Isolated Voltage Sensor

This circuit measures an isolated DC voltage. First is a difference amplifier that scales the voltage to a low range. The difference amp. also includes a single-pole filter. The signal then goes to an optical isolation circuit based on Broadcom's HCNR201 integrated circuit. The isolation circuit provides closed loop linear isolation and low pass filtering. The current controlled current source is a LED and photo-diode pair. Next a low-pass passive RC network further filters the signal. A voltage follower and another RC filter complete impedance matching in preparation for sampling.