Correct digital temperature measurements are vital in a spread of purposes together with wearables, medical monitoring gadgets, well being and health trackers, chilly chain and environmental monitoring, and industrial computing programs. Whereas broadly utilized, the implementation of extremely correct digital temperature measurements often includes temperature sensor calibration or linearization, in addition to greater energy consumption which might be a difficulty for compact, ultra-low energy purposes with a number of acquisition modes. The design challenges can shortly mount, inflicting value overruns and delayed schedules.
Complicating the matter, some purposes contain a number of temperature sensors sharing a single communication bus. As well as, some manufacturing check setups should be calibrated in keeping with the U.S. Nationwide Institute of Requirements and Expertise (NIST), whereas verification gear must be calibrated by an ISO/IEC-17025 accredited laboratory. Immediately, what appeared an easy perform turns into each intimidating and dear.
This text briefly describes the necessities for high-accuracy temperature measurements in cell and battery-powered well being monitoring purposes. 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 suggestions, an analysis board, and a Bluetooth-enabled demo equipment with a companion app that makes it attainable to switch sensor settings and observe the impression on energy consumption.
Necessities for high-accuracy temperature monitoring
Accuracy is obligatory in well being monitoring purposes. As manufactured, digital temperature sensors exhibit part-to-part variations in efficiency that should be addressed. As in-house calibration is pricey and utilizing uncalibrated sensors will increase the price of reaching the specified accuracy, designers ought to contemplate sensors which are totally calibrated and linearized. It’s, nonetheless, vital to make sure that the sensor maker makes use of calibration devices traceable to NIST requirements. Utilizing devices with traceable calibration ensures an unbroken chain again to the essential NIST requirements, with the uncertainties at every hyperlink within the chain recognized and documented to allow them to be addressed within the machine maker’s high quality assurance system.
The first customary for testing and calibration laboratories is ISO/IEC 17025 “Basic necessities for the competence of testing and calibration laboratories.” ISO/IEC 17025 relies on technical rules targeted particularly on calibration and testing laboratories, is used for his or her accreditation, and supplies the idea for creating steady enchancment plans.
Digital temperature sensor with NIST-traceable manufacturing testing
To satisfy the numerous design and certification necessities, designers can flip to the AS6211 digital temperature sensor from ams OSRAM that gives accuracy as much as ±0.09°C and requires no calibration or linearization. Designed to be used in healthcare gadgets, wearables and different purposes that require high-performance thermal info, the AS6211’s manufacturing testing is calibrated by an ISO/IEC-17025 accredited laboratory in keeping with NIST requirements. The calibrated manufacturing testing speeds the method of gaining certification to EN 12470-3, which is required for medical thermometers within the European Union.
The AS6211 is an entire digital temperature sensor in a six-pin, 1.5 x 1.0-millimeter (mm) wafer stage chip scale bundle (WLCSP), prepared for system integration. An orderable half quantity instance, the AS6221-AWLT-S, is delivered in plenty of 500 items on tape & reel. The AS6211’s measurements are delivered by way of a regular I²C interface, and it helps eight I²C addresses, thereby eliminating considerations about bus conflicts in multi-sensor designs.
Excessive accuracy plus low energy
The AS6221 delivers excessive accuracy with low energy consumption over its full provide vary from 1.71 to three.6 volts DC, which is very vital in purposes powered by a single battery cell. It features a delicate and correct silicon (Si) bandgap temperature sensor, an analog-to-digital converter, and a digital sign processor with related registers and management logic. The built-in alert perform can set off an interrupt at a particular temperature threshold, which is programmed by setting a register worth.
The AS6221 consumes 6 microamperes (µA) when making 4 measurements per second, and in standby mode, energy consumption is just 0.1 µA. The usage of the built-in alarm perform to get up the appliance processor solely when a temperature threshold has been reached can cut back system energy consumption much more.
Wearables integration choices
In wearable purposes, the higher the thermal connection between the sensor and the pores and skin, the extra correct the temperature measurement. Designers have a number of choices for optimizing the thermal connection. A method is to place a thermally conductive pin between the pores and skin and the sensor (Determine 1). To attain dependable outcomes, the pin must be remoted from any exterior sources of thermal power, such because the machine case, and a thermal paste or adhesive must be used between the pin and the AS6211. This method advantages from utilizing a versatile (flex) printed circuit board (PCB) to hold the AS6221, enabling extra freedom in finding the sensor.
Determine 1: A flex PCB and thermal adhesive can be utilized to supply a low thermal impedance path between the pores and skin and the sensor.
In designs that profit from having the sensor on the principle PCB, the thermal connection might be made utilizing a contact spring or a thermal pad. If the sensor is mounted on the underside of the PCB, a contact spring can be utilized to make a thermal connection between the contact pin and thermal vias on the PCB which are related to the sensor (Determine 2). This method may end up in a cheap machine that helps longer distances between the sensor and the pores and skin, but it surely requires cautious consideration of the a number of thermal interfaces to realize excessive ranges of sensitivity.
Determine 2: When the sensor is mounted on the underside of a PCB, thermal vias and a contact spring can be utilized to hook up with the contact pin.
A 3rd possibility is to make use of a thermal pad to attach the pin to a sensor mounted on the highest of the PCB (Determine 3). In contrast with utilizing a spring contact or flex PCB, this method requires a pad with excessive thermal conductivity and cautious mechanical design to make sure minimal thermal impedance between the contact pin and the sensor. This may end up in a less complicated meeting whereas nonetheless delivering excessive ranges of efficiency.
Determine 3: A thermal pad can join a top-mounted sensor to the contact pin. This supplies less complicated meeting, whereas nonetheless delivering excessive efficiency.
Bettering thermal response time
As a way to receive quick thermal response occasions, it’s vital to reduce the exterior influences on the measurement, particularly by the portion of PCB instantly adjoining to the sensor. Two viable design recommendations are to make use of cutouts to reduce any copper planes within the neighborhood of the sensor on the highest of the PCB (Determine 4, high), and to cut back thermal loading from the underside of the PCB by utilizing a cutout space beneath the sensor to cut back general PCB mass (Determine 4, backside).
Determine 4: Cutouts on the highest and backside of the PCB can reduce the PCB mass across the sensor and enhance its response time.
Along with minimizing PCB results, different methods that may assist enhance measurement pace and efficiency embody:
- Maximizing the contact space with the pores and skin to extend the warmth obtainable to the sensor.
- Utilizing skinny copper traces and minimizing the dimensions of energy and floor planes.
- Utilizing batteries and different elements reminiscent of shows which are as small as attainable to realize the machine efficiency necessities.
- Designing the bundle to thermally isolate the sensor on the PCB from the encircling elements and the skin atmosphere.
Sensing environmental temperature
Further concerns apply when utilizing a number of temperature sensors, reminiscent of in designs that use each pores and skin temperature and the temperature of the encircling atmosphere. A separate sensor must be used for every measurement. The thermal design of the machine ought to maximize the thermal impedance between the 2 sensors (Determine 5). A better intervening thermal impedance supplies higher isolation between the sensors and ensures that the measurements won’t intervene with each other. The machine bundle must be fabricated with supplies which have low thermal conductivities, and a thermal isolation barrier must be inserted between the 2 sensor sections.
Determine 5: For correct environmental temperature sensing, there must be a excessive thermal resistance between the pores and skin and environmental temperature sensors.
Eval equipment kickstarts AS6221 improvement
To hurry utility improvement and time to market, ams OSRAM affords designers each an eval equipment and a demo equipment. The AS62xx Eval Equipment can be utilized to shortly arrange the AS6221 digital temperature sensor, enabling a fast analysis of its capabilities. This eval equipment connects on to an exterior microcontroller (MCU) that can be utilized to entry temperature measurements.
Determine 6: The AS62xx eval equipment can be utilized to arrange and consider the AS6221
Demo equipment for the AS6221
As soon as the essential analysis is accomplished, designers can flip to the AS6221 demo equipment as an utility improvement platform. The demo equipment consists of an AS6221 temperature button and a CR2023 coin cell battery. Downloading the companion app from the App Retailer or Google Play Retailer helps connection to as much as three sensor buttons at one time (Determine 7). The app communicates with the sensor buttons over Bluetooth, making it attainable to switch the entire sensor settings, together with the measurement frequency, and observe the impression on energy consumption. The app can file measurement sequences, thereby enabling comparisons of the efficiency of assorted temperature sensor settings. Designers can even use the demo equipment to experiment with the alert mode and find out how it may be used to enhance resolution efficiency.
Determine 7: The AS6221 demo equipment serves as a temperature sensor utility improvement platform for the AS6221.
Designing high-accuracy digital temperature sensing programs for healthcare, health, and different wearables is a fancy course of with respect to design, check, and certification. To simplify the method, decrease value, and get to market extra shortly, designers can use extremely built-in, low-power, high-accuracy sensors.