RTDs are straightforward devices: simply a small strip of platinum that measures precisely 100Ω or 1000Ω at 0°C. Bonded to the PT100/PT1000 are two, three, or four wires.

Thus, the 4-wire RTD has two wires attached to each side of the sensor. Each wire has about 1Ω of resistance. When connected to the amplifier, the innovative amp measures the voltage across the RTD and across the wire pairs.

For example, the approximate resistances of a 4-wire PT100 RTD at 0 °C are as follows. (For a PT1000, the middle resistance would be about 1002Ω rather than 102Ω).

When the amp measures this sensor, it measures the resistance between one set of red and blue wires. It then measures the resistance between the red wires and the blue wires. Next, divide those resistances by two, since there are two wires and we only want the resistance of one wire. The final result is 102 - 1 - 1 = 100Ω.	These are very similar to the 4-wire type, but there is only one 'pair' of connected wires. The reason for this is that the wires for the RTD are all of the same gauge and length; therefore, instead of having two pairs, the amplifier will read one pair and use that resistance for both wires.	It is as simple as it gets, with only one wire on each side. You may need to calibrate the sensor by placing it in an ice bath to measure the resistance at 0°C (around 102Ω) and then subtract 100Ω to determine the total resistance of the connection wires!

The two ends of the PT100/PT1000 resistor must be connected to the RTD+ and RTD- terminals of the sensor module; in the example above, a resistance of 102 Ohms can be measured. The wire connections for the 3-wire or 4-wire configuration are connected to the F+ and F- terminals. These connections may differ by only a few Ohms from the resistance values of the respective side. That is, the resistance between F+ and RTD+ or F- and RTD- may only be a few Ohms.