Or just take a resistor of 250 ohms.(4 resistors of 1K in parrallel)
Connect 1 point of the resistor to an analog input of the processor and the + output of the current device. The other point of the resistor to the gnd of the processor and the - output of the current device.
The you will have 0 to 5 volt for 0 to 20 mA.
The 4 mA (1volt in this case) you subtract in the programm and the rest is your value.