How to use high accuracy digital temperature sensors in health monitoring wearables
programmed by setting a register value. The AS6221 consumes 6 microamperes (µA) when making four measurements per second, and in standby mode, power consumption is only 0.1 µA. The use of the integrated alarm
ready for system integration. An orderable part number example, the AS6221-AWLT-S , is delivered in lots of 500 pieces on tape & reel. The AS6211’s measurements are delivered through a standard I²C interface, and it supports eight I²C addresses, thereby eliminating concerns about bus conflicts in multi-sensor designs.
continuous improvement plans.
Figure 1: A flex PCB and thermal adhesive can be used to provide a low thermal impedance path between the skin and the sensor. Image source: ams OSRAM
Digital temperature sensor with NIST-traceable production testing To meet the many design and certification requirements, designers can turn to the AS6211 digital temperature sensor from ams OSRAM that provides accuracy up to ±0.09°C and requires no calibration or linearization. Designed for use in healthcare devices, wearables and other applications that require high- performance thermal information, the AS6211’s production testing is calibrated by an ISO/IEC-17025 accredited laboratory according to NIST standards. The calibrated production testing speeds the process of gaining certification to EN 12470-3, which is required for medical thermometers in the European Union. The AS6211 is a complete digital temperature sensor in a six-pin, 1.5 x 1.0-millimeter (mm) wafer level chip scale package (WLCSP),
High accuracy plus low power
function to wake up the application
instruments traceable to NIST standards. Using instruments with traceable calibration ensures an unbroken chain back to the basic NIST standards, with the uncertainties at each link in the chain identified and documented so they can be addressed in the device maker’s quality assurance system. The primary standard for testing and calibration laboratories is ISO/ IEC 17025 “General requirements for the competence of testing and calibration laboratories.” ISO/ IEC 17025 is based on technical principles focused specifically on calibration and testing laboratories, is used for their accreditation, and provides the basis for developing
health monitoring applications. It then introduces a low-power, high-accuracy digital temperature sensor IC from ams OSRAM that doesn’t require calibration or linearization. It finishes with integration recommendations, an evaluation board, and a Bluetooth- enabled demo kit with a companion app that makes it possible to modify sensor settings and observe the impact on power consumption. Requirements for high- accuracy temperature monitoring Accuracy is mandatory in health monitoring applications. As manufactured, digital temperature sensors exhibit part-to-part variations in performance that need to be addressed. As in- house calibration is expensive and using uncalibrated sensors increases the cost of achieving the desired accuracy, designers should consider sensors that are fully calibrated and linearized. It is, however, important to ensure that the sensor maker uses calibration
processor only when a temperature threshold has been reached can reduce system power consumption even more.
The AS6221 delivers high accuracy with low power consumption over its full supply range from 1.71 to 3.6 volts DC, which is especially important in applications powered by a single battery cell. It includes a sensitive and accurate silicon (Si) bandgap temperature sensor, an analog-to-digital converter, and a digital signal processor with associated registers and control logic. The integrated alert function can trigger an interrupt at a specific temperature threshold, which is
Figure 4: Cutouts on the top and bottom of the PCB can minimize the PCB mass around the sensor and improve its response time. Image source: ams OSRAM
measurement. Designers have several options for optimizing the thermal connection. One way is to put a thermally conductive pin between the skin and the sensor (Figure 1). To achieve reliable results, the pin needs to be isolated from any external sources of thermal energy, such as the device case, and a thermal paste or adhesive should be used between the pin and the AS6211. This approach benefits from using a flexible (flex) printed circuit board (PCB) to carry the AS6221, enabling more freedom in locating the sensor. In designs that benefit from having the sensor on the main PCB, the thermal connection can be made using a contact spring or a thermal pad. If the sensor is mounted on the bottom of the PCB, a contact
Wearables integration options In wearable applications, the better the thermal connection between the sensor and the skin, the more accurate the temperature
Figure 3: A thermal pad can connect a top-mounted sensor to the contact pin. This provides simpler assembly, while still delivering high performance. Image source: ams OSRAM
Figure 2: When the sensor is mounted on the bottom of a PCB, thermal vias and a contact spring can be used to connect to the contact pin. Image source: ams OSRAM
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