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PRINCIPLE OF OPERATION
The Freescale accelerometer is a surface-micromachined
integrated-circuit accelerometer.
The device consists of a surface micromachined
capacitive sensing cell (g-cell) and a CMOS signal
conditioning ASIC contained in a single integrated circuit
package. The sensing element is sealed hermetically at the
wafer level using a bulk micromachined “cap'' wafer.
The g-cell is a mechanical structure formed from
semiconductor materials (polysilicon) using semiconductor
processes (masking and etching). It 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 ( Figure 3 ).
As the beams attached to the central mass 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 (). As
the central mass moves with acceleration, the distance
between the beams change and each capacitor's value will
change, (C = NA ε /D). Where A is the area of the facing side
of the beam, e is the dielectric constant, D is the distance
between the beams, and N is the number of beams.
The CMOS ASIC uses switched capacitor techniques to
measure the g-cell capacitors and extract the acceleration
data from the difference between the two capacitors. The
ASIC also signal conditions and filters (switched capacitor)
the signal, providing a high level output voltage that is
ratiometric and proportional to acceleration.
SPECIAL FEATURES
Filtering
Freescale accelerometers contain an onboard 2-pole
switched capacitor filter. Because the filter is realized using
switched capacitor techniques, there is no requirement for
external passive components (resistors and capacitors) to set
the cut-off frequency.
Self-Test
The sensor provides a self-test feature that allows the
verification of the mechanical and electrical integrity of the
accelerometer at any time before or after installation. A fourth
“plate'' is used in the g-cell as a self-test plate. When the user
applies a logic high input to the self-test pin, a calibrated
potential is applied across the self-test plate and the
moveable plate. The resulting electrostatic force
(Fe = 1 / 2 AV 2 /d 2 ) causes the center plate to deflect. The
resultant deflection is measured by the accelerometer's
control ASIC and a proportional output voltage results. This
procedure assures that both the mechanical (g-cell) and
electronic sections of the accelerometer are functioning.
Status
Freescale accelerometers include fault detection circuitry
and a fault latch. The Status pin is an output from the fault
latch, OR'd with self-test, and is set high whenever the
following event occurs:
? Parity of the EPROM bits becomes odd in number.
The fault latch can be reset by a rising edge on the self-test
input pin, unless one (or more) of the fault conditions
continues to exist.
Acceleration
Figure 3. Transducer
Physical Model
Figure 4. Equivalent
Circuit Model
MMA2260D
Sensors
4
Freescale Semiconductor
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