Precision

Current Source

13

TMP64

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SNIS212B – DECEMBER 2019 – REVISED JUNE 2020

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Typical Application (continued)

The engineer can use a polynomial equation or a LUT to extract the temperature reading based on the ADC

code read in the microcontroller. The Thermistor Design Tool should be used to translate the TMP64 resistance

to temperature.

output voltage of the divider circuit cannot increase much. Therefore, not all of the ADC codes are used due to

the small voltage output range compared to the FSR. This application is very common, however, and is simple to

implement.

The engineer can use a current source-based circuit, like the one shown in Figure 13, to have 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 100 µA is used with the TMP64, 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

utilization of the ADC codes and full-scale range. Similar to the ratiometric approach above, 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 13. TMP64 Biasing Circuit With Current Source

In comparison to the non-linear NTC thermistor in a voltage divider, the TMP64 has an enhanced linear output

linear curve across the entire temperature range while the NTC curve is only linear across a small temperature

more linear but greatly affects the output voltage range.