2 circuits with different power sources
However are they connected, just add another wire which connects one board's "ground" to the other boards "ground". It's that simple! Your DAC signal probably goes through a wire and into the other board, so along with that wire do this:

Attach (solder, screw terminal, whatever) another wire on the first board to GND, twist that along with the DAC signal wire so they form a twisted pair (this helps shield the DAC wire too which is handy) and at the second board where the DAC wire goes, find the closest GND point and solder/attach the wire there. 

This means they both know what "0V" is, with reference to each others supply. 

Without a common ground, the board with the DAC output will be sending for example: 1V with reference to board 1's ground reference. The second board won't know what 1V means in the context of a ground reference, and it will cause incorrect results. By connecting the two Grounds together, they will both become 0V, and the second board will be able to read 1V with reference to the common ground (0V) without issue. 

edit: As some others have hinted, multiple power sources may not always safely be joined in this way, especially if the supplies are using fancy grounding techniques, split supplies, or one is grounded at mains and the other is not. If you have a virtual ground in one device, joining it to a true ground in another device will blow things up. If one device is properly grounded/earthed and another is not, the wire joining them may short out if there is a ground fault in the AC circuit of the non-grounded one. I am not 100% certain on the AC side of things, as I mostly deal with DC circuits in my line of work.