I used an op amp because it gives the board the ability to read 0 to 1024. I recently made a horse power calculator where I had to take a mVdc reading across a shunt, I found that using an amp makes a mVdc reading more accurate if max mVdc = 5.
I guess in this scenario I don't need to convert 1024 to 5vdc.
@TomGeorge. the op amp is getting powered from the +and- from the back of the ammeter. it receives a voltage of 75mvdc. the ammeter is powered from a converter and the converter gets its signal from a shunt in the motor circuit.
in the circuit diagram I sent you I didn't draw a power supply to the Arduino. I figured that was a given.
I pretty much just drew sensor in and digital out.