Asahi Kasei Announces Small, Thin, 3-Axis Electronic Compass
Asahi Kasei Microsystems (AKM), a wholly owned subsidiary of Asahi Kasei EMD, has completed development of the AK8973S, which the company claims is the world’s smallest and thinnest 3-axis electronic compass with chip-size packaging of 2.5 x 2.5 x 0.5 mm. The small size is made possible by forming Si monolithic Hall elements together with amplifier and logic circuitry on a single chip. Applications foreseen for the digital-interface AK8973S include pedestrian navigation systems in cell phones and personal navigation devices, and motion input in video game controllers.
The development of Si monolithic Hall elements to serve as magnetic sensors, and their integration with amplifier and logic circuitry on a single chip, is claimed by the company to be a major advance both conceptually and in terms of applied technology. Any magnetic sensor, Hall element or otherwise, can only measure a magnetic field in a single axis. This is why conventional 3-axis sensors were of necessity multi-chip modules incorporating three separate magnetic sensors, emplaced in three different planar orientations corresponding to the spatial axes of X, Y, and Z. Because the Hall element, unlike other magnetic sensors, measures magnetic field in the axis perpendicular to the plane of the chip, many had considered it to be an unfavorable choice for 3-axis sensors, as two of the Hall elements needed to stand vertically with respect to the plane of the chip package. With the development of the Si monolithic Hall element, the perpendicular measurement direction of Hall elements becomes an advantage that enables sensors for all three spatial axes to be formed on a single wafer plane.
Chip-size packaging (CSP), unlike QFN packaging, enables the 3-axis electronic compass to be used in circuit board designs with extremely high mounting density. For many portable electronics, cellular handsets in particular, recent years have seen a rapidly expanding adoption of CSP technology. The AK8973S uses wafer-level CSP, or WLCSP, in which LSIs are encapsulated in resin before the wafer is cut into individual chips.
Software compatibility with previous AKM electronic compasses enables customers to utilize existing software assets, including the patented DOE algorithm for automatic adjustment of magnetic offset and software for calculations to compensate for tilt. DOE has been provided to customers since before the beginning of volume shipment of AKM’s first electronic compass product. With the AK8973S, as before, DOE enables greatly improved productivity by eliminating the need for adjustment to be performed individually for each handset before shipment, and makes it unnecessary for the consumer to perform adjustments when the ambient magnetic field changes.
The AK8973S also features: digital interface with internal A/D converter compatible with 2-line I2C bus; internal oscillator circuitry – no clock input required; low power consumption – 6.4 mA during sensor operation, 0.8 mA average with measurement at 100 ms intervals; internal temperature sensor with 8-bit digital output; and an internal EEPROM for recording geomagnetic sensor sensitivity and adjustment data.
