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Current monitoring circuit for Arduino

I’m considering using the current monitoring circuit at here
Looking into the Arduino connections and the provided code, things kind of make sense. But I don’t understand what C1 is for. It mentions diverting the AC current, but why is that needed? I searched here, and have been Googling it, but no clear explanations yet. Any thoughts on what it’s for? Thanks.

It is providing a low impedance path to the common for a.c. current, in effect, it is anchoring the mid-point so that it is an a.c. ground. Without it, the impedance of the midpoint (given the resistor values shown: 5 kΩ - 235 kΩ) would be in series with the input signal, and that would attenuate it so giving invalid readings.