9.2.2.3 Thermistor Measurement

The temperature measurement using a 10-kΩ thermistor is implemented using a ratiometric measurement approach to achieve best accuracy. The analog supply voltage, AVDD, is used as the excitation voltage for the thermistor in a resistor divider configuration, as well as the external reference voltage, V_REF, for the ADS1119.

The relationship between output codes of the ADS1119 and the thermistor resistance, R_Thermistor, is derived using the following equations. Equation 10 expresses the input voltage at input AIN0 as the voltage across R_Thermistor, whereas Equation 11 shows how the ADC converts the voltage at AIN0 into corresponding digital codes.

Setting Equation 10 equal to Equation 11 and solving for R_Thermistor yields the relationship between thermistor resistance and ADC code.

Equation 13 proves that the output code and thus the accuracy of the thermistor measurement is independent of the excitation voltage. The accuracy of the reference resistor, R_REF, is typically dominating the measurement accuracy in such a ratiometric circuit implementation. A high-precision, low-drift resistor is therefore required for R_REF. For best performance, the value of R_REF is chosen such that the ratio between R_REF and R_{Thermistor_Max} equals the ratio between R_{Thermistor_Min} and R_REF. Equation 14 is therefore used to calculate R_REF.

At the two temperature measurement extremes, –40°C and +125°C, a typical 10-kΩ NTC exhibits a resistance of R_{Thermistor_Max} = 239.8 kΩ and R_{Thermistor_Min} = 425.3 Ω, respectively. Using Equation 14, R_REF calculates to 10.1 kΩ. A 10-kΩ resistor is chosen for this example. Consequently, when using Equation 10, the voltage at the ADC input ranges from 0.13 V to 3.17 V. Thus, an ADC gain = 1 must be used for the measurement.

The microcontroller interfacing to the ADS1119 converts R_Thermistor into a corresponding thermistor temperature by either solving the Steinhart-Hart equation or leveraging a look-up table.