> > is this way acceptible? any example?

http://www.avr-asm-tutorial.net/avr_en/AVR_DAC.html

The problem is the resistor tolerance and matching. Save
yourself a lot of work and buy a proper 12-bit DAC

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist