The shunt temperature in this example is measured using the TMP61-Q1, a linear 10-kΩ PTC, in a typical voltage divider configuration using the analog supply (AVDD) as excitation. The PTC resistance is calculated using Equation 11, which is derived from Equation 10:

The maximum input voltage on AIN3P is limited to 1.2 V when using gain = 1 for channel 3 with AIN3N connected to AGND. Therefore the value of the precision resistor, R_BIAS, must be chosen so that the voltage on AIN3P stays below 1.2 V for the value range of the PTC across the temperature range that needs to be measured. The TMP61-Q1 has its largest resistance at the most positive temperature, approximately 18 kΩ at +125°C. Following Equation 11, that means R_BIAS ≥ 31.5 kΩ. A value of 36.5 kΩ is chosen for R_BIAS to allow variation in the AVDD supply voltage up to 3.6 V without exceeding the maximum AIN3P voltage of 1.2 V.