RTD is a resistor whose resistance varies with temperature. I suppose you want to measure the temperature using an RTD. Normal method uses a constant current source(DC). Common RTDs are PT100s i.e, it has 100 ohms resistance at degree Centigrade. So, if you pass 1mA constant current through the PT100, it will read 100mV across with a multimeter. In your system there should be an amplifier present in the form of hardware or firmware, that has two parameters to adjust-offset and gain. Adjust offset (preset or software depending on your hardware) to read zero on display for the PT100 at 100 ohms which would be 100mV when 1mA is passed through it. Similarly adjust gain with PT100 at the resistance that corresponds to the temperature of your choice.Now, to get slightly serious with instrumentation, although 1mA is a matter of convenience, it generates internal heating of RTD thereby giving error during longer periods. So engineers use constant current 100uA(0.1mA) or 10uA(0.01mA) to minimize the error due to internal heating.