Looking at
Fast Settling Time of 4.5 ?s
I'd expect 1/4.5 = 222 kHz max for this DAC. May be more, with less accuracy -- higher distortion. I don't think arduino can drive with this speed, depends on the code in interrupt subroutine. Even update with 200 kHz, sine wave max - 100 kHz. Don't call writedata from ISR, better do it inside. Look up the size of "word" and "double", I don't use them for a while, probably, word is 16-bit on arduino, too small for phase accumulator. Unsigned long, or long long .