Why does my wave signal from AD9850 / AD9851 DDS get cut off?
Several things:


Make sure that your ADC minus(-) signal is GND. If it is by default something else you may see clipped signals like the one you are observing. 
your ADC might not be able rail to rail input. It means that value lower than the minumum input voltage of the ADC would get clipped
There is a software bug somewhere.


Could you test with a DIY oscilloscope? (such as the one that uses your audio card of the PC. (if the frequency range falls in the audio range)