The AD9650 data sheet quotes a gain thermal drift specification of 15 ppm/C. Assuming this is with the internal reference, what fraction of that number is the internal reference (which appears in Fig. 85 to approximately match this number) and what is due to other parts in the IC? In particular, to what could it reasonably be expected to improve with an arbitrarily precise external reference? The discussion of this on page 32 of the datasheet is somewhat nebulous.