I've seen a couple of application notes recommend keeping led drive supply and ADC supplies separate. To me it seems that since the LED will illuminate the photodiode that ADC is reading anyway, keeping this two close would make sense. Including the reference voltage for current source and the reference voltage for ADC.
I'm referring to this datasheet. Page 78 "11.1 Layout Guidelines" http://www.ti.com/lit/ds/symlink/afe4400.pdf
TXP, TXN are fast-switching lines and should be routed away from sensitive reference lines as well as from the INP, INN inputs.
Is is possible that this are just general recommendations and the author did not consider this special case where LED illuminates the photodiode from the same circuit? Or is this to prevent oscillations.