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When the thermistor is excited by a constant


voltage, the current through it will scale automatically as its resistance changes


the signal can be measured by the electronics. But, there is a simpler option: When the thermistor is


excited by a constant voltage, the current through it will scale automatically as its resistance changes. Rather than using a reference resistor, use a precision sense resistor, its purpose being to calculate the current fl owing through the thermistor. With the excitation voltage being used as the ADC reference also, there’s no need for a gain stage. There is no workload for the processor in terms of monitoring the voltage across the thermistor, determining whether the signal level can be measured by the electronics, and calculating what the gain/ excitation current value should be adjusted to.


Thermistor-resistance range/excitation If the thermistor’s nominal resistance and range are small, either voltage or current excitation can be used, in which case the excitation current and gain can be fi xed; see Figure 3. This method is useful since the current through the sensor and reference resistor can be controlled, which is valuable in low-power applications and thermistor self-heating is minimised. Using voltage excitation for a thermistor with low nominal resistance can also be used. However, it must be ensured that the current through the sensor at no time is too large for the sensor or the application. When using thermistors with large nominal resistance


and temperature range, voltage excitation allows an easier implementation. The larger nominal resistance ensures that the nominal current is at a reasonable level.


Thermistor circuit configuration Whether choosing an excitation current or voltage, it’s best to use a ratiometric confi guration wherein the reference and sensor voltages are derived from the same excitation source, which will preserve the accuracy of the measurement. Figure 5 shows a constant excitation current


supplying the thermistor and a precision resistor RREF the voltage generated across RREF


, being the reference


voltage for the thermistor measurement. The excitation current doesn’t have to be accurate and could be less stable since any errors in the excitation current will be


cancelled in this setup. An excitation current is usually preferred over voltage excitation since it allows better control over sensitivity and is more noise-immune for remote sensors. This type of biasing technique is commonly used for RTDs or thermistors with low resistance values. However, for thermistors with higher resistance values and sensitivity, the generated signal levels will be larger per change in temperature, so voltage excitation is better. For example, a 10kΩ thermistor has a resistance of 10kΩ at 25°C. At −50°C the NTC thermistor resistance is 441.117kΩ. The smallest excitation current of 50µA provided by the AD7124-4/AD7124-8 generates a voltage of 441.117kΩ × 50µA = 22V, which is too high and outside the operating range of most available ADCs. Thermistors are also usually located near the electronics, so the noise immunity advantage of an excitation current is not needed. Figure 6 shows a constant excitation voltage used to


generate a voltage across the NTC thermistor. Adding a series sense resistor in the form of a voltage divider circuit will limit the current fl ow across the thermistor at its minimum resistance value. In this confi guration, the value of the sense resistor, RSENSE


, must be equal


to the magnitude of the thermistor’s resistance at the base temperature of 25°C so that the output voltage will be set at the mid-scale value of the reference voltage when it is at its nominal temperature of 25°C. So, again, if using a 10kΩ thermistor with resistance of 10kΩ at 25°C, the RSENSE


must be 10kΩ. When the temperature


changes, the resistance of the NTC thermistor and the fraction of the excitation voltage across the thermistor also change, producing an output voltage proportional to the NTC thermistor resistance. If the selected reference voltage used to supply the is the same reference as that


thermistor and/or RSENSE


of the ADC used for the measurement, then the system is confi gured in a ratiometric measurement (Figure 7) so that any errors associated with the excitation voltage source will be removed. Note that the sense resistor (voltage excitation) or


reference resistor (current excitation) need to have low initial tolerance and low drift since both variables contribute to the overall system accuracy. When multiple thermistors are used, a single excitation


voltage can be applied; however, each thermistor must have its own precision sense resistor; see Figure 8. Another option is to use an external mux or switches with low on-resistance, which allows sharing of a single precision sense resistor. When using this confi guration, each thermistor requires some settling time during measurement.


www.analog.com www.electronicsworld.co.uk February 2024 09


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