Hi Maziar,

using internal DAC was only best choice for big noise :-(
On 8 bit accuracy no problem, but for real 12 bit you should be very experienced in EMI and pcb design.

The best way, use a serial DAC, with 100 Ohm decoupling resistors in the 3 control lines (data, clock and strobe) and separate analog VCC and you can easy reach good resolution and low noise.
At the best use 3 opto couplers and noise should easy be rule up to 16 bit.


Peter