Sensors
4 Freescale Semiconductor
MMA7260QT
PRINCIPLE OF OPERATION
The Freescale accelerometer is a surface-micromachined
integrated-circuit accelerometer.
The device consists of two surface micromachined
capacitive sensing cells (g-cell) and a signal conditioning
ASIC contained in a single integrated circuit package. The
sensing elements are 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 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
(Figure 3). As the center beam moves with acceleration, the
distance between the beams changes and each capacitor's
value will change, (C = Aε/D). Where A is the area of the
beam, ε is the dielectric constant, and D is the distance
between the beams.
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.
Figure 3. Simplified Transducer Physical Model
SPECIAL FEATURES
g-Select
The g-Select feature allows for the selection among 4
sensitivities present in the device. Depending on the logic
input placed on pins 1 and 2, the device internal gain will be
changed allowing it to function with a 1.5g, 2g, 4g, or 6g
sensitivity (Table 3). This feature is ideal when a product has
applications requiring different sensitivities for optimum
performance. The sensitivity can be changed at anytime
during the operation of the product. The g-Select1 and
g-Select2 pins can be left unconnected for applications
requiring only a 1.5g sensitivity as the device has an internal
pull-down to keep it at that sensitivity (800mV/g).
Sleep Mode
The 3 axis accelerometer provides a Sleep Mode that is
ideal for battery operated products. When Sleep Mode is
active, the device outputs are turned off, providing significant
reduction of operating current. A low input signal on pin 12
(Sleep Mode) will place the device in this mode and reduce
the current to 3 μA typ. For lower power consumption, it is
recommended to set g-Select1 and g-Select2 to 1.5g mode.
By placing a high input signal on pin 12, the device will
resume to normal mode of operation.
Filtering
The 3 axis accelerometer contains onboard single-pole
switched capacitor filters. 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.
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. Offset ratiometric
error can be typically >20% at V
DD
= 2.2 V. Sensitivity
ratiometric error can be typically >3% at V
DD
= 2.2 V. Consult
factory for additional information.
Acceleration
Table 3. g-Select Pin Descriptions
g-Select2 g-Select1 g-Range Sensitivity
0 0 1.5g 800 mV/g
0 1 2g 600 mV/g
1 0 4g 300 mV/g
1 1 6g 200 mV/g
