The signals from field sensors can be affected by noise generated by power surges, lightning strikes or other EMI (Electromagnetic Interference) sources and also by ground potential differences. One method to avoid most of these problems is to use a complete isolation from the field.
The isolation of an input sensor will require a separate power supply to power the field device and the circuit that realize the insulation itself.
So I think up to use this circuit also with Arduino. The diagram shows the circuit that accepts an input voltage from about 20mV to 5V or a current of 4 to 20 mA (with the jumper W1 inserted). The two resistors in parallel R2 and R3 give a value of about 250 ohms, in order to have 1V to 5V for 4mA to 20mA current input.
More details, components and the program listing are visible in the post on my blog: http://ardupiclab.blogspot.it/