Unkown DC offset of an isolation amplifier circuit design
You have no DC path to ground on the AD8513 input. A resistor to ground or whatever other DC reference you want to use will provide that. The bias current of the 8513 is pA or nA over a reasonable temperature range, but it still needs a path. Also the (likely bigger) factor of the DC leakage through a 220uF capacitor needs to be dealt with. The resistor value needs to be low enough that the sum of the leakage and bias currents does not cause excessive offset.
220uF does not really provide isolation the way most would think of it, but I won't address that issue.