PMMA7450LR2 Freescale Semiconductor, Inc, PMMA7450LR2 Datasheet - Page 5

• Manufacturer Part Number: PMMA7450LR2
• Description: ?2g/?4g/?8g Three Axis Low-g Digital Output Accelerometer
• Manufacturer: Freescale Semiconductor, Inc

Sensors
Freescale Semiconductor
integrated-circuit accelerometer. The device consists of a
surface micromachined 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. 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
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
moves with acceleration, the distance between the beams
changes and each capacitor's value will change, (C = A
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 digital output voltage that is
proportional to acceleration.
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
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. When the self test function is initiated through the
mode control register, accessing the “self test” bit, an
electrostatic force is applied to each axis to cause it to deflect.
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.
The Freescale accelerometer is a surface-micromachined
As the beams attached to the central mass move, the
The ASIC uses switched capacitor techniques to measure
The sensor provides a self test feature that allows the
Figure 3. Simplified Transducer Physical Model
Acceleration
FEATURES
(Figure
3). As the center beam
(Figure
PRINCIPLE OF OPERATION
3).
ε
/D).
g-Select
sensitivities for measurement. Depending on the values in
the Mode control register ($16), the MMA7450L’s internal
gain will be changed allowing it to function with a 2g, 4g or 8g
measurement sensitivity. This feature is ideal when a product
has applications requiring two or more sensitivities for
optimum performance and for enablilng multiple functions.
The sensitivity can be changed during the operation by
modifying the two GLVL bits located in the mode control
register.
$16: Mode control register (Read/Write)
Standby Mode
standby mode that is ideal for battery operated products.
When standby mode is active, the device outputs are turned
off, providing significant reduction of operating current. When
the device is in standby mode the current will be reduced to
5 µA typical. In standby mode the device can read and write
to the registers with the I
measurements can be taken in this mode as all current
consuming parts are off. The mode of the device is controlled
through the mode control register by accessing the two mode
bits as shown in
D7
--
0
The g-Select feature enables the selection between 3
This digital output 3-axis accelerometer provides a
GLVL [1:0]
00: 8G is selected for measurement range.
01: 4G is selected for measurement range.
10: 2G is selected for measurement range.
DRPD SPI3W STON GLVL[1] GLVL[0] MODE[1] MODE[0] Function
Table 4. g-Select Description for 8-Bit Mode
D6
0
Table 5. Mode Descriptions
GLVL [1:0]
00
01
10
MODE [0:1]
D5
0
Table
00
01
10
11
D4
0
5.
2
C/SPI available, but no new
g-Range
D3
0
8g
4g
2g
Pulse Detection Mode
Level Detection Mode
Measurement Mode
D2
Standby Mode
0
Function
Sensitivity
D1
0
16 LSB/g
64 LSB/g
32 LSB/g
MMA7450L
D0
0
Default
Bit
5