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Analog Front End
10.3.2
Accelerometer principle of operation
The Freescale accelerometer is a surface-micromachined, integrated-circuit accelerometer.
The device consists of three surface micromachined capacitive sensing cells (g-cells) and a CMOS signal
conditioning ASIC contained in a single, integrated-circuit package. The sensing elements are sealed
hermetically at the wafer level using a bulk micromachined “cap” wafer.
The g-cells are fabricated as a mechanical structure formed from semiconductor materials (polysilicon)
using semiconductor processes (masking and etching). They can be modeled as a set of beams attached to
a movable central mass that moves between fixed beams. The movable beams can be deflected from their
rest position by subjecting the system to an acceleration. This is shown for a single dimension in
Figure 10-2 (A).
When the beams attached to the center masses move, the distance from them to the fixed beams on one
side will increase by the same amount that the distance to the fixed beams on the other side decreases. The
change in distance is a measure of acceleration. The g-cell beams form two back-to-back capacitors
Figure 10-2 (B). As the center plate moves with acceleration, the distance between the beams change and
each capacitor ’s value changes.
The CMOS ASIC uses switched-capacitor techniques to measure the g-cell capacitors and extract the
acceleration data from the difference between each set of two capacitors. The ASIC also signals conditions
and filters the signal, providing a high-level output voltage that is ratiometric and proportional to
acceleration.
A
d
B
C 1
x
d
C 2
Figure 10-2. Transducer physical model (A) and equivalent circuit model (B)
Normally, the center mass in Figure 10-2 (A) is equidistant from both upper and lower plates such that C 1
and C 2 are equal. However, if the mass is displaced, the two capacitance values move in opposite
directions. C 1 and C 2 form a capacitive divider. Figure 10-3 shows the outer legs of the divider (N 1 and
N 2 ) driven with square waves that are 180 degrees out of phase from one another. 1
1. Actual waveforms may vary in the actual application, but the theory presented here is still applicable from a high-level
perspective.
MMA955xL Intelligent, Motion-Sensing Platform Hardware Reference Manual, Rev. 1.0
184
Freescale Semiconductor, Inc.
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