I'm trying to desing simple DDS waveform generator using DDS IC, but I'm a bit lost about designing output filter for it. My requirements are 1 Hz - 25 MHz sine, square and triangle/ramp wave. Using Analog Device's great online evaluation tool I've came up with a fact that I need to use ~50 MHz lowpass filter, but I'm struggling about how will it behave with square and triangular waves of maximum required frequency. Won't it just try to drop them down to sine waves. If it's true, then do I have to create 3 different filters for different waveforms and switch them when switching waveforms? How this problem is solved in production-quality devices?
If you are trying to push the Dds chip to its max frequency, then you do need a sharp cutoff LP filter. This can get you nice sine waves. To get square waves, use an analog comparator fed with the sine waves. To get triangles, perhaps use a switched integrator-type circuit, switched with the square waves. (The triangles are more of a challenge.)
Note: some Dds chips come with a comparator.
A commercial instrument would likely not use a Dds chip, as these are set up for sine waves, but would digitally generate the 3 kinds of waves using 3 different look up tables in the Dds ROM.