Please check the guide of IO-AO6X:
http://www.unitronics.com/docs/io_expansions/io-ao6x.pdf?sfvrsn=0
Resolution at 4-20mA
819 to 4095 (3277 units)
X min: 819 ( 4mA)
X max: 4095 (20mA)
4080 or 4095 depends of the impedance input of the inverter, or the gain frecuency, etc.