Operational amplifier +Vcc <5V for protect arduino input and gnd to -vcc
Unidirectional current only
Arduino aref 2,5V (Voltage divider , R1=R2 on 5V)
Since it is in inverting configuration, analogread 1023 for 0Amp and 0 for max Amp
Link at falstad circuit simulator
if you just want to measure from 0 to 2.5A you can amplify 2 times and thus further improve resolution.
Just put an R2 = 2 * R1