ROHM’s Waterproof Barometric Pressure Sensor Is More Than a Weather Forecaster
The MEMS piezoresistive pressure sensor stands apart from traditional barometers with a waterproof package, built-in temperature compensation, and high tolerance to stress.
While the earliest barometers have been around for over 300 years, modern barometric pressure sensors can do far more than traditional weather forecasting. Shrinking chip sizes now allow these sensors to function in car engines and even smartphones.
ROHM Semiconductor recently introduced the BM1390GLV barometric pressure sensor. The new device, which is a MEMS piezoresistive sensor, features a waterproof rating of IPX8 and targets home appliances, industrial equipment, and compact IoT devices.
What's unique about this often commonplace sensor?
How Does a MEMS Piezoresistive Pressure Sensor Work?
To start, it may be useful to briefly review an underlying principle of the BM1390GLV: the piezoresistive effect. This effect refers to a change in the electrical resistance of a material that occurs in response to an external mechanical force. The piezoresistive effect is much more pronounced for semiconductors than for metals.
As shown below, a piezoresistive pressure sensor creates a Wheatstone bridge by attaching piezoresistive sensing elements to a flexible diaphragm.
Piezoresistive pressure sensor creating a Wheatstone bridge. Image used courtesy of ROHM
Under the influence of pressure, the diaphragm deflects, changing the resistance of the piezoresistive elements. The change in resistance produces a voltage signal proportional to the applied pressure at the output of the bridge.
Today’s sensors commonly have built-in electronics that further process the bridge output signal before delivering it to the next link in the signal conditioning chain.
MEMS: A Key Enabling Technology
Many sensors, including pressure sensors, rely on having a movable part in the system. MEMS (short for micro-electro-mechanical systems) is the key technology that enables movable parts on a silicon chip next to the signal conditioning electronics.
The following figure shows how this technology can be used to create a miniature mass-damper-spring system on a silicon chip to implement an accelerometer sensor.
Scanning electron micrographs (SEM) of a CMOS MEMS accelerometer. Image used courtesy of K. Zhang
In fact, the huge advances in micro-fabrication technologies have enabled many of today’s small, low-cost sensors. For example, ROHM’s new pressure sensor based on MEMS technology comes in a compact 2.0 mm × 2.0 mm × 1.0 mm package.
The following figure compares the BM1390GLV with conventional designs.
Image used courtesy of ROHM
The MEMS and control circuitry employed in the BM1390GLV might not be very different from other similar products; however, the new sensor uses a proprietary structure with a special waterproof gel that makes it useful for industrial equipment and home appliances that require waterproof performance.
The BM1390GLV is offered in a waterproof IPX8-rated package—the highest waterproof rating.
Built-in Temperature Compensation
The properties of the strain gauge change with temperature and hence, temperature compensation is required to increase the accuracy of a piezoresistive pressure sensor, especially if the device is to be used over an extensive temperature range.
According to ROHM, the BM1390GLV employs a proprietary temperature compensation algorithm to minimize temperature-induced errors.
The following figure compares the performance of the new sensor with other similar products.
Performance comparison of BM1390GLV against similar products. Image used courtesy of ROHM
The company claims that the new sensor (the red curve above) outperforms similar products with internal temperature compensation (blue curve) as well as those solutions that trim out the error in an external MCU (the green curve). Also, note that since an external MCU is not required for temperature calibration, the new device can offer a lower design complexity.
Tolerance to Stress
The BM1390GLV uses a ceramic package that minimizes the error from mounting stress. The figure below shows the stress simulation for both the ceramic package used for the new sensor and the resin package commonly used in conventional sensors.
Stress simulation for ceramic packaging vs. resin packaging. Image used courtesy of ROHM
Employing a more robust package eliminates the layout restrictions faced by designers when using resin products and consequently, provides greater design flexibility.
Applications of Barometric Pressure Sensors
A barometric pressure sensor measures the absolute air pressure. In addition to traditional weather forecasting applications, these sensors also find use in smartphones for navigation. Since air pressure decreases with altitude, the information from a high-resolution barometric pressure sensor can be used to determine height above sea level.
Typical application circuit and block diagram of BM1390GLV. Image used courtesy of ROHM
This information can be used to assist satellite navigation or \
in the event of a lost satellite signal. Dead-reckoning calculates one's position by estimating the direction and distance traveled. In some cases, barometric pressure sensors can distinguish altitude changes as small as the height of one step.
These sensors are suited for tracking fitness activities and recognizing gestures. Barometric pressure sensors are also used in car engines to enable optimum efficiency when driving at different altitudes.
Use cases for the new BM1390GLV barometric pressure sensor. Image used courtesy of ROHM
ROHM’s new sensor is mainly designed for home appliances, such as rice cookers and vacuum cleaners; industrial equipment that need waterproof performance; small IoT devices operating outdoors; and drones.