Hello Ben,
sure, this has been discussed intensively, also on:
http://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/
85°C is NOT commonly used neither for LTZ1000A, nor for LTZ1000.
Only in the HP3458A, the LTZ1000A is mistreated, running on about 95°C.
Usually, if the LTZ is run w/o further heat sources in the surrounding
applications, you may run the
LTZ1000 : 45°C, 12k / 1k (gives about -0.8ppm/year stability)
LTZ1000A: 55°C, 12k5 / 1k
LTZ1000x: 65°C, 13k / 1k (see datasheet)
50°C will be somewhere in between.
LTZ1000 and LTZ1000A both use the very same die (chip), but a different
die attach epoxy.
The A version therefore is thermally more isolated from the TO99 case
(400°C/K) than the non-A version (80°C/K).
Therefore, the A version heats up 10°C higher than the non-A, due to the
self heating effect caused by the reference circuit itself, and
therefore must run 10°C higher; 45°C or even 40°C is not feasible for
the A version.
I always recommend the LTZ1000, non-A version, on 45°C for best
stability, and it's also much cheaper.
Frank
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