I suggest to visit this thread because this has something to do with your inquiry: http://ez.analog.com/thread/39519. Upon reading this,you'll know that the appropriate divisor to be used is 2^16 since the width of the POW register is 16 bits.
This follows that function mode 0100b and 0101 has no problems because it fits to the 16 bit POW register. Now for function modes 1001b and 0111b, you'll notice that POW is using an 8 bit word but with different bit placements;POW[7:0] and POW[15:8]. This means that using these modes, the DDS device still uses the 16 bit wide register. However, you can only fill in 8 bits of it. This could be from 0 to 7 or 8 to 15.
Best Regards
Louijie
