With new fast 7.5 digit meters around the market for the 3458 is now smaller than it was 30 years ago, when it also was the fastest at 7.5 digits. Today the 34470 or DMM7510 may be a good alternative in some cases, when it is more about speed or digital RMS than highest resolution.
The 7 1/2 digit DMMs named above have A/D converters derived from older 6 1/2 digit DMMs and we cannot call these
instruments metrology grade 7 1/2 digit DMMs.
e.g. The DMM7510 has a very stable (ovenized) reference, however its linearity is not good enough to call it 7 1/2 digits worthy and units leave factory with sloppy calibration: [+10V/-10V] symmetry is poor and instrument suffers from DC offset once DC inputs shorted. Here the advise is: Try before you buy and compare results with a true 8 1/2 digit DMM.
Improved A9 reference board - 2ppm/YR - lower TC @ lower cost using recent technology...
A 2ppm reference for the 3458A is/was already available from Keysight, for a sweet $2000, it can be purchased today, but I believe you’ll have to install yourself and send for calibration. Not available as an option during purchase.
The legacy HFL reference board 03458-66529 is still available from Keysight - that is correct.
The 2ppm / YR reflects long term drift - these are binned boards - screened over a few months.
However semiconductor companies make advances in BZ silicon - Passive component suppliers like VPG, etc. ... make advances in vacuum sealed BMF resistors ... so taking advantage of that knowledge KS could upgrade the 8ppm board 03458-66509 and create a new reference board with better TC and <2ppm long term stability as standard 3458A product at affordable cost. Currently the yield of the 2ppm binning process is not high enough to make it part of the standard product.
KS should be agile that it does not loose market share in this metrology arena by believing their 3458A - B.E. is still good enough to beat competition.
There was a live demonstation here at PTB.
Any shots made at that demonstration?
Some shots were taken - but attendees were asked not to divulge these pictures as it would alert competition ... and also reveal their identity ...
On one of the occasions the unit had still engineering colours - not the final RAL colors that were chosen by marketing.
Stay tuned ...
Herzliche Grüße/Meilleures salutations/Best regards
ScoobyDoo