I would like to generate a simple sine waveform. The input to this block is a 50% duty-cycle clock from a CPU.

From fourier transform I get the odd harmonics, can I use a four order LPF to produce the sine wave? what amplifier you suggest for this application?

Thanks

It depends on how good your sine wave needs to be in terms of harmonic content (distortion). In theory you can attenuate the harmonics by filtering to arbitrarily small levels, but in practice that may not be realistic. What is the frequency range of the square wave input and how quickly does it change?