Arduino Due analog output has 500mV offset
I don't know how old this thread is, anyway I thought it doesn't hurt to give some insight because I experienced the same problem. Look at: 
http://www.atmel.com/Images/Atmel-42187-ATSAM3X-and-ATSAM3A-Series-Checklist_AP-Note_AT03462.pdf 
page 13: DAC0 and 1 voltage lays between 1/6 ADVREF and 5/6 ADVREF which corresponds to approximately ADVREF=3.2v; Span is (4/6)*3.2 = 2.1v
for periodic signals, Offset can be removed by using a small decoupling capacitor which will act as a high pass filter. 
Hope this helps