Circuit #24 : Analog Optoisolation
Optoisolators are an easy way to send digital data across an isolation barrier, but what about analog signals? The circuit below shows how a 0V-10V analog signal can be sent across an isolation barrier. The IL300 dual optocoupler is the key. It contains two photodiodes, on either side of the isolation barrier, both illuminated by a single LED driven by the input analog signal.
The resistor R3 is nominally half the value of R2, or about 38k. The IL300 comes in various gain ratios (called K3 on the datasheet). R3 can be varied to account for various K3 values and trim the output voltage to best fill the 0V-5V input range of the A/D converter.