If you really need a optical isolation you can build a instrumentation amplifier based on a il300.
There is a datasheet about it from infineon and intersil .
http://saturn.uni-mb.si/~bojan/pdf/app50.pdf
Although i have to add that for a non-inverting lineair isolator you need a + & - powersupply .
Otherwise you will have a dc offset (I know its a bummer)
Recently i been playing with those things and it works like a charm .

Good luck Walkura