Hi Kevin,
Unfortunately I had to leave this project aside during August. We are a small team and I've had to take care of other things that required my attention during the past few weeks.
Anyways, I got some of those REF5050 Mr. Jonathan suggested and I'm about to try them, along with joining both grounds. All the caps are already there, although I preferred to leave them out on my original schematic for simplicity.
I'll upload my whole, complete schematic diagram once I've added the voltage reference. Hopefully tomorrow I'll get everything setup again to keep debugging this little DAC.
Thanks for your interest and further help. I'll get back with updates soon.
Abel.