Hello,
the complete HP Journal Feb. 1986 can be found here:
http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1986-02.pdf.Then, I was asked by the TO to comment the 7th digit performance, although I do not own that instrument.
Well, most of the newer HP DMM deliver one additional digit directly over the GPIB, beginning with the 3458A, the 34401A, and so forth.
By proper averaging, and especially in higher ranges, that could be stable enough for short term stability measurements.
This way, I have measured successfully the < 1ppm linearity (worth 7 digits) of the 34401A's A/D on the 10V range.
It's a little bit complicated to get the 7th digit from the 3457A, though.
The statistical distribution for the 3V range shows, that you get +/- 3 digits of fluctuation from reading to reading, which can be averaged over 100 samples to a standard deviation of 110nV, that means, the 7th digit of the 3V range will be stable enough for short term measurements. (*)
But it also shows, that manual reading of the 7th digit math register over the front panel will NOT be stable enough, as there is no averaging and the +/- 3 digits fluctuation will apply. To use the 7th digit from the front panel, it is necessary to use the averaging math function of the instrument, instead. (*)
As the source of the +/- 3digit noise is not further described in the article, i.e. whether it originates mainly from the A/D, or from the DCV input conditioner, the 30, 300mV may not produce a stable enough 7th digit, or may require much more averaging, i.e. over 10,000 samples.
Under the mentioned preconditions, the 7th digit is for sure usable in the 3, 30, and 300V ranges.
If you look into the specifications of the 3457A, neither the performance of the 7th digit is specified, not even mentioned, nor the so called Transfer Stability, or 10min stability.
For that parameter, the internal reference (LM299H) would be stable enough.
That's a pity, as latter parameter can be easily derived from the statistical distribution in the HP Journal. I think, it would be identical to the 1 sigma value of that average.
Frank
(*) Rem.: If I understand the HP Journal article right, for usage of the 7th digit with 110nV sigma stability, it is necessary to use 100 NPLC and additionally to average over 100 of those readings.
That would require 166 sec measuring time for 7 stable digits.
An analogous statistical test should be done with NPLC 10, if that already yields that level of stability.