LTZ1000 project build
I monitor two DIY LTZ1000 references, standard circuit from LT
datasheet, since about 7 years.
These are running on 45°C, and are using 5 precision wirewound resistors
(PWW), T.C. ~0.2 .. 4ppm/K.
BOM cost are currently 42.85$ for the LTZ1000 (non A!!) directly from LT
(recommended, beware of chinese fakes), and about 7-10$ for each resistor.
So each reference may be built for about 100$ in total.
By avoiding some pitfalls, like excessive heating, causing hysteresis
effects, these reference will have an annual stability of typ.
-0.8ppm/year, without any special treatment, like "ageing", slots in the
PCB, or usage of extremely sophisticated and expensive metal foil resistors.
One only has to monitor the drift of each reference if it's having this
typical drift rate, or not.
Andreas Jahn currently makes a great job in investigating on the overall
T.C. performance of his LTZ references, in terms of selecting these PWW
resistors, trimming by 400k resistor, and LTZ lead length.
I'm doing that drift monitoring by comparison of a total of 4
references, that's an additional LTZ1000A in a 3458A, pimped to 65°C,
and not running continuously (practically no drift), and two stacked
SZA263 in a Fluke 5442A, which have typ. +1ppm/yr, but this instrument
also is not powered continuously.
The differences within this group are still within +1/-2ppm after 7 years.