Manfred,

if You want to do a good job you use a 100 Ohm resistor followed by a opamp stage (noninverting with filter): now the signal is quite clean and can be amplified to your need.If you dont want to remove the 20% offset by software you can do this in an amplifier stage....
You also may consider opto-isolation in which case a V/F converter is needed.
Be careful with GROUND....
Viel Spass

Edgar