LIS244ALH Datasheet, PDF (11/16 Pages) STMicroelectronics – MEMS inertial sensor high performance 2-axis ±2/±6g ultracompact linear accelerometer

LIS244ALH
3
Functionality
Functionality
The LIS244ALH is an ultra compact low-power, analog output two-axis linear accelerometer
packaged in a LGA package. The complete device includes a sensing element and an IC
interface able to take the information from the sensing element and provide an analog signal
to the external world.
3.1
Sensing element
A proprietary process is used to create a surface micro-machined accelerometer. The
technology allows to carry out suspended silicon structures which are attached to the
substrate in a few points called anchors and are free to move in the direction of the sensed
acceleration. To be compatible with the traditional packaging techniques a cap is placed on
top of the sensing element to avoid blocking the moving parts during the moulding phase of
the plastic encapsulation.
When an acceleration is applied to the sensor the proof mass displaces from its nominal
position, causing an imbalance in the capacitive half-bridge. This imbalance is measured
using charge integration in response to a voltage pulse applied to the sense capacitor.
At steady state the nominal value of the capacitors are few pF and when an acceleration is
applied the maximum variation of the capacitive load is in the fF range.
3.2
IC interface
The complete signal processing uses a fully differential structure, while the final stage
converts the differential signal into a single-ended one to be compatible with the external
world.
The first stage is a low-noise capacitive amplifier that implements a Correlated Double
Sampling (CDS) at its output to cancel the offset and the 1/f noise. The produced signal is
then sent to two different S&Hs, one for each channel, and made available to the outside.
All the analog parameters (output offset voltage and sensitivity) are ratiometric to the
voltage supply. Increasing or decreasing the voltage supply, the sensitivity and the offset will
increase or decrease linearly. This feature provides the cancellation of the error related to
the voltage supply along an analog to digital conversion chain.
3.3
Factory calibration
The IC interface is factory calibrated for sensitivity (So) and Zero-g level (Voff).
The trimming values are stored inside the device by a non volatile structure. Any time the
device is turned on, the trimming parameters are downloaded into the registers to be
employed during the normal operation. This allows the user to employ the device without
further calibration.
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