This document explain the additive phase noise due to quantization noise
in a theoretically way. Moreover, it provides phase noise values using
the default mounted DAC (AD5662, 16 bits), taking into account also the
voltage noise due to the DAC output stage. The result of the calculation
is that AD5662 is still fine for cheap, AT-cut, voltage controlled
crystal oscillators (VCXOs). In order to optimally control an OCXO with
SC-cut crystal, a DAC resolution of 20+ bits is recommended.