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.
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