(Maximum ratings are the limits to which the device can be exposed without causing permanent damage.)
Rating
Maximum Acceleration (all axis)
Supply Voltage
Drop Test
(1)
Storage Temperature Range
1. Dropped onto concrete surface from any axis.
Symbol
g
max
V
DD
D
drop
T
stg
Value
±5000
–0.3 to +3.6
1.8
–40 to +125
Unit
g
V
m
°C
ELECTRO STATIC DISCHARGE (ESD)
WARNING: This device is sensitive to electrostatic
discharge.
Although the Freescale accelerometer contains internal
2000 V ESD protection circuitry, extra precaution must be
taken by the user to protect the chip from ESD. A charge of
over 2000 volts can accumulate on the human body or
associated test equipment. A charge of this magnitude can
alter the performance or cause failure of the chip. When
handling the accelerometer, proper ESD precautions should
be followed to avoid exposing the device to discharges which
may be detrimental to its performance.
MMA7331LC
2
Sensors
Freescale Semiconductor
Table 2. Operating Characteristics
Unless otherwise noted:
-
40°C < T
A
< 85°C, 2.2 V < V
DD
< 3.6 V, Acceleration = 0g, Loaded output
(1)
Characteristic
Operating Range
(2)
Supply Voltage
(3)
Supply Current
(4)
Supply Current at Sleep Mode
(4)
Operating Temperature Range
Acceleration Range, X-Axis, Y-Axis, Z-Axis
g-Select: 0
g-Select: 1
Output Signal
Zero g (T
A
= 25°C, V
DD
= 2.8 V)
(5), (6)
XY
Z
(7)
Zero g Temperature Coefficient
(8)
X
Y
Z
Sensitivity (T
A
= 25°C, V
DD
= 2.8 V)
4g
9g
Sensitivity
(4)
Bandwidth Response
XY
Z
Output Impedance
Self Test
Output Response
X
OUT
, Y
OUT
Z
OUT
Input Low
Input High
Noise
Power Spectral Density RMS (0.1 Hz – 1 kHz)
(4)
Control Timing
Power-Up Response Time
(9)
Enable Response Time
(10)
Self Test Response Time
(11)
Sensing Element Resonant Frequency
XY
Z
Internal Sampling Frequency
Output Stage Performance
Full-Scale Output Range (I
OUT
= 3 µA)
Nonlinearity, X
OUT
, Y
OUT
, Z
OUT
Cross-Axis Sensitivity
(12)
Symbol
V
DD
I
DD
I
DD
T
A
g
FS
g
FS
Min
2.2
—
—
-40
—
—
Typ
2.8
400
3
—
±4
±9
Max
3.6
600
10
+85
—
—
Unit
V
μA
μA
°C
g
g
V
OFF
V
OFF
TCO
4g
TCO
9g
TCO
9g
TCO
9g
S
4g
S
9g
S,T
A
f
-3dBXY
f
-3dBZ
Z
O
1.316
1.45
-2
-12
-7.9
-8.4
289.5
75.2
-0.0075
—
—
24
1.4
1.4
±0.5
-6.4
-0.9
0
308
83.6
±0.002
400
300
32
1.484
1.484
2
-0.6
5.2
8.5
326.5
91.9
+0.0075
—
—
40
V
V
mg/°C
mg/°C
mg/°C
mg/°C
mV/g
mV/g
%/°C
Hz
Hz
kΩ
Δg
STXY
Δg
STZ
V
IL
V
IH
n
PSD
t
RESPONSE
t
ENABLE
t
ST
f
GCELLXY
f
GCELLZ
f
CLK
V
FSO
NL
OUT
V
XY, XZ, YZ
+0.05
+0.5
V
SS
0.7 V
DD
—
—
—
—
—
—
—
V
SS
+0.1
-1.0
-5.0
-0.1
+1.75
—
—
350
1.0
0.5
2.0
6.0
3.4
11
—
—
—
—
+3.0
0.3 V
DD
V
DD
—
2.0
2.0
5.0
—
—
—
V
DD
–0.1
+1.0
+5.0
g
g
V
V
μg/
Hz
ms
ms
ms
kHz
kHz
kHz
V
%FSO
%
1. For a loaded output, the measurements are observed after an RC filter consisting of an internal 32 kΩ resistor and an external 3.3 nF capacitor
(recommended as a minimum to filter clock noise) on the analog output for each axis and a 0.1
μF
capacitor on V
DD
- GND. The output sensor
bandwidth is determined by the Capacitor added on the output. f = 1/2π * (32 x 10
3
) * C. C = 3.3 nF corresponds to BW = 1507 Hz, which is the minimum
to filter out internal clock noise.
2. These limits define the range of operation for which the part will meet specification.
3. Within the supply range of 2.2 and 3.6 V, the device operates as a fully calibrated linear accelerometer. Beyond these supply limits the device may
operate as a linear device but is not guaranteed to be in calibration.
4. This value is measured with g-Select in 4g mode.
5. The device can measure both + and – acceleration. With no input acceleration the output is at midsupply. For positive acceleration the output will
increase above V
DD
/2. For negative acceleration, the output will decrease below V
DD
/2.
6. For optimal 0g offset performance, adhere to AN3484 and AN3447.
7. Product performance will not exceed this minimum level, however, measurement over time will not be equal to time zero measurements for this specific
parameter.
8. X, Y, Z = 0g
9. The response time between 10% of full scale V
DD
input voltage and 90% of the final operating output voltage.
10. The response time between 10% of full scale Sleep Mode input voltage and 90% of the final operating output voltage.
11. The response time between 10% of the full scale self test input voltage and 90% of the self test output voltage.
12. A measure of the device’s ability to reject an acceleration applied 90° from the true axis of sensitivity.
MMA7331LC
Sensors
Freescale Semiconductor
3
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 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 (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.
Acceleration
g-Select
The g-Select feature allows for the selection between two
sensitivities. Depending on the logic input placed on pin 10,
the device internal gain will be changed allowing it to function
with a 4g or 9g sensitivity (Table
3).
This feature is ideal 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 4g
sensitivity as the device has an internal pull-down to keep it
at that sensitivity (308 mV/g).
Table 3. g-Select Pin Description
g-Select
0
1
g-Range
4g
9g
Sensitivity
308 mV/g
83.6 mV/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 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 4g mode. By placing a high
input signal on pin 7, the device will resume to normal mode
of operation.
Filtering
The 3-axis accelerometer contains 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.
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.
Figure 3. Simplified Transducer Physical Model
SPECIAL FEATURES
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 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, 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.
MMA7331LC
4
Sensors
Freescale Semiconductor
BASIC CONNECTIONS
Pin Descriptions
Top View
N/C
14
13
PCB Layout
POWER SUPPLY
N/C
X
OUT
Y
OUT
Z
OUT
V
SS
V
DD
Self Test
N/C
N/C
Accelerometer
V
DD
V
SS
Sleep
g-Select
Self Test
X
OUT
Y
OUT
Z
OUT
C
C
C
C
C
1
V
RH
P0
P1
P2
A/D
IN
A/D
IN
A/D
IN
Microcontroller
V
DD
V
SS
11
3
12
C
2
10
9
g-Select
N/C
N/C
6
5
4
Sleep
Figure 4. Pinout Description
Table 4. Pin Descriptions
Pin No. Pin Name
1
2
3
4
5
6
7
8
9
10
11
12
13
14
N/C
X
OUT
Y
OUT
Z
OUT
V
SS
V
DD
Sleep
N/C
N/C
g-Select
N/C
N/C
Self Test
N/C
Description
No internal connection
Leave unconnected
X direction output voltage
Y direction output voltage
Z direction output voltage
Power Supply Ground
Power Supply Input
Logic input pin to enable product or Sleep Mode
No internal connection
Leave unconnected
No internal connection
Leave unconnected
Logic input pin to select g level
Unused for factory trim
Leave unconnected
Unused for factory trim
Leave unconnected
Input pin to initiate Self Test
Unused for factory trim
Leave unconnected
7
8
Figure 6. Recommended PCB Layout for Interfacing
Accelerometer to Microcontroller
NOTES:
1. Use 0.1 µF capacitor on V
DD
to decouple the power
source.
2. Physical coupling distance of the accelerometer to
the microcontroller should be minimal.
3. Place a ground plane beneath the accelerometer to
reduce noise, the ground plane should be attached to
all of the open ended terminals shown in
Figure 6.
4. Use a 3.3 nF capacitor on the outputs of the
accelerometer to minimize clock noise (from the
switched capacitor filter circuit).
5. PCB layout of power and ground should not couple
power supply noise.
6. Accelerometer and microcontroller should not be a
high current path.
7. A/D sampling rate and any external power supply
switching frequency should be selected such that
they do not interfere with the internal accelerometer
sampling frequency (11 kHz for the sampling
frequency). This will prevent aliasing errors.
8. 10 MΩ or higher is recommended on X
OUT
, Y
OUT
and
Z
OUT
to prevent loss due to the voltage divider
relationship between the internal 32 kΩ resistor and
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