Electronic Design

Accelerometer Offers Economical Low-G Sensing

Designed for consumer portable handhelds, this tri-axis device gets low in all the right places: footprint, cost, and power dissipation.

Portable handheld consumer electronics have proven successful market-wise for microelectromechanical-system (MEMS) sensors. This is particularly true for sensing low levels of gravity and protecting data in devices that fall and crash on the ground.

Low-g sensing is a tough technical challenge for design engineers, requiring extremely sensitive sensors with low noise levels and very small footprints. Because these sensors target consumer items, they also must be cost-conscious while dissipating very little power.

The MMA7260Q three-axis acceleration sensor combines low-level sensing (selectable), low power dissipation, low noise, low cost, and a very low profile (Fig. 1). Freescale Semiconductor's single-package, two-chip MMA7260Q features the sensing unit on top of the control chip. Cost is $4.25 each in 1000-unit lots.

In comparison, Oki Electric's ML8950 5- by 5- by 1.4-mm three-axis accelerometer, set to sense ±3-g acceleration, costs $50 each in sample quantities (Japanese market only). This includes an evaluation board. Kionix's KXP74 three-axis accelerometer, which measures 5 by 5 by 1.2 mm and senses ±2 g, goes for $7 each in 10,000-unit lots. (See the table for a comparative listing of commercially available three-axis MEMS IC sensors.)

Also, STMicroelectronics' three-axis LIS3L02D, measuring 7 by 7 by 1.8 mm while sensing ±2 g, costs about $5 each in 100,000-unit lots. This June, Analog Devices will release a three-axis single-chip MEMS sensor that's 4 by 4 by 1.45 mm. Hitachi Metals America Ltd. is developing a single-chip, three-axis, 3-g accelerometer in a 4.8 by 4.8 by 1.25 mm. European Technology for Business Ltd. is working on a three-axis single-chip accelerometer as well.

Housed in a 16-lead, 6- by 6- by 1.45-mm QFN package, the MMA7260Q features four selectable g ranges of 1.5, 2, 4, and 6. Also, it consumes 500 mA and operates from a 2.2- to 3.6-V supply. In power-saving mode, it dissipates a mere 5 mA. It can turn on in just 1 ms, and noise is rated at 4.2 mV rms (from 0.1 Hz to 1 kHz).

The four selectable g ranges suit it for various movement, vibration, shock, tilt, and positioning applications (Fig. 2). Set at 1.5 g, it can be used for freefalls and accurate tilt compensation. At 2 g, it fits handheld motion detection and gaming controllers. At 4 g, it can be used for low-vibration monitoring, shipping, and handling. And at 6 g, it suits high-vibration monitoring and high shock readings.

The development of low-cost and miniature three-axis accelerometers raises the possibility of putting gyroscopes in handheld consumer electronics, such as cell phones. Even though there may not seem to be a strong demand in today's consumer market, falling chip prices may soon change this scenario. Gyroscopes are primarily used to provide rotational detection in military, aviation, industrial, and biomedical applications.

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