How to get rid of the 0.55 Vdc from the DAC output---
You will have to waste an amplifier on this to do it right, otherwise a series capacitor will do the trick. I'll show you in text so everyone can see it:
----|(---- use a nice big capacitor; 47uf will pass just about all frequencies, 0.047pf will block all frequencies.
Better to use an amplifier, Freescale has one that works in front of a comparator; here's the link:
http://cache.freescale.com/files/microcontrollers/doc/app_note/AN3552.pdf
Best regards