TMP64

BIAS

BIAS

TMP64

n

n

TMP64

BIAS

BIAS

TMP64

R

V

R

R

R

ADC Code

2

2

V

R

R

§

·

§

·

u

¨

¸

¨

¸

§

·

©

¹

¨

¸ u

u

¨

¸

¨

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©

¹

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(4)

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.

The cancellation of VBIAS 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, 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 9-3, 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.

VTemp

RTMP64

IBias

Precision

Current Source

Figure 9-3. 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

characteristic. The two voltage divider circuits with and without a linearization parallel resistor, RP, are shown in

Figure 9-4. Consider an example where VBIAS = 5 V, RBIAS = 47 kΩ, and a parallel resistor (RP) is used with the

NTC thermistor (RNTC) to linearize the output voltage with an additional 47-kΩ resistor. The TMP64 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 (RP) is added to the NTC circuit, the added resistor makes the curve much

more linear but greatly affects the output voltage range.

RBias

VBias

VTemp

RTMP64

VTemp

RNTC

RP

RBias

VBias

Figure 9-4. TMP64 vs. NTC With Linearization Resistor (RP) Voltage Divider Circuits

TMP64

SNIS212C – DECEMBER 2019 – REVISED SEPTEMBER 2020

www.ti.com

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