Robust and precise barometric pressure sensor

1By Theresa Möhrle, Product Marketing Manager( Pressure Sensors), Infineon Technologies

Ideal for wearables in harsh environments

Smart watches and wearables are often used in harsh environments and its features like motion and activity tracking request highest precision, fast read-out and low power consumption. Related portable devices integrate barometric pressure sensors for accurate elevation gain,vertical speed and motion detection. The new DPS368 is ideal for these applications as it saves up to 80% space compared to other waterproof pressure sensors, offers a precision of +/-2 cm and up to 50% power savings compared to piezo-resistive technology. The digital barometric air pressure sensor is robust against water, humidity and dust, as the pads and membranes are protected by gel (figure 1). It is IPx8 certified and can withstand 50m under water for one hour.

Fig 1: The ASIC and MEMS chip of the DPS368 are protected by gel in a very robust package
Fig 1: The ASIC and MEMS chip of the DPS368 are protected by gel in a very robust package
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The pressure sensor element of the DPS368 uses a capacitive sensing principle which guarantees high precision during temperature changes. The DPS368 is based on the proven DPS310, but with a very robust and waterproofed package. This combination makes the DPS368 ideal for a variety of applications(figure 2)in harsh environments. Target applications are smart watches, wearables and smart phones (e.g. fitness tracking, step counting, fall detection, navigation, altitude detection); home appliances (e.g. air flow control in HVAC / vacuum cleaner, water level detection in washing machine, intruder detection); drones (e.g. flight stability, height control), eCigarettes (heater control) and health care (e.g. fall detection, air flow monitoring).

Fig 2: The DPS368 is ideal for precise pressure sensing in various mobile devices and wearables often used in harsh environments
Fig 2: The DPS368 is ideal for precise pressure sensing in various mobile devices and wearables often used in harsh environments

Robust and sensitive

The DPS368 offers superior resolution, temperature stability, energy efficiency and high robustness. This combination of benefits makes it particularly attractive in battery powered applications where sensing of very small pressure changes is needed – even in harsh environments. The DPS368 integrates both barometric pressure and temperature sensing (figure 3) into a single, extremely compact 8-pin LGA package that measures just 2.0 mm x 2.5 mm x 1.1 mm. Average current consumption is 1.7 μA for pressure measurement (only 0.5 μA in standby mode)..

Fig 3: Block diagram of the DPS368 architecture
Fig 3: Block diagram of the DPS368 architecture

Capable of precision to +/-0.002hPa (equating to +/-2 cm), the device can measure air pressure between 300hPa and 1200hPa at a temperature of -40 to 85 °C and has a pressure temperature sensitivity of less than 0.5Pa/K.  The temperature accuracy is specified with ± 0.5°C. The sensor is able to detect single stairs, body motions or gestures. Every sensor is individually calibrated during production with the calibration coefficients stored in one-time programmable (OTP) memory. Raw data can be transferred using an I2C or SPI interface, with compensated pressure values being calculated in the host device.

The high efficiency device provides up to 50% less power consumption then competitor products when running in full speed due its capacitive technology (AC biasing), which leads to longer battery lifetime. Further system power savings are provided by allowing the host processor to remain in sleep mode for long periods between read. The high measurement rate (up to 200Hz) and fast read-out enables a quick sensor feedback.

Capacitive technology

Typical small form factor MEMS (Micro Electro Mechanical System) pressure sensors are built around piezo-resistive measurement techniques. In these cases, the flexing of a diaphragm in relation to changes in pressure is sensed via a strain sensor. However, piezo-resistive sensing elements are particularly susceptible to variation with temperature changes and they do not respond linearly to temperature. For this reason, piezo-resistive sensors have a need for more complex calibration compared to a capacitive element. In addition, resistive measurement comes with a relatively high current consumption – a particularly important consideration when the target application is battery powered and operating life time is critical.

Because of the limitations of piezo-resistive technology, Infineon developed a capacitive MEMS technology for its pressure sensors. The cell structure and capacitive bridge configuration developed is illustrated in figure 4.The capacitive barometric pressure sensor consists of four arrays of sensing and reference cells. The sensing cells have a flexible membrane which reacts to pressure change and provides the air pressure measurement. The reference cells have a stiff membrane which does not react to pressure changes and provide a stable measurement reference. The benefit of this type of structure is that the pressure measurement can be differential and both sensing and reference cells are exposed to the same temperature changes negating temperature drift effects. The cell size is optimized for high sensitivity and mechanical reliability. Based on the small MEMS cell there is no gravity effect. Other key features besides the very good temperature stability over wide temperature and pressure ranges are low noise and low power consumption.

Fig 4: The cell structure of the capacitive sensor enables differential measurements with very low temperature drift
Fig 4: The cell structure of the capacitive sensor enables differential measurements with very low temperature drift

Optimized system design

For flexible system designs the sensor is configurable for different use cases to optimize the resolution in balance with the energy consumption. Different operation modes (high precision, standard, low power and ultra-low power) coordinate with different precision (4 to 50cm) and measurement rates (single shot and up to 200 Hz). For example, one-time measurement can be configured for GPS altitude accuracy, while the option to take several measurements per second will address the needs of gesture recognition or fall detection. The configurable modes (figure 5) also lead to an optimized efficiency, as the power consumption is direct proportional tothe measurement frequency. Operating in low-power mode for example the current consumption is 2.7μA with one measurement/second and less than 1.0μA at standby mode. Operating with maximum resolution the sensor has a current consumption of about 38 μA.

Fig 5: Different operation modes enable optimized energy consumption and precision, depending on the related application
Fig 5: Different operation modes enable optimized energy consumption and precision, depending on the related application

Summary

Barometric pressure sensors are increasingly becoming an important part of mobile and smart devices. With regard to IoT, pressure sensors constitute a key component for integrated sensor solutions in the field of consumer electronics. The DPS368 will be available in the first half of 2019. It is based on proven semiconductor processes and combines ultra-high resolution, fast read-out speed, excellent temperature stability and low power consumption with a small and very robust package. It allows developers to enhance functionality and ease-of-use in applications such as indoor and outdoor assisted localization and navigation, sports and fitness apps, drone flight stability, real-time weather monitoring, IoT devices, and also in healthcare such as for automatic fall detection.

Further information on the Infineon pressure sensor portfolio can be found at: http://www.infineon.com/pressuresensor

Fast Prototyping

Infineon supports quick evaluation and prototyping with comprehensive IoT solutions, evaluation boards and software. The sensor board Shield2Go DPS310 can either be used together with the microcontroller board XMC2Go or with popular IoT hardware platforms such as Arduino and Raspberry Pi for fast and easy development using the DPS310, DPS368 or DPS422 sensors. A free, out-of-the-box Arduino library is also available through GitHub (https://github.com/Infineon). This allows fast and cost-effective evaluation of applications and speeds up the production of prototypes.

The sensor board Shield2Go DPS310
The sensor board Shield2Go DPS310

Furthermore, the Sensor Hub Nano is available for developments with the barometric sensors. The standalone board, measuring just 30 mm x 15 mm x 10 mm (including battery), incorporates a pressure sensor that can be evaluated for a variety of applications. Bluetooth connectivity is used to transmit the data to the host.

As an alternative to using the SES2G sensor evaluation software, the Infineon Pressure SensorAndroid app is available free of charge. Compatible with the related sensor hubs, this app connects via Bluetooth and provides access to key sensor functionality to speed the evaluation and testing of sensor performance in a target application.

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