Sensors
4 Freescale Semiconductor
MMA2204
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 move
between fixed beans. The movable beams can be deflected
from their rest position by subjecting the system to an
acceleration (Figure 3).
When the beams attached to the center 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
(Figure 4). As the center plate moves with acceleration, the
distance between the beams change and each capacitor's
value will change, (C = NAε/D). Where A is the area of the
facing side of the beam, ε is the dielectric constant, and 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
The Freescale 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. 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. 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 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.
Ratiometricity
Ratiometricity simply means that the output offset voltage
and sensitivity will scale linearly with applied supply voltage.
That is, as you increase supply voltage the sensitivity and
offset increase linearly; as supply voltage decreases, 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 cancellation of supply induced errors
in the analog to digital conversion process.
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 one (or
more) of the following events occur:
• Supply voltage falls below the Low Voltage Detect (LVD)
voltage threshold
• Clock oscillator falls below the clock monitor
minimum frequency
• Parity of the EPROM bits becomes odd in number.
The fault latch can be reset by a falling 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
