David,
my 3458A has a T.C. for DCV, 10V of about +0.45ppm/K, i.e. about
+0.25ppm/K for the A/D and +0.2ppm/K for the LTZ1000A reference.
These are typical, specified values for this instrument. Other 6.5 ..
8.5 digit DMMs have very similar T.C. specifications.
Therefore a temperature stability of 0.2 °C is required to achieve
0.1ppm stability, which is a reasonable criterion for 7.5 and 8.5digit DMMs.
For ratio and indirect comparisons of references that temperature
constancy is necessary on a minutes timescale.
For time and temperature stability measurements on DUTs, that needs to
be maintained over up to 24h, during which I monitor the 3458A internal
temperature.
An example is the characterization of my DIY LTZ1000 references, which
are all trimmed to < 0.03ppm/K by means of the 3458A.
These measurements also run over several hours at +/- 0.1°C stable room
temperature, but varying the DUTs temperature over +/- 10°C.
Only our basement allows such stable conditions at 21 +/- 1 °C seasonal
changes.
Temperature variations greater than +/- 0.5°C make precise DCV
measurements nearly impossible.
Frank
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