The really strange thing I found was that they recommend to join the two split grounds at the ADC... Why do they take out of the package two separate pins for ground, if then you have to externally connect them just out the chip?
This is a great enigma to me.
They have two pins just because two grounding paths is better than just one. Bond wires inside IC package are extreme thin, and is a bit different situation than in PCB environment. Of course, much more ground pins would be even better but return benefit diminishes as number of ground pins increase, and larger packaging will cost more. If anything is high volume product, ICs certainly classify as one. So perhaps two ground pins is the optimum amount.
Complex digital chips like processors and FPGAs have very large number of ground pins too, to handle the massive return current produced by IO pin drivers. Too small number of ground pins would seriously degrade the SSN (simultaneous switching noise) performance. But usually there is a statement in the datasheet that all the ground pins should be kept at same potential for good performance (quite obvious).
Never ADCs seem to have more sensible recommendations in their datasheets, like in
AD9650 (16 bit, upto 105 MSPS ADC) datasheet:
A single PCB ground plane should be sufficient when using the AD9650. With proper decoupling and smart partitioning of the PCB analog, digital, and clock sections, optimum performance is easily achieved.
I think that most of the classic statements found in ADC and DAC datasheets has been made in the era where multilayer boards were not so common. With only two layers, dedicating one side of just common ground plane with absolutely no slots in it, is difficult. Nowadays it makes more sense to just design a multilayer board.
AD5755 datasheet states that:
A microstrip technique is by far the best, but not always possible with a double-sided board. In this technique, the component side of the board is dedicated to ground plane, while signal traces are placed on the solder side.
Regards,
Janne