Electronics Forum / Basics / January 2007

Take a look at instrumentation and difference amplifiers.  Analog
Devices has a pretty good website.  I believe that both types of amps
can give you a pretty high degree of isolation when used properly.  This
is especially true of the instrumentation amps, but I also think that
they are usually designed with a high gain, so they may or may not work
for your application.

You could still use a simple optical coupler - just calibrate against
a lookup table in memory.

If the two batteries can share a common ground there's no problem.

If they have to be separated - ask yourself why you need isolation?

Just to keep the motor supply from talking to the data supply?   If
that's the case you can use a high impedance op amp (something like a
Jfet input amp) and use relatively high series resistance into the
differential inputs - it won't be "isolated" strictly speaking, but no
significant current will be exchanged by the two systems, and no
noise.

A cheap and dirty  isolation trick is to use something like a two pole
double throw relay and charge a cap with the motor voltage then switch
the relay and read the voltage on the cap.

Another technique is to use a voltage to frequency converter on the
motor side to send a string of pulses over to an optical coupler whose
frequency is proportional to the motor voltage.  You read the
frequency at the micro controller and calculate the voltage or convert
the pulse train back to a voltage level with a frequency to voltage
converter.

Flying capacitor input to the ADC: (View in Courier)

      +----------->\    <--------[ADC]--->DATA
      |             \              |
      |              O             |
      |              |             |
[MOTOR BATTERY]  [CAPACITOR]        |
      |              |             |  
      |              O             |
      |             /              |
      +----------->/    <----------+----GND