Analog Devices has come up with a new low-power design for a three-axis accelerometer with in-built memory and decision-making. Memory and decision-making has been added to cut system power consumption.
“I think the FIFO memory is of one of the most important parts of the design, and we have added some motion detection features: tap and double tap detection, activity and inactivity monitoring, and free-fall detection,” Analog Devices field application engineer Christoph Wagner told
EW.
This means the host processor can set detection parameters within the accelerometer, then go to sleep to save power.
When local acceleration meets the set criteria, or the FIFO is full, the accelerometer wakes its host. At this point 32 three-axis acceleration samples taken near the trigger value are available to the host.
“The FIFO can be set to detect data from all three axes and the customer can supply the threshold levels,” said Wagner. “You can use all five detection features at the same time.”
Sample rate can be set between 0.1Hz and 3.2kHz. Current consumption depends on this and is typically from 25 to 130µA at 2.5V. “At 3.6V it might go up to 160µA or 170µA, and at 1.8V it is a little less,” said Wagner.
Sensitivity has been designed for consumer products. “This part is targeted for consumer applications, not industrial or automotive,” said Wagner.
It is user-selectable, with fixed 10-bit resolution or 4mg/LSB scale factor in all g-ranges, up to 13-bit resolution at ±16g.
This is the first Analog accelerometer on which data is available over a digital bus. Configuration is user-selectable between three or four wire SPI, or I2C.
The ADXL345 is a fourth-generation
MEMS product. While previous MEMS products had the accelerometer and signal conditioning circuits on the same silicon, This product has two dies in the package.
“For Analog, this is the first time the signal conditioning circuit has not been on the MEMS die. Right now, one or two die is part of the design decision. Which is the smallest and the most cost effective, and what is needed to get a robust signal out?” said Wagner.
“You can now get signal conditioning circuits in CMOS from a foundry like TSMC,” he said. The accelerometer is made in polysilicon by ADI at its own foundry.