SNIS211D October 2019 – November 2020 TMP63
Existing thermistors, in general, have a non-linear temperature vs. resistance curve. To linearize the thermistor response, the engineer can use a voltage linearization circuit with a voltage divider configuration, or a resistance linearization circuit by adding another resistance in parallel with the thermistor, RP. Section 9.2.1 highlights the two implementations where RT is the thermistor resistance. To generate an output voltage across the thermistor, the engineer can use a voltage divider circuit with the thermistor placed at either the high side (close to supply) or low side (close to ground), depending on the desired voltage response (negative or positive). Alternatively, the thermistor can be biased directly using a precision current source (yielding the highest accuracy and voltage gain). It is common to use a voltage divider with thermistors because of its simple implementation and lower cost. The TMP63 has a linear positive temperature coefficient (PTC) of resistance such that the voltage measured across it increases linearly with temperature. As such, the need for a linearization circuits is no longer a requirement, and a simple current source or a voltage divider circuit can be used to generate the temperature voltage.
This output voltage can be interpreted using a comparator against a voltage reference to trigger a temperature trip point that is either tied directly to an ADC to monitor temperature across a wider range or used as feedback input for an active feedback control circuit.
The voltage across the device, as described in Equation 2, can be translated to temperature using either a lookup table method (LUT) or a fitting polynomial, V(T). The Thermistor Design Tool must be used to translate Vtemp to Temperature. The temperature voltage must first be digitized using an ADC. The necessary resolution of this ADC is dependent on the biasing method used. Additionally, for best accuracy, tie the bias voltage (VBIAS) to the reference voltage of the ADC to create a measurement where the difference in tolerance between the bias voltage and the reference voltage cancels out. The application can also include a low-pass filter to reject system level noise. In this case, place the filter as close to the ADC input as possible.