the latter seems more plausible.  I think that I may have answered my own question.

I presume that the result of this comparison is not used directly to adjust the output of the reference, but just used to adjust later measurement results.  Right?

Related to this, how does the 3458A convert the 7.2V reference into the +/-12V references used by the converter?  Is it as simple as a couple of opamps and some precision gain setting resistors?  Wouldn't the stability of those other components then dominate the overall stability of the converter?  Is something similar done in other LTZ1000-based voltage standards (such as the Datron 491x)?

Thanks.

Randy.