MMA6260Q
Sensors
4 Freescale Semiconductor
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 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 plates form two back-to-back capacitors
(Figure 4). As the center plate moves with acceleration, the
distance 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
between the plates.
The 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
These Freescale accelerometers contain an onboard
single-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 allowing 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 a logic
high input to the self-test pin is applied, 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 ASIC and a proportional output
voltage results. This procedure assures both the mechanical
(g-cell) and electronic sections of the accelerometer are
functioning.
Freescale accelerometers include fault detection circuitry
and a fault latch. Parity of the EEPROM bits becomes odd in
number.
Self-test is disabled when EEPROM parity error occurs.
Ratiometricity
Ratiometricity simply means the output offset voltage and
sensitivity will scale linearly with applied supply voltage. That
is, as supply voltage is increased, 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.
Acceleration
Figure 3. Transducer
Physical Model
Figure 4. Equivalent
Circuit Model
