Why are people expecting the 34470A to match the metrology grade 3458A?
The 34470A is designed as a cheap-ish good value bench/system 7.5 digit meter in the TrueVolt range. It is not a replacement for the 3458A for metrology applications.
Hi Dave,
I simply would have expected stability parameters, which are adapted to a (worst case) 6 1/2 digits reading (at e.g. 1.201000V), to really make use of the 10 times higher resolution.
That would require 0.1ppm linearity of range. As it is 1 ppm of range instead, this 1.201000V measurement is uncertain to 10 digits already.
Further on, a 24h stability of < 1ppm per 24h would be required, 10ppm give additional 10 digits on a 6 1/2 digits reading.
A T.C. of 1ppm/°C (instead of 0.1ppm/°C) additionally gives 5 digits at the usual +/- 5°C interval.
That sums up to 25 digits for a 1.2010
00 V reading, making the additional digit absolutely useless!
Instead, all these 3 crucial parameters should have been 10 times better.
That could have been easily and cheaply achieved, if the Keysight engineers would have put a little bit more brain into the analogue design, and more test and validation time into the development.
Instead they obviously simply recycled a lot.
And the 34470A is not really a cheap instrument.
The design of the 3458A is already 27 years old, so a lot of clever improvements of its circuitry elements could have been done in the meantime, at even lower cost, to be incorporated into lower resolution instruments. (The 34401A was a spin-off from the 3458A techniques)
Although promoted as a metrology grade instrument, the 3458A in practise was mainly intended for rough industrial / military environment (high Tamb. of 55°C) and not really optimized for metrology (typ. 15...35°C specification)
It is in fact designed to 7 1/2 digits performance (at 1/10 of full scale), i.e. 0.02 ppm linearity of range, 0.15ppm / °C T.C. and 1ppm 24h stability.
But in the end, 8ppm/yr. for DCV and 10ppm/yr. for OHM are really mediocre for metrology use, but that can easily be improved by proper component selection, as the Fluke 8508A / Wavetek-Datron 1271 / 1281 demonstrate.
Such an improved level of stability I would have already expected for the "new" LTZ1000 reference in the 34470A. Both the key elements, i.e. the volt and Ohm references, already have the potential for greatly improved performance ..
As a conclusion, the 34470A is really a nice instrument, with additional statistical and graphical features, and some improved characteristics.
Without competing against the still top-of-the-notch 3458A, Keysight could have designed it better, more appropriate to 7 1/2 digits, anyhow.
Frank