AIS1120SX Datasheet, PDF (9/58 Pages) STMicroelectronics – Embedded self-test

AIS1120SX / AIS2120SX
Block diagrams and pin description
1.1.1
Mechanical element
A proprietary process is used to create a surface micromachined accelerometer. This
technology allows processing suspended silicon structures which are attached to the
substrate in few points called anchors and are free to move in the direction of the sensed
acceleration thanks to flexible springs. In order to be compatible with standard packaging
techniques, a cap is placed at wafer level on the top of the sensing element.
From an electrical point of view, the sensor can be represented as a differential capacitive
system (see below for a dual-axis element). When the acceleration is applied to the sensor,
the proof mass displaces from its nominal position, causing an unbalance in the capacitive
half-bridge. This unbalance is measured using charge integration in response to a voltage
pulse applied to the sense capacitor:
Figure 3. Differential capacitive system (dual axis)
The differential capacitive change towards acceleration can be expressed, in small
displacements approximation, as:
1.1.2
Where C₀ is the at-rest capacitance, m is the inertial mass, a the acceleration, k stiffness of
the springs and g the distance between capacitor electrodes.
Sigma-Delta converter
A 2^nd order sigma-delta modulator is used to convert the differential voltage that comes from
the charge-to-voltage converter to a pulse-density modulated (PDM) data stream. The data
stream will be further processed through on-chip digital filters.
Figure 4. 2^nd order sigma-delta modulator
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DocID028312 Rev 3
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