>        I think CP Claire (or something like that) has an opto designed
>for linear operation.  As I recall, there might be two LEDs and two
>detectors that are matched so you can get rid of nonlinearities by
>putting stuff in an op-amp feedback loop.
>        They've also got some telephone DAAs that don't use a
>transformer, which is pretty neat!



I did this a while back, using a pair of optos, with the LEDs in series.
The idea is that the photocurrent in the "B" transistor is the same as in
the "A"
transistor, since the LEDs have the same current. It is important to feed
the
LEDs with current, rather than voltage.  I expected it to be very finicky,
but in
fact it was easy, stable, and very repeatable.  No fancy pricey special
optos.

Four optos and a dual opamp, and I had a reasonable hybrid.