Datasheet

RTD Wiring & Config

RTDs are really very simple devices: just a small strip of Platinum that measures 100Ω or 1000Ω exactly at 0°C. Bonded

to the PT100/PT1000 are 2, 3 or 4 wires.

4-Wire RTDs

We'll explain the 4-wire version since that's the most complex. Normally if you want to measure a resistor you just

connect your multimeter to each side of the resistor. The multimeter puts a small current through the resistor and

measures the voltage generated across it (remember V = I * R). This works great for just about all resistors. However,

for

very precise readings

of low-resistance resistors, you

also

have to account for the wires connected! For basic

resistors, they are only good to 5% anyways so we don't mind the resistance of the wires.

For RTDs, the wires, especially the 1 meter long ones, are 1, 2 maybe even 4Ω of extra resistance! That can add up to

half or even a full °C! No good, we want to make sure that resistance is not included in our measurement

Thus, the 4-wire RTD. Each side of the RTD has

two

wires attached. Each wire is maybe 1Ω of resistance. When

connected to the amplifier, the smart amp will measure the voltage across the RTD and also across the wire pairs.

For example, here's the approximate resistances of a 4-Wire PT100 RTD at 0°C (for a PT1000, the middle resistance

would be (1002Ω rather than 102Ω)

(Remember that the middle resistance - 102 or 1002 Ω - will vary with temperature, but the 2Ω wires will not) When the

amp measures this sensor, it will measure the resistance between one set of red and blue wires. It will then measure

the resistances between the red wires and blue wires. Then divide those resistances by half - since there's two wires

and we just want the resistance of one wire. The final result is 102 - 1 - 1 = 100Ω

3-Wire RTDs