Here's a precision linear optoisolator from TI, TIL300A
It's a shame it's been obsoleted only after five years
of being launched.
Maybe it would be interesting to have a look at its
datasheet...

Other (status:active) linear optos: www.cpclare.com.
There is one of these, a LOC211 smt lin opto soldered in
a SocketModem around my table...

Txip.