AGND and DGND connection using flash ADC, totally stumped
The whole separation of grounds thing is a common source of misunderstanding and cargo cult engineering.  Except for devices that are specifically designed to provide isolation (IE, digital isolators, isolating amplifiers), all of the ground pins on an IC, regardless of what they're labeled, are really the same ground.  They are labeled differently because they carry the ground currents for different parts of the IC, so in the case of an ADC, the AGND pins carry ground currents from the analog section, and the digital ground pins carry ground currents from the digital section.  Likewise with power devices like amplifiers, you may have 'power ground' and 'signal ground', which simply tells you that some pins carry large currents from the power stage, whereas other pins only carry the much lower currents used in the front end.

So the reason that the AGnd/DGnd bonding point is right at the ADC in your diagram, is that the two pins really need to be at the same potential.  If you had a long path between the ADC's DGND to some far away star ground point, and then a long path back through the analog ground plane to the ADC's AGND pin, there's the opportunity for a potential difference to appear between the two pins which can lead to disruptive or damaging currents flowing between the analog and digital sections of the IC.  

There is of course a larger question implied in your post, which is why and how you should separate the analog and digital grounds overall to begin with.  Generally, even though this goes against the prevailing wisdom of a LOT of app notes, you will be better off not separating the analog and digital ground planes in most cases.  Instead, you should carefully separate the analog and digital sections via component placement, and carefully design your layout to control the path of ground return currents to minimize their effect on sensitive subsystems.  The first couple of answers to this question have some more information, and there are a few other related questions that are probably worth looking at.