Sort of to do with EFI (more electronics)

[Orin Eman]

> << Op-amps.  You can scale and invert, even deal with shifted grounds 
>  and stuff like that. >>


> But you need to get -12/+12 volts for the inversion, a bit tricky on +12 volt
> only system.  (Learded this the hard way).

True if you really want a -ve output... but often you want to use
an inverting configuration to sum some inputs.  In this case, you can
reference the signals to some other voltage, say half the supply voltage.
Follow with a second 'inverting' stage referenced to the same voltage
and it all works out nicely.

Remember that when you do use split supplies, there is no direct
connection of ground to the op-amp, it doesn't know or care...
It sees +/- 6V supplies with its non-inverting input connected to
ground the same as a 12V supply with its non-inverting input
connected to 6V as long as the 6V point is of low impedance relative
to the input impedance of the op-amp.

BTW, the O2 sensor on my car is only sampled 100 times per second
and 10Hz for the period of the rich/lean swings sounds correct...
an op-amp is more than capable of following this.

Orin.