Linear opto-isolator

The fact that the Texas Instruments TIL300 opto-isolator contains two photodiodes is exploited here to endow the device with a virtually linear transfer characteristic. The trick is to include one of the photodiodes in the feedback circuit of the LED driver, while the other is used to drive an output buffer as usual. Assuming that the two photodiodes are virtually identical, the feedback circuit irons out any non-linearity of the transmit diode and the photodiode.