digital analog ground separation
DGND is a "noissy" signal (it is not perfect 0V, but has "ripples" around 0V due to switching of logic circuits)
Analog circuits on the other hand, are very sensitive to noisse.
Therefore AGND must be keeped "clean" (little ripples around 0V).
If you have a single batery, you are forced to short-cut AGND with the noissy DGND.
You can insert a LPF between DGND and AGND to keep noisse coupling from DGND to AGND small.
LPF can be made using C, R and/or L .
Important is to keep AGND clean!!!