How to use high accuracy digital temperature sensors in health monitoring wearables
spring can be used to make a thermal connection between the contact pin and thermal vias on the PCB that are connected to the sensor (Figure 2). This approach can result in a cost-effective device that supports longer distances between the sensor and the skin, but it requires careful consideration of the several thermal interfaces to achieve high levels of sensitivity. A third option is to use a thermal pad to connect the pin to a sensor mounted on the top of the PCB (Figure 3). Compared with using a spring contact or flex PCB, this approach requires a pad with high thermal conductivity and careful mechanical design to ensure minimum thermal impedance between the contact pin and the sensor. This can result in a simpler assembly while still delivering high levels of performance.
of the device should maximize the thermal impedance between the two sensors (Figure 5). A higher intervening thermal impedance provides better isolation between the sensors and ensures that the measurements will not interfere with one another. The device package should be fabricated with materials that have low thermal conductivities, and a thermal isolation barrier should be inserted between the two sensor sections. Eval kit kickstarts AS6221 development To speed application development and time to market, ams OSRAM offers designers both an eval kit and a demo kit. The AS62xx Eval Kit can be used to quickly set up the AS6221 digital temperature sensor, enabling a quick evaluation of its capabilities. This eval kit connects directly to an external microcontroller (MCU) that can
be used to access temperature measurements.
Designers can also use the demo kit to experiment with the alert mode and learn how it can be used to improve solution performance. Conclusion Designing high-accuracy digital temperature sensing systems for healthcare, fitness, and other wearables is a complex process with respect to design, test, and certification. To simplify the process, lower cost, and get to market more quickly, designers can use highly integrated, low-power, high-accuracy sensors. As shown, the AS6221 is one such device. It does not require calibration or linearization, and the production testing equipment is calibrated according to NIST standards by an ISO/IEC-17025 accredited laboratory, speeding the design and approval process for medical devices.
Demo kit for the AS6221 Once the basic evaluation is completed, designers can turn to the AS6221 demo kit as an application development platform. The demo kit includes an AS6221 temperature button and a CR2023 coin cell battery. Downloading the companion app from the App Store or Google Play Store supports connection to up to three sensor buttons at one time (Figure 7). The app communicates with the sensor buttons over Bluetooth, making it possible to modify all of the sensor settings, including the measurement frequency, and observe the impact on power consumption. The app can record measurement sequences, thereby enabling comparisons of the performance of various temperature sensor settings.
Figure 5: For accurate environmental temperature sensing, there should be a high thermal resistance between the skin and environmental temperature sensors. Image source: ams OSRAM
In order to obtain fast thermal response times, it’s important to minimize the external influences on the measurement, especially by the portion of PCB directly adjacent to the sensor. Two viable design suggestions are to use cutouts to minimize any copper planes in the vicinity of the sensor on the top of the PCB (Figure 4, top), and to reduce thermal loading from the bottom of the PCB by using a cutout area below the sensor to reduce overall PCB mass (Figure 4, bottom).
■ Maximizing the contact area with the skin to increase the heat available to the sensor ■ Using thin copper traces and minimizing the size of power and ground planes ■ Using batteries and other components such as displays that are as small as possible to achieve the device performance requirements ■ Designing the package to thermally isolate the sensor on the PCB from the surrounding components and the outside environment Sensing environmental temperature Additional considerations apply when using multiple temperature sensors, such as in designs that use both skin temperature and the temperature of the surrounding environment. A separate sensor should be used for each measurement. The thermal design
Improving thermal response time
In addition to minimizing PCB effects, other techniques that can help improve measurement speed and performance include:
Figure 7: The AS6221 demo kit serves as a temperature sensor application development platform for the AS6221. Image source: ams OSRAM
Figure 6: The AS62xx eval kit can be used to set up and evaluate the AS6221.
Image source: ams OSRAM
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