Can Earth be utilized as the whole circuit's common ground?
Common ground or circuit ground is the point you choose as a reference in your circuit, usually designated 0V.
Earth ground, the &quot;earth&quot; on the AC socket is a physical connection to our planet. You don't need or want to connect the two. Earth ground can, for instance, be used to &quot;ground&quot; a metallic enclosure that holds, but isn't connected to, your circuit; should, by accident, a high voltage make its way onto your enclosure, this earth grounding prevents you yourself becoming &quot;earth&quot; for a current that could kill you, much like a lightning rod protects a building.
Unfortunately, the same schematic symbol is often used for all kinds of ground.
TL;DR: you don't need to, and usually don't want to, connect common ground to earth ground.
Also, your microcontroller can't measure AC, but that is another matter.