I know the normal way of optically isolating an analog signal is to use an analog optocoupler with two detectors, and using an op-amp to servo the emitter voltage until the detector output matches the input, like this:
However, there are some issues with this, particularly the need to have some kind of supply voltage on the input side to power the op-amp and provide a voltage reference for the detector to use.
Would it be possible to build a circuit that didn't need an input-side supply voltage?
One idea I had is to use two separate optocouplers, driving one directly from the input and driving the second from the output of an op-amp, servoing until the second optocoupler is driven identically to the first, like this:
But I haven't ever seen a schematic like this in any appnotes or documentation. Would this actually work?
Also, how would I find such a "matched" pair of optocouplers that could be used in this way? Potentially, the two couplers in a single-package dual channel optocoupler would be reasonably similar to each other, but I've yet to see a datasheet that actually gives any kind of specification for that beyond the general tolerances for the individual elements.