Hi Ioannis,
OK, you lost me there.... I'm no math expert either but isn't differentiation the same as derivative ie. the difference between two samples with respect to time. (in this case)

Let's say we read a voltage once every second first reading is 2.2V second reading is 2.5V the rate of change is then 0.3V/second third reading is 4.0V, rate of change is now 1.5V/second and so on.

It may very well be that I misunderstood the original question, if so you'll have to excuse me, but he said he was looking for the rate of change.

/Henrik Olsson.