Many applications can successfully be solved by the following concept:

Use an individual local analog ground plane at each ADC and/or DAC, while the digital sides of all ADCs and/or DACs are connected to the one digital ground plane, in which also the micro sits. Make a connection between these local analog grounds and digital ground at each chip, like it's explained in datasheet, means connect analog ground pin and digital ground pin of same chip at only one point, and this with each chip. This will finally connect all the individual local analog grounds to the one digital ground. Do not make any further connections from the local analog grounds to any other ground, not even between them.

Make a further local analog ground plane (chassis plane) at the entry of enclosure, where the cables carrying the analog signals enter or leave the enclosure, and connect it to the metal enclosure. Connect the cable shields to this point. (You can also make a 'soft ground' connection between this local analog ground and metal enclosure by the help of 10...100nF capacitors if needed. But this will give a poorer performance.)

Now you have one analog ground plane and a more or less big number of individual local analog ground planes at the ADC and/or DAC chips. Due to the digital noise running across the digital ground plane all these individual analog grounds are not identical but differ due to the digital noise being superimposed to the individual grounds. If you would route a certain analog signal from the chassis plane to any local analog ground plane, this digital noise would be added to the signal and signal integrity would be eroded. So, what's needed is an differential amplifier at the input of each ADC, which takes this noise into consideration and subtratcts it from the signal. This is done by not only routing the analog signal (V2), but also the according analog ground (V1), while the differential amplifier fabricates V2 - V1, which eliminates the superimposed noise to a high degree. Put the differential amplifier directly at input of each ADC, concretely onto the local analog ground.
The same for the DACs. But here you put the differential amplifier not onto the local ground plane at DAC, but onto the chassis plane.

A look at datasheet of AD830 shows what I mean:

http://www.analog.com/UploadedF...D830_b.pdf

Of course, such a concept stands and falls with the performance of differential amplifiers, which must not only show extremely low THD + noise characteristic (THD = total harmonic distortion) but also a superior CMRR (common mode rejection ratio).

The digital ground plane should be connected to the chassis plane, where the cable enters the enclosure. The micro and fast digital busses should not be routed close to this connection point to keep this point quiet as much as possible, free from digital noise!
At this point all the voltage regulators should be placed.

Sometimes ADC and/or DAC performance suffers, if digital side of these chips see rapidely changing bus signals. In such a case buffers should be introduced, which isolate the digital sides of ADCs/DACs from the digital bus traffic.

Kai