Can any OP amp experts help me out with something please?

I want to make an "absolute value" amp to output the same positive
voltage for a negative input as it does for a positive input.

My back EMF idea:

Both sides of the motor are fed to the two inputs of a
differential amp.  The output of the differential amp is fed
to the input of the "absolute value" amp.  The output of the
"absolute value" amp is fed to the input of a sample & hold amp which
is gated by the inverted PWM gate signal. The output of the sample & hold
amp is fed to a voltage to frequency converter.  The output
frequency of the converter is sampled or timed using a single
pin on the PIC (RA4?).  The period measured is inversely proportional
to the motor speed, hopefully....

This sounds a bit "Rube Goldberg'ian" to me though.  Is there
a simpler way?  I have not seen any other schematics of anything
like this yet so I can't decide myself....

If the PIC had an A/D I could skip the F-V step. Then the sample
& hold amp could be dropped because the PIC knows when to read
the voltage from the "absolute value" since it generates the PWM
signal.

But how do I simply this with a 16F84 with no A/D?

Anyway, I need an op amp circuit that an perform an absolute
value operation so I can experiment with this idea.  I'd
ask for a sample & hold amp too, but I am sure I can look one
up somewhere else.



Thanks in advance for your feedback.

Dan