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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-Q1 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.

8.2.1.2 Detailed Design Procedure

The resistive circuit divider method produces an output voltage (VTEMP) scaled according to the bias voltage

(VBIAS). When VBIAS is also used as the reference voltage of the ADC, any fluctuations or tolerance error due to

the voltage supply are canceled and do not affect the temperature accuracy (as shown in Figure 8-5). Equation 2

describes the output voltage (VTEMP) based on the variable resistance of the TMP63-Q1 (RTMP63-Q1 ) and bias

resistor (RBIAS). The ADC code that corresponds to that output voltage, ADC full-scale range, and ADC

resolution is given in Equation 3.

Bias

TMP63

Filter

REF

GND

ADC

Figure 8-5. Voltage Divider With an ADC

TMP63

TEMP

BIAS

TMP63

u ¨

(2)

TEMP

ADC Code

FSR

(3)

where

• FSR is the full-scale range of the ADC, which is the voltage at REF to GND (VREF)

• n is the resolution of the ADC

Equation 4 shows whenever VREF = VBIAS, VBIAS cancels out.

TMP63-Q1

SNIS215C – JANUARY 2020 – REVISED SEPTEMBER 2020

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