4
Motorola Sensor Device Data
PRINCIPLE OF OPERATION
The Motorola accelerometer is a surface–micromachined
integrated–circuit accelerometer.
The device consists of a surface micromachined capaci-
tive 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 us-
ing a bulk micromachined “cap’’ wafer.
The g–cell is a mechanical structure formed from semicon-
ductor materials (polysilicon) using semiconductor pro-
cesses (masking and etching). It can be modeled as two
stationary plates with a moveable plate in–between. The
center plate can be deflected from its rest position by sub-
jecting the system to an acceleration (Figure 2).
When the center plate deflects, the distance from it to one
fixed plate will increase by the same amount that the dis-
tance to the other plate decreases. The change in distance is
a measure of acceleration.
The g–cell plates form two back–to–back capacitors (Fig-
ure 3). As the center plate moves with acceleration, the dis-
tance between the plates changes and each capacitor’s
value will change, (C = Aε/D). Where A is the area of the
plate, ε is the dielectric constant, and D is the distance be-
tween the plates.
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 ratio-
metric and proportional to acceleration.
Acceleration
Figure 2. Transducer
Physical Model
Figure 3. Equivalent
Circuit Model
SPECIAL FEATURES
Filtering
The Motorola accelerometers contain an onboard 2–pole
switched capacitor filter. A Bessel implementation is used
because it provides a maximally flat delay response (linear
phase) thus preserving pulse shape integrity. Because the fil-
ter 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 ver-
ification of the mechanical and electrical integrity of the ac-
celerometer at any time before or after installation. This
feature is critical in applications such as automotive airbag
systems where system integrity must be ensured over the life
of the vehicle. 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 elec-
trostatic force (Fe =
1
/
2
AV
2
/d
2
) causes the center plate to
deflect. The resultant deflection is measured by the accel-
erometer’s control ASIC and a proportional output voltage re-
sults. This procedure assures that both the mechanical
(g–cell) and electronic sections of the accelerometer are
functioning.
Status
Motorola 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 fol-
lowing 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.
Frees
cale Semiconductor,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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