Someone please correct me if I'm wrong on any of this.

Your ammeter is used to measure electric current flowing through a circuit. This can be done a lot of different ways. The way your schematic is set up is using a shunt to provide an very small voltage drop which you can measure. A shunt is just a very low resistance resistor with a high power level as all of the current to your outside system has to flow through this. You want to pick a very small resistor (Typically 0.01 ohms) so you ensure the drop is small enough to not take away power from anything you are trying to run. Now you have to read the drop across this known resistance. Typically the drop is very small, in the mV range. That is why you see operational amplifier on the output of the shunt otherwise your pic will never be able to read the voltage. All of these parts really depend the upper limit you are trying to read current to, but the sch you have should work out ok.

Wiring it up-

+Vin/Vout is the plus side of the power supply
-Vin is the negative terminal from the extrnal item you are trying to read current from
-Vout is the negative termial of the power supply

It looks this the schematic you are using is also used measure the voltage, which is maybe what "tensione" means. I agree with chuck on the voltage reading though, as those zeners get closer to thier max voltage the output becomes less linear giving you a not so perfect reading. I didn't actually go through all of the circuit and figure out each of the points, but this is a typical design for reading current. They do have IC's now that you can place over a trace or run the current through that do everything for you. Maybe look at those if it's low currents your looking for. I hope this helps.

Anthony