TMP61 datasheet(13/27 Pages) TI1

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Temperature (qC)

-60

-40

-20

100 120 140 160

d013

IBIAS = 50 PA

IBIAS = 100 PA

IBIAS = 200 PA

IBIAS = 300 PA

IBIAS = 400 PA

TMP61-Q1

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SNIS210 – APRIL 2019

Product Folder Links: TMP61-Q1

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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 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 will be 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 400 µA is used with the TMP61, the output voltage will span approximately

5.5 V and will have 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. Based on the bias current, the temperature voltage is shown in

Figure 14. 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. TMP61-Q1 Biasing Circuit With Current Source

Figure 14. TMP61-Q1 Temperature Voltage With Varying Current Sources

In comparison to the non-linear NTC thermistor in a voltage divider, the TMP61-Q1 has an enhanced linear

output characteristic. The two voltage divider circuits with and without a linearization parallel resistor, RP, is

shown in Figure 15. For example, consider an example where VBIAS = 5 V, RBIAS = 10 kΩ, and a parallel resistor

(RP) is used with the NTC thermistor (RNTC) to linearize the output voltage with an additional 10-kΩ resistor. The

output characteristics of the voltage dividers are shown in Figure 16. The TMP61-Q1 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.

TMP61 Datasheet(PDF) 13 Page - Texas Instruments

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