is it the ACS712 sensor? it has been said before but you do want an opamp circuit. Putting a reference voltage of 2.5V on one of the input pins should null the 2.5V in to near 0V output.
I suggest that your opamp should also be a "rail to rail" design so you can scale all the way to 5V if that's what you wish to do. Some op-amps will start to chop the signal as low as ...say 3.7V max output for one example.