Problem with mesuring value from current sensig pin on VNH2SP30
I've seen that people in other forums have faced the same problem, so I'll leave it here:

I found a solution to the problem (in my case). It was necessary to connect the ground of Arduino with the earth of the battery (from which the motor worked). Of course grounds  was connected through Monster Motor Shieled, but apparently it's not so simple with it. The extra wire on the minus battery solved the problem.

By the way, the capacity on the image is incorrectly signed. Its real capacity is 33 nF.