The resistive circuit divider method produces an output voltage (V_TEMP) scaled according to the bias voltage (V_BIAS). When V_BIAS is also used as the reference voltage of the ADC, any fluctuations or tolerance error due to the voltage supply are cancelled and do not affect the temperature accuracy (as shown in Figure 8-5). Use Equation 2 to calculate the output voltage (V_TEMP) based on the variable resistance of the TMP61 (R_TMP61) and bias resistor (R_BIAS). Use Equation 3 to calculate the ADC code that corresponds to that output voltage, ADC full-scale range, and ADC resolution.

Figure 8-5 TMP61 Voltage Divider With an ADC

Equation 2.

Equation 3.

where

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

Equation 4 shows when V_REF = V_BIAS, V_BIAS cancels out.

Equation 4.

Use a polynomial equation or a LUT to extract the temperature reading based on the ADC code read in the microcontroller. Use the Thermistor Design Tool to translate the TMP61 resistance to temperature.

The cancellation of V_BIAS is one benefit to using a voltage-divider (ratiometric approach), but the sensitivity of the output voltage of the divider circuit cannot increase much. Therefore, this application design does not use all of the ADC codes due to the small voltage output range compared to the FSR. This application is very common, however, and is simple to implement.

A current source-based circuit, such as the one shown in Figure 8-6, offers better control over the sensitivity of the output voltage and achieve higher accuracy. In this case, the output voltage is simply V = I × R. For example, if a current source of 40 µA is used with the device, the output voltage spans approximately 5.5 V and has a gain up to 40 mV/°C. Having control over the voltage range and sensitivity allows for full use of the ADC codes and full-scale range. Figure 8-7 shows the temperature voltage for various bias current conditions. Similar to the ratiometric approach, if the ADC has a built-in current source that shares the same bias as the reference voltage of the ADC, the tolerance of the supply current cancels out. In this case, a precision ADC is not required. This method yields the best accuracy, but can increase the system implementation cost.

Figure 8-6 TMP61 Biasing Circuit With Current Source

Figure 8-7 TMP61 Temperature Voltage With Varying Current Sources

In comparison to the non-linear NTC thermistor in a voltage divider, the TMP61 has an enhanced linear output characteristic. The two voltage divider circuits with and without a linearization parallel resistor, R_P, are shown in Figure 8-8. Consider an example where V_BIAS = 5 V, R_BIAS = 100 kΩ, and a parallel resistor (R_P) is used with the NTC thermistor (R_NTC) to linearize the output voltage with an additional 100-kΩ resistor. The output characteristics of the voltage dividers are in Figure 8-9. The device produces a linear curve across the entire temperature range while the NTC curve is only linear across a small temperature region. When the parallel resistor (R_P) is added to the NTC circuit, the added resistor makes the curve much more linear but greatly affects the output voltage range.

Figure 8-8 TMP61 vs NTC With Linearization Resistor (R_P) Voltage Divider Circuits

Figure 8-9 NTC With and Without a Linearization Resistor vs TMP61 Temperature Voltages