Today’s smartphones and wearable monitors, such as smartwatches and fitness bands, allow people to capture more and more data about their lives, activities, and physical condition. New services enabled by this type of data are emerging, ranging from online group fitness training to tele-health and care for the elderly.
Demand for Better Temperature Monitoring
While monitoring vital signs like your heart rate is now more user-friendly, keeping track of other signs, such as body temperature, remain rather more difficult. Thermometers that remain in contact with the body are inconvenient and can be difficult to keep in place. On the other hand, non-contact far infrared (FIR) sensors can be influenced by heat from sources other than the object being monitored, causing temperature measurements to be inaccurate.
To overcome this, current state-of-the-art non-contact FIR sensors are typically provided in a TO-can package. The TO-can has significant thermal mass and high thermal conductivity, which combine to mitigate the effects of rapid thermal gradients and shocks. However, these packaged sensors are physically large and heavy, and this mitigation strategy has its limitations when faced with very dynamic thermal environments. They are not well suited to use in devices such as consumer wristwear and could preclude temperature monitoring as a feature in emerging products, such as hearables that are designed to be worn in the ear.
Non-Contacting Far Infrared Sensors
Continuous non-contact monitoring of body temperature that is both convenient and accurate has been a challenge. FIR sensors are convenient but subject to false readings because of heat from sources such as adjacent components like microprocessors or power transistors. Melexis NV (leper, Belgium) has met this challenge by taking a unique approach. Although small sensors are generally subject to unstable readings, Melexis developed a non-contact FIR sensor that is significantly smaller in size, yet at the same time is stable and accurate. Its temperature sensing element is a thin, thermally isolated membrane that has a low thermal mass and is therefore able to rapidly change its temperature when the amount of incoming heat radiation changes.
Signal processing based on careful modeling and characterization of different heat disturbance scenarios, along with sophisticated compensation algorithms, is applied to remove unwanted thermal effects from the output of the sensor. In this way, active compensation implemented by electronics and software can effectively replace the effects achieved passively using the metal TO-can.
The Melexis MLX90632 non-contact FIR sensor comes in a 3×3×1 mm surface mount QFN package, which makes it especially well-suited for wearables, hearables, and portable medical body-temperature monitoring. Despite its extremely small size, it is highly accurate — the medical grade version is optimized for the normal human body temperature range and calibrated in the factory to ensure an accuracy of +/- 0.2°C. It is also relatively stable in the presence of ambient heat-disturbance scenarios.
What’s Inside the Package?
The temperature sensing element is a suspended membrane that is only a few micrometers (μm) thick. The membrane responds very quickly to heat radiating to its surface. Although it is stationary, the membrane is built using an existing MEMS type process. It is specially designed to have a very low thermal mass and is thermally isolated from the rest of the chip.
One of the advantages of this approach, is that it can be mass produced using a well-established CMOS backend process. A gate oxide, intermetal oxide, and passivation layers are deposited on top of a silicon wafer. All of these are CMOS compatible. The bulk silicon is then removed, and the other materials stay behind.
As shown in Figure 2, there is a thermopile, which is a group of series-connected thermocouples, in contact with the membrane at different points. The thermocouples in this sensor use P- and N-doped silicon for the temperature-sensing junction, allowing it to be produced with large-scale standard CMOS processing. The P-N junctions are connected in series to improve the signal to noise ratio, since each junction only produces an analog output in the range of μV/°C. The thermopile measures the temperature difference (∆T) between the membrane and the surrounding chip material. Since the membrane has an extremely small thermal mass, it heats very rapidly. The chip also contains a thermistor, which serves as an absolute temperature reference. Knowing the absolute temperature and ∆T, it is easy to calculate the absolute membrane temperature.
The thermopile voltage signal is amplified, digitized, and digitally filtered before being stored in RAM. The reading from the on-board temperature reference is processed and stored in the same way. A state machine controls the timing and functionality of the sensor, and the results of each measurement and conversion are made available to the host system via an I2C connection. The host processor can readily calculate the target and sensor temperatures from the raw data.
The sensor is calibrated by placing it in front of a super-stable “black-body” reference plate to calculate a calibration coefficient for the absolute temperature reading. The coefficient is then written into the EEPROM for each sensor.
Window on the World
In the end, the device needs a package and a window to look through to receive the radiation from the outside world. Since silicon is transparent to heat radiation, it makes sense to use a silicon window for the heat input. For the MLX90632, the window takes the form of a lens. This silicon lens helps to focus more energy onto the membrane. If you focus more energy you get more signal, thereby improving the signal to noise ratio. It also helps restrict the field of view. If you look under a small angle, you can see a smaller spot at a larger distance — a critical function when measuring forehead temperature to determine whether someone is ill.
Figure 3 compares the response of the MLX90632 and a state-of-the-art TO-can sensor, both of them monitoring a reference source at a stable temperature of 40°C. A strong external heat source was placed close to the sensors. Figure 3a shows that the sensor temperature was around 2°C at the start of the experiment, and the external heat source delivered a thermal shock of approximately 60°C/min.
Figure 3b shows that, despite the thermal inertia due to the TO-can package, the conventional sensor’s output is heavily disturbed by the external heating. In contrast, the output of the actively compensated MLX90632 deviates by no more than 0.25°C, demonstrating considerably better stability.
A Uniquely Important Temperature Sensor for Our Times
With its small size and digitally optimized thermal response, the MLX90632 is suited to use in wearables and hearables. Moreover, it can also be used in portable medical devices for continuous body-temperature monitoring, which is often used in preventive medicine to detect early critical health conditions, as well as more classic devices such as forehead or in-ear thermometers. Finally, it seamlessly fits the point-of-care trend to move diagnosis from the lab closer to the patient.