Layout Considerations for Wearable Temperature Sensing

Introduction
For wearable applications, such as fitness trackers
and baby monitors, measuring skin temperatureaccurately can be challenging due to two major
concerns: accuracy and speed.
Accuracy can be addressed by choosing a highprecision temperature sensor, as well as employing
good PCB layout techniques.
A temperature sensor’s thermal response is how fast
the device responds to a sudden change intemperature. The speed of the response is determinedby the amount of thermal mass—a material’s ability tostore heat energy—surrounding the sensor. By using
good layout techniques, engineers can maintain asmall thermal mass to improve the thermal responsetime.
PCB Layout Techniques
To accurately measure skin temperature, it isimportant to:• follow good layout techniques• understand the dominant thermal path
• isolate the sensor package• place the device away from interfering heatsources
Another important technique is to minimize thetemperature resistance between the measured surface
and the temperature sensor—which can help decrease
temperature response time—while also maximizing thethermal resistance from the PCB to the surrounding
air—which can help reduce "leakage" from the sensor
to the surrounding air. Employing these techniques will
result in a faster response time and a more accurate
temperature measurement.
Figure 1 shows the cross section of a TMP116
mounted on a PCB. The TMP116 measurestemperature by measuring its own die temperature.
The thermal bipolar junction transistor (BJT) within thedie acts as the transducer. The transistor converts athermal quantity into an electrical signal which is thenmeasured and converted to temperature. The heatflows from the skin to the die through the via andexposed pad, which helps decrease thermalresistance and provide the most direct heat flow path.

Figure 1. TMP116 Heat Flow Path
Placing the temperature sensor in a closed-ended
stainless steel protective probe, as shown in Figure 2,
may help prevent contamination or potential damage.
This technique may be applied to wearable
applications by placing the bottom of the PCB between
the thermal-sensing contact and the skin.

Figure 2. TMP116 Protective Temperature Probe
Another method is to control the thermal mass of thePCB. A thick PCB with a high thermal mass will
respond to temperature fluctuations slower than a thin
PCB with a lower thermal mass. TI advises to either
use a thin PCB of 0.8mm thickness (rather than thestandard 1.6 mm FR4 thickness), or use a flex PCB asshown in Figure 3 and Figure 4. These PCB designs
will respond to changes in temperature more rapidly
than a thicker PCB design with a high thermal mass.

Figure 3. Thin PCB Design

Figure 4. Flex PCB Design

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Layout Considerations for Wearable Temperature Sensing
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The plot from Figure 5 shows the thermal responses
from different PCB designs. The thin and flex PCB
substrates have the fastest thermal response due totheir small thermal mass.

Figure 5. PCB Substrate Thermal Response
Comparison
In addition to PCB layout, good mechanical andthermal contact is important to achieve a fast thermalresponse.
Thermal Response of Different Mediums
With wearable applications, it is important to choose alocation to measure skin temperature that will result ina fast thermal response.
An experiment conducted using the TMP116 in astainless steel probe investigates the thermal responsetime of the TMP116 to a sudden change intemperature using different mediums and contact
locations, as shown in Figure 6. The stirred oil test is
used as a reference for the other mediums because
the test provides the fastest thermal response. The
oral, underarm, and stirred oil experiments yield afaster thermal response time compared to moving airand still air.

Figure 6. Thermal Response Comparison for
Different Mediums
Device Recommendations
The TMP116 and TMP117 are digital temperature
sensors designed for low-power, high-accuracy
applications. Both devices provide a 16-bit
temperature result with a resolution of 0.0078°C.
The TMP116 and TMP117 provide factory calibrated
performances of ±0.2°C and ±0.1°C, respectively,
across the human body temperature range, making
these devices ideal for skin temperature
measurements in wearable applications. The TMP117
sensor is designed to exceed ASTM E1112
requirements for electronic patient thermometers,
making it an excellent choice for wearable healthdiagnostic applications. In addition, their compact 2.00mm × 2.00-mm WSON packages have a uniquethermal pad that aids in heat transfer from the skin to
the temperature sensor.
To ensure optimal performance and device longevity,
additional layout recommendations are discussed in
the collateral in Table 1.
Table 1. Recommended Collateral

2

COLLATERALDESCRIPTION

Application Report

Wearable Temperature Sensing Layout
Considerations Optimized for ThermalResponse

Application Report

Precise Temperature Measurements withTMP116

Application Report

Temperature Sensors: PCB Guidelines forSurface Mount Devices

Layout Considerations for Wearable Temperature Sensing
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