pito:
For example (just an idea, needs fine tuning may be). The op-amp type is just an example.
Arduino is isolated from the power lines by a separate battery. Do not connect Arduino GND and power GND together !
Or, keep it simple, and use the AD8215 like I suggested on the last page - that way the Arduino can share the power supply (with suitable regulation of course), ground, etc, and it's just one chip and a capacitor or two (decoupling).