As stated by others, grounding can be a subject of much discussion and disagreement. But there are some simple rules.
All signals have a return path. That return path generally follows the signal path as closely as it can along the nearest ground (or power) plane or trace. It is a simple matter of a signal following the lowest impedance route.
Digital signals have high frequency components - think of the Fourier representation of the leading and trailing edges. Those high frequency components of the pulse edges will readily capacitively couple into nearby components or signal traces. Low signal level analog circuits can thereby be subjected to the "noise" of digital signals.
You don't necessarily need separate gound planes for analog and digital, but you do need to look at both the signal path and the return path. You don't want the digital return path to cross or parallel the sensitive analog signal return path. You can control this with component placement such that the digital and analog circuits are physically separated, and each has a clearly defined path to return to the power supply.
You will hear and read arguments that separate ground planes should be used with a single point common "mecca". This is pure garbage when you look at both the theory and real world experiments that have been conducted. Single point analog/digital ground plane connections are, in fact, inviting introduction of noise onto the analog plane at the point of connection. They also ignore the real world fact that some return paths are through the power planes, and not the ground planes.
Learn to visualize the entire signal loop, including the return path, and keep the digital return away from the analog. You won't have any problems.