Actually good datasheet section on planes

[T3sl4co1l]

http://www.ti.com/lit/ds/symlink/ads1246.pdf
10.3 (page 86) and beyond.

The underlying lesson is: learn what's actually going on so you can determine for yourself what's best, but failing that -- at least take these documents with a huge grain of salt.

Oftentimes you'll see "separate analog and digital grounds", and FYI, that advice is almost always incorrect.   Fig.123 in particular is the key that a lot of appnotes are lacking.

Tim

[iMo]

Similar from Maxim:
https://www.maximintegrated.com/en/app-notes/index.mvp/id/5450