4 Surface Mount Micromachined Accelerometer MOTOROLA
MMA3202D
PRINCIPLE OF OPERATION
The Motorola accelerometer is a surface-microma-
chined integrated-circuit accelerometer.
The device consists of a surface micromachined ca-
pacitive sensing cell (g-cell) and a CMOS signal condi-
tioning 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 semi-
conductor materials (polysilicon) using semiconductor
processes (masking and etching). It can be modeled as
a set of beams attached to a movable central mass that
move between fixed beams. The movable beams can be
deflected from their rest position by subjecting the sys-
tem to an acceleration (Figure 2).
As the beams attached to the central mass move, the
distance from them to the fixed beams on one side will in-
crease by the same amount that the distance to the fixed
beams on the other side decreases. The change in dis-
tance is a measure of acceleration.
The g-cell plates form two back-to-back capacitors
(Figure 2). As the central mass moves with acceleration,
the distance between the beams change and each ca-
pacitor's value will change, (C = NAε/D). Where A is the
area of the facing side of the beam, ε is the dielectric con-
stant, D is the distance between the beams, and N is the
number of beams. The X-Y device contains two struc-
tures at right angles to each other.
The CMOS ASIC uses switched capacitor techniques
to measure the g-cell capacitors and extract the acceler-
ation data from the difference between the two capaci-
tors. The ASIC also signal conditions and filters
(switched capacitor) the signal, providing a high level out-
put voltage that is ratiometric and proportional to acceler-
ation.
Figure 1. Simplified Transducer Physical Model
SPECIAL FEATURES
Filtering
The Motorola accelerometers contain an onboard 4-
pole switched capacitor filter. A Bessel implementation is
used because it provides a maximally flat delay response
(linear phase) thus preserving pulse shape integrity. Be-
cause the filter is realized using switched capacitor tech-
niques, 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.
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 ap-
plied 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 as-
sures that both the mechanical (g-cell) and electronic
sections of the accelerometer are functioning.
Ratiometricity
Ratiometricity simply means that the output offset volt-
age and sensitivity will scale linearly with applied supply
voltage. That is, as you increase supply voltage the sen-
sitivity and offset increase linearly; as supply voltage de-
creases, offset and sensitivity decrease linearly. This is
a key feature when interfacing to a microcontroller or an
A/D converter because it provides system level cancella-
tion of supply induced errors in the analog to digital con-
version process.
Status
Motorola accelerometers include fault detection cir-
cuitry and a fault latch. The Status pin is an output from
the fault latch, OR'd with self-test, and is set high when-
ever one (or more) of the following events occur:
• Supply voltage falls below the Low Voltage Detect
(LVD) voltage threshold
Acceleration
Frees
cale Semiconductor,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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