Because the AD9915 and AD9914 have a 16 Bit Phase to 12 Bit Amplitude Converter, there will be Phase Truncation Spurs if the FTW'S least significant 16 bits are not all at zero.
Other than using the rightmost 16 bits of the FTW for having finer than 16 bit frequency resolution, one may also use the Programmable Modulus Mode if I understood it correctly. Usually both are used in order to achieve super-fine frequency resolution, but for medium-fine resolution one of both alternatives should work.
The question is now, what kind of spurs would be generated by usin the Programmable Modulus Mode. Is there a rule of thumb? Is there a calculator available anywhere? Can someone who already tried this comment on wether Programmable Modulus Mode is better or worse than using the full width of the FTW and therefore introducing phase truncation spurs?