Where to ground digital and analog regulators when using separate planes?
Analog Ground is closest to signal input being measured by ADC such that no digital loop currents are shared in the path of the analog current loop so that supply noise is decoupled and ground ripple currents do not cause ground ripple voltage on analog Vref. 
This is done by physical layout of star grounds and isolated ground copper pours and active guarding of sensitive sensor sources.
Each digital current path must consider inductive and capacitive effects that may couple to analog loop elements.