I want to read a kind of rectified square wave in a circuit with PIC 12F675 ADC input. The rectified voltage can be read without problem by a multimeter, but when I use the common ground of the circuit the value changes dramatically. I must have a seperate ADC ground in order to connect to this floating ground. But the ADC input has no seperate ground in PIC. What can I do to measure the potential difference between two points in the circuit with PIC 12F ADC without using common ground?