Preliminary Datasheet
SMB365
Three-axis accelerometer
Bosch Sensortec
5
Operation
5.1
General Description
The SMB365 is a triaxial low-g acceleration sensor. It allows measurements of static as well as dynamic
accelerations in all three dimensions. Due to its three perpendicular axes it gives the absolute orientation
in a gravity field. The sensor is set up as a two-chip arrangement in a standard mold package. An
application-specific IC evaluates the output of a three-channel micromechanical acceleration-sensing
element that works according to the differential capacitance principle.
The ASIC is produced in a standard CMOS process. The production of the sensing element is based on
standard semiconductor process steps followed by special steps providing the functional structure. These
steps start with the deposition of a thick epitaxial layer on a sacrificial oxide. The large thickness allows
the design of working capacitances of up to 1pF. This in turn enables the hybrid two-chip assembly. The
poly layer is patterned by deep reactive ion etching in an inductive coupled plasma (DRIE-ICP, the so-
called Bosch process). A large aspect ratio and a very high anisotropy is achieved by periodic passivation
of the side walls in between the etch intervals. Afterwards the sacrificial layer is removed. Eventually the
sensing element is hermetically sealed by a bulk mircromachined cap to prevent damages of the
structure by dicing, packaging and operation of the device. The encapsulation is performed in a
cleanroom environment to reduce the risk of particle contamination to a minimum. Finally, the sensor is
packaged in a standard surface mountable micro leadframe QFN housing. Here the ASIC is stacked onto
the sensing element.
In the ASIC, three different so called self-balancing bridges convert the change of capacitances into
voltages. These signals are preconditioned by first order 50Hz filters and then multiplexed, amplified and
converted into a digital output signal.
Self-Balancing
Bridge X-axis
Self-Balancing
Bridge Y-Axis
Self-Balancing
Bridge Z-Axis
SPI
EEPROM
Amplifier
SCK
CSB
MISO
MOSI
VDDD
VDDA
VSS
MUX
Low-Pass
Filter (30Hz)
Low-Pass
Filter (30Hz)
Low-Pass
Filter (30Hz)
C
1x
C
2x
C
1y
C
2y
Sensing Element
ADC
ADC
PTAT
INT
C
1z
C
2z
ASIC
(50Hz)
(50Hz)
(50Hz)
5.2
Operation Modes and Sensitivity Resolution
The sensor can be operated in three different modes:
•
normal mode
•
low-power mode
•
standby mode.
The different modes are chosen via the SPI interface (see EEPROM mapping). In normal mode the
sensor reaches its full performance. In the low-power (interrupt) mode the sensor is fully functional but
Rev. 1.2
Page 10
© Bosch Sensortec GmbH reserves all rights even in the event of industrial property rights. We reserve all rights of disposal such as
copying and passing on to third parties. BOSCH and the symbol are registered trademarks of Robert Bosch GmbH, Germany.
7211RB11
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