The HCNR200/201 is a high-linearity analog optocoupler which can be used to isolate analog signals. It offers good stability, linearity, bandwidth, simple design and low cost. With choice of suitable application circuit it can also incorporate amplification, attenuation, offset, inversion among others to the input signal.
Details of the IC along with datasheet can be found in the link below.
https://www.broadcom.com/products/optocouplers/industrial-plastic/specific-function/high-linearity-analog/hcnr200
To get complete isolation, make sure that grounds as well as bias supplies of input and output stage are isolated. Note the label used in above circuit for input stage are GND, VCC, VEE and that of output stage are GND2, VCC2 and VEE2. I generally use two isolated DC to DC converters each powering one stage at a time.
Now coming back to working of the circuit. Lets us analyze the input stage first.
The input side op-amp always tries to force same input voltages at its two input terminals in close loop connection. Thus, the input side photodiode PD1 (terminal 3,4 of HCNR200) will have zero voltage across it. A positive voltage at inverting terminal of U1 will swing the output to negative rail causing current flow through LED (terminal 1,2 of HCNR200). Also the positive voltage will cause a current through R1 which will eventually flow through photodiode PD1.
IPD1 = Vin/ R1
Current is linearly related to input voltage.
Since photodiode PD1 and PD2 are identical to each other, IPD2 should be equal to IPD1 ideally. Practically the relation is
IPD2 = K x IPD1
where K is gain coefficient.
Output voltage can be given as Vout = R2 x IPD2
