@Andreas:
Thank you for the explanation of the trimming process. Indeed, if trimming is done only for three fixed temperatures,
the TC for other temperature points may have a systematic deviation from zero.
Would you agree that, in theory, and for a large enough population of devices, the TC at these three trimming temperatures should have zero average?
(The average is never exactly zero of course, but presumably it should be significantly smaller than the datasheet value valid across the entire temperature range.)
Using a DAC to generate the trim voltage sounds like a neat idea! A processor can easily interpolate (polynomially to any order) between a number of predetermined table entries.
If the trim value is determined at, say, 5 C intervals from 0 to, say, 80 C, the table contains only 17 values. Assuming that the TC is a relatively smooth function of temperature,
third order interpolation of four table entries should suffice (hopefully!). That is a lot less work than determining the optimal 3rd order polynomial per interval.
But I guess one runs into the question of how much the DAC output drifts? I guess the DAC resistor chain be set by the reference to be trimmed?? (Similar to bootstrapping a zener...)