The AD9859 cannot generate a square wave clock.
In order to use on of our DDS devices, (some of which have a resolution of 48
bits), the reference frequency will need to be at least 3 times the o/p desired
frequency, and then use a comparator to
square the sinewave.
Please have a look at the AD9852 instead of the AD9859. For the AD9852, the
(extenally filtered) cosine wave output can be converted to a square wave by
the internal comparator for agile clock generator applications.