[sdiy] 3 position switch help

[JH.]

>Put two diodes inside the feedback loop of two op-amps. One for
>positive, one for negative.
>Then you have two precision halfwave rectifiers.

Or, put two diodes inside the feedback loop of *one* op-amp.
Think of the ordinary half wave rectifier with inverting opamp: Two diodes 
usually branch the feedback path into a feedback resistor (from which you 
derive the half wave), and the other normally goes directly to the virtual 
GND summing node from the diode.
Put another resistor in there, and voila, the other half wave.
A second opamp is then configured for calculating the difference of two 
inputs (the simple, single-opamp instrumentation amp with 4 resistors, only 
that you can omit two of them here), to derive the full wave rectified 
signal.
Dual opamp solution for the whole thing!
(see http://www.jhaible.heim.at/fs1a/jh_fs1a_schematics_board_1.pdf , the 
part around U3 and U4. I don't make use of the half wave signals here - I 
need magnitude and sign instead - but it's the same circuit.)

JH.