Q1.) Pg 9 - Pin 2 is DNC
Pg 47 - Pin 2 is labeled as LPF in the application circuit.
2.) Pg 55 - Must the thermal pad be connected to AGND or DGND?
A1) DNC = Do Not Connect
My guess is that figure 88, page 47 is a throwback to the fact that we had an
on chip PLL clock multiplier
(requiring a loop filter) on the original AD9786.
(Those who want that feature need to look instead at the AD9779).
Therefore please leave pin 2 unconnected.
2) The exposed thermal pad should be connected to DGND, for best DAC
performance (noise, distortion).