Using Differential Inputs on Arduino Mega 2560

Here's my project. I am measuring the current flow to a motor. To do this I am measuring the voltage across a shunt resistor in series with that motor. The problem that I have is the voltage measured across that resistor is a max of 7mvolts, and that obviously cannot be measured with any kind of accuracy using the normal "analogread". I noticed the Mega has differential inputs and I was wondering if there were any libraries already made that I could use or if I would have to start from scratch? If I would have to start from scratch where would be a good place to start to get me headed in the right direction.

Thanks for any help!

(deleted)

well at the positive and negative of the resistor the voltage is about 48 volts, so, according to what I understand, you cant really use an op amp. It needs to be a step down of some sort to go into the Mega.

(deleted)

I understand there are other possible(and possibly even easier) ways to do this. However, I would like to accomplish this with the arduino. Are there any differential input libraries out there or existing projects that could give me any direction?