Piezoresistive Sensing: A New Miniature MEMS Water-Resistant Digital Barometer from STMicro

November 27, 2017 by Nick Davis

STMicroelectronics showcases their new ultra-compact piezoresistive pressure sensor which functions as a digital barometer.

STMicroelectronics showcases their new ultra-compact piezoresistive pressure sensor which functions as a digital barometer.

STMicroelectronics has a new miniature high-resolution MEMS pressure sensor (the LPS33HW) that targets applications such as wearables, GPS, weather station equipment, and altimeters and barometers for portable devices. This little device—it’s truly a little device, measuring in at only 3.3 × 3.3 × 2.9 mm (see image below)—allows for either I2C or SPI digital communications.

And depending on which STMicroelectronics information you’re reading, this sensor is either “water-resistant”—as described in the datasheet—or, according to their press release, “waterproof.” Hmm….I think I’ll go with water-resistant since very few things in this world are actually waterproof. Nonetheless, this robust pressure sensor looks to be impressive, and Samsung seems to agree since they have chosen to use it in their high-performance wearables.


Figure 1. The LPS33HW. Image courtesy of STMicroelectronics.

Piezoresistive Sensing Element

This metal-lid pressure sensor uses a sensing element based on a piezoresistive Wheatstone bridge approach (see the block diagram below). As you can see in the image above, the package/metal lid has a hole in it which allows external pressure to reach the sensing element.

Also, ST’s secret-sauce process uses a silicon membrane that protects the electronics from water, and, according to the press release, the device is “resistant to chemicals like chlorine, bromine, and salt water, it is ideal for pool or sea swimming, and will also resist soaps or detergents used when showering or cleaning.” Nice! This sounds like a very versatile and robust pressure sensor.


Figure 2. Block diagram for the LPS33HW. Image taken from the datasheet (page 7).

No Need for Manual Calibration

One rather helpful feature when using this absolute pressure sensor is that it comes pre-calibrated from the factory. Specifically, the device uses non-volatile memory, or a “non-volatile structure,” as ST refers to it, for storing trimming values. When power is applied to the sensor, these calibration values are loaded into the “normal operation” registers. Sounds super easy!

Some Mechanical and Electrical Characteristics

The PAccRel (relative accuracy over pressure) has a typical value of ±0.1 hPa. An interesting thing about this mechanical specification is its footnote, which states, “By Design”. What exactly does this mean? Isn’t it obvious that it should be “by design?” Why even list this footnote? Weird.

Also, the typical value—and this is the case with all the typical values—has footnote 1, which says, “Typical specifications are not guaranteed.” Okay, this makes sense…sort of. Of course, typical specifications aren’t guaranteed. After all, they are typical…that’s the key word, right? And besides, it’s always a good idea to review the minimum and maximum specifications that are (or should be) listed in conjunction with the typical values. Oh wait…not all specifications listed in the datasheet have minimum and maximum values; most of the mechanical specs don’t (see image below). 


Figure 3. Mechanical specifications: most without minimum and maximum values, and “no guarantee” with the typical specs (footnote 1). Image taken from the datasheet (page 9).


And some of the electrical specs—for example, the supply currents (see image below)—are missing their minimum and maximum values too. Perhaps, with time, ST will update the datasheet to include these values. If not, well, I wouldn’t bet your job on these specs.


Figure 4. Electrical specifications: supply currents are missing minimum and maximum values, and “no guarantee” with the typical specs (footnote 1). Image taken from the datasheet (page 10).

I2C and SPI Bus and Protocol Communications

This device provides a digital output, so you’ll need to know how (or learn how) to use I2C or SPI for digital communication. I have used both the I2C bus and the SPI bus, and I find them both to be very powerful and useful. And fortunately, ST has provided ample I2C and SPI digital communication information regarding this pressure sensor in section 6 of the datasheet (the "Digital Interfaces" section).

Application Hints

Whenever a manufacturer provides application information, I place a gold star next to the company’s name. ST gets a gold star for providing power supply decoupling capacitor hints as well as some soldering information. See section 5 of the datasheet ("Application Hints").

The LPS33HW also has the (now rather common) advantage of requiring very few external components. In fact, according to the “electrical connections” diagram, the bare minimum is one part (the decoupling cap):


Figure 5. (Very) few external components required. Diagram taken from the datasheet (page 23).


Have you had a chance to use ST’s new MEMS water-resistant digital pressure sensor? If so, leave a comment and tell us about your experiences.