Operation in a Voltage Divider

If a PT100 measuring resistor is operated in a voltage divider with a fixed resistor of R = 100 Ω on a fixed voltage source U₀ = 3 V, we have the following circuit:

Voltage divider of 100 Ω and PT100 on the voltage U_0 with sensor voltage U_S

\[ \text{Voltage divider: } U_S = U_0 \cdot \frac{R_{\mathrm{PT100}}}{100\,\Omega + R_{\mathrm{PT100}}} = U_0 \cdot \frac{100\,\Omega + 0.4\,\frac{\Omega}{°\mathrm{C}} \cdot T}{100\,\Omega + 100\,\Omega + 0.4\,\frac{\Omega}{°\mathrm{C}} \cdot T} \]

Simulation

This is a non-linear equation; with the values given above, the voltage plotted against temperature looks like this:

Characteristic at the voltage divider: U_S rises slightly curved from 1.5 V at 0 °C to about 1.85 V at 150 °C

This does not correspond to our ideal of proportional behaviour. Starting from an initial value of 1.5 V, the sensor voltage rises slowly and non-linearly with temperature. There are better solutions.