mma7361l Freescale Semiconductor, Inc, mma7361l Datasheet - Page 4

• Manufacturer Part Number: mma7361l
• Description: ?1.5g, ?6g Three Axis Low-g Micromachined Accelerometer
• Manufacturer: Freescale Semiconductor, Inc

integrated-circuit accelerometer.
capacitive sensing cell (g-cell) and a signal conditioning ASIC
contained in a single package. The sensing element is sealed
hermetically at the wafer level using a bulk micromachined
cap wafer.
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
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.
(Figure
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 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.
0g-Detect
high signal when all three axes are at 0g. This feature
enables the application of Linear Freefall protection if the
signal is connected to an interrupt pin or a poled I/O pin on a
microcontroller.
Self Test
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 hard disk drive
4
MMA7361L
The Freescale accelerometer is a surface-micromachined
The device consists of a surface micromachined
The g-cell is a mechanical structure formed from
As the beams attached to the central mass move, the
The g-cell beams form two back-to-back capacitors
The ASIC uses switched capacitor techniques to measure
The sensor offers a 0g-Detect feature that provides a logic
The sensor provides a self test feature that allows the
Figure 3. Simplified Transducer Physical Model
3). As the center beam moves with acceleration, the
(Figure
Acceleration
SPECIAL FEATURES
3).
PRINCIPLE OF OPERATION
protection where system integrity must be ensured over the
life of the product. Customers can use self test to verify the
solderability to confirm that the part was mounted to the PCB
correctly. To use this feature to verify the 0g-Detect function,
the accelerometer should be held upside down so that the
z-axis experiences -1g. When the self test function is
initiated, an electrostatic force is applied to each axis to
cause it to deflect. The x- and y-axis are deflected slightly
while the z-axis is trimmed to deflect 1g. This procedure
assures that both the mechanical (g-cell) and electronic
sections of the accelerometer are functioning.
g-Select
sensitivities. Depending on the logic input placed on pin 10,
the device internal gain will be changed allowing it to function
with a 1.5g or 6g sensitivity
when a product has applications requiring two different
sensitivities for optimum performance. The sensitivity can be
changed at anytime during the operation of the product. The
g-Select pin 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
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 7
(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-Select to 1.5g mode. By placing a high
input signal on pin 7, the device will resume to normal mode
of operation.
Filtering
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.
Ratiometricity
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.
The g-Select feature allows for the selection between two
The 3 axis accelerometer provides a Sleep Mode that is
The 3 axis accelerometer contains an onboard single-pole
Ratiometricity simply means the output offset voltage and
Table 3. g-Select Pin Description
g-Select
0
1
g-Range
(Table
1.5g
6g
3). This feature is ideal
Freescale Semiconductor
Sensitivity
800 mV/g
206 mV/g
Sensors