ADI slashes power consumption of 3-axis accelerometer design

With motion sensing technology increasingly being considered for a wide array of portable consumer electronic devices, such as mobile gaming systems, cell phones and digital still cameras, Analog Devices has devised a three-axis MEMS accelerometer with integrated ADC and FIFO memory, that specifically caters to the already tight power budgets of battery-operated systems.

Explained Bill Murphy, product line director, micromachined products group, Analog Devices: “ADI's newest iMEMS product addresses two important pain points in these areas by lowering the current consumption of the ADXL345 to as little as 25 microamps and increasing the resolution across the entire measurement range.”

Analog Devices claims the ADXL345, properly implemented, could achieving up to an 80 percent power saving compared to competing three-axis inertial sensors. The on-chip ADC simplifies hardware configuration, but more importantly is said to cut overall system power. Similarly, an on-chip FIFO (first-in/first-out) memory block samples input data to determine if the system should be actively responding to a change in movement or acceleration. This offloads that function from the host processor, allowing it to remain in sleep mode as long as possible and leading to a dramatic reduction in overall power usage (it is the host processor that typically consumes the dominant portion of the system power).

The ADXL345 is one of ADI's 4th generation of accelerometers ” previous accelerometers combined mems and signal conditioning on one die. This consists of two die though, a pure mems die wired to a separate asic that provides all signal conditioning. According to Analog Devices' field application engineer Christoph Wagner, this is the most cost-efficient partitioning for this kind of product.

Users can select the measurement range, choosing to implement 10, 11, 12 or 13bit resolution to result in either a 2, 4, 8 or 16g part. This enables a single MEMS sensor to be designed into multiple products. With current consumption depending on
data rate, ultimate power consumption is also effectively user selectable. The chip operates from between 25 to 130mA, which scales to data rate. Explains Wagner: “Depending on whether the customer wants a very wide bandwidth, up to 1.6kHz, or very low at 10Hz, for example, they can set the data rate and accordingly optimise power consumption.” Supply voltage is 1.8 to 3.6V.

This 3 x 5 x 1mm part includes 3 and 4 wire SPI and I2C digital interfaces, and some additional motion detection functions, such as tap/double tap detection and free-fall detection. Sampling has started, and ADI plans to go to volume with the product imminently.

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