Question:
Why is there power supply decoupling between VDDA25 and VNEG12 on the AD9162 eval board? And what is the purpose of the SA_GND?
Answer:
The DAC core swings between VDDA25 and VNEG12, so decoupling on both rails is a good idea. The rest of the DAC is referenced to ground, so decoupling from them to ground (VSS) is also a good idea.
The SA_GND is a holdover from an earlier board. It is not needed, so on future designs, that domain will be eliminated and shorted to GND.