direct digital synthesis low pass filter
It's not the frequency of the output signal your filter is designed for, but rather the clock thats running through the tabulated entries inside the DDS. 

The external clock signal that is provided is what's driving the change in output. You're low pass filter will be designed for this frequency. Take a look at the following section from Analog's guidelines for their DDS products:

http://www.analog.com/en/analog-dialogue/articles/dds-generates-high-quality-waveforms-efficiently.html