Freescale Semiconductor
Data Sheet: Technical Data
Document Number: MMA1251
Rev 1, 01/2012
Low-g
Micromachined Accelerometer
The MMA series of silicon capacitive, micromachined accelerometers feature
signal conditioning, a 2-pole low-pass filter and temperature compensation.
Zero-g offset full scale span and filter cutoff are factory set and require no
external devices. A full system self-test capability verifies system functionality.
Features
•
•
•
•
•
•
•
•
Integral Signal Conditioning
Linear Output
2nd Order Bessel Filter
Calibrated Self-test
EPROM Parity Check Status
Transducer Hermetically Sealed at Wafer Level for Superior Reliability
Robust Design, High Shock Survivability
Qualified AECQ100, Rev. F Grade 2 (-40°C/+105°C)
MMA1251
16-LEAD SOIC
Pb-FREE
CASE 475-01
Typical Applications
•
•
•
•
•
•
•
Vibration Monitoring and Recording
Appliance Control
Mechanical Bearing Monitoring
Computer Hard Drive Protection
Computer Mouse and Joysticks
Virtual Reality Input Devices
Sports Diagnostic Devices and Systems
ORDERING INFORMATION
Device Name
MMA1251EG
MMA1251EGR2
MMA1251KEG*
MMA1251KEGR2*
Temperature Range
–40°
to 105°C
–40°
to 105°C
–40°
to 105°C
–40°
to 105°C
Case No.
475-01
475-01
475-01
475-01
Package
SOIC-16
SOIC-16, Tape & Reel
SOIC-16
SOIC-16, Tape & Reel
V
SS
V
SS
V
SS
V
OUT
STATUS
V
DD
V
SS
ST
Top View
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
N/C
N/C
N/C
N/C
N/C
N/C
N/C
N/C
Pin Connections
*Part number sourced from a different facility.
© 2012 Freescale Semiconductor, Inc. All rights reserved.
V
DD
g-Cell
Sensor
Integrator
Gain
Filter
Temp Comp
and Gain
V
OUT
ST
Self-test
Control Logic &
EPROM Trim Circuits
Oscillator
Clock
Generator
V
SS
STATUS
Figure 1. Simplified Accelerometer Functional Block Diagram
Table 1. Maximum Ratings
(Maximum ratings are the limits to which the device can be exposed without causing permanent damage.)
Rating
Powered Acceleration (all axes)
Unpowered Acceleration (all axes)
Supply Voltage
Drop Test
(1)
Storage Temperature Range
1. Dropped onto concrete surface from any axis.
Symbol
G
pd
G
upd
V
DD
D
drop
T
stg
Value
1500
2000
–0.3 to +7.0
1.2
–40 to +125
Unit
g
g
V
m
°C
ELECTRO STATIC DISCHARGE (ESD)
WARNING: This device is sensitive to electrostatic
discharge.
Although the Freescale accelerometers contain internal
2 kV 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.
MMA1251
2
Sensors
Freescale Semiconductor, Inc.
Table 2. Operating Characteristics
(Unless otherwise noted: –40°C
≤
T
A
≤
+105°C, 4.75
≤
V
DD
≤
5.25, Acceleration = 0g, Loaded output.
(1)
)
Characteristic
Operating Range
(2)
Supply Voltage
(3)
Supply Current
Operating Temperature Range
Acceleration Range
Output Signal
-1g (T
A
= 25°C, V
DD
= 5.0 V)
(4)
-1g
Sensitivity (T
A
= 25°C, V
DD
= 5.0 V)
(5)
Sensitivity
Bandwidth Response
Nonlinearity
Noise
RMS (0.1 Hz – 1.0 kHz)
Spectral Density (RMS, 0.1 Hz – 1.0 KHz)
(6)
Self-Test
Output Response
Input Low
Input High
Input Loading
(7)
Response Time
(8)
Status
(9), (10)
Output Low (I
load
= 100
μA)
Output High (I
load
= 100
μA)
Output Stage Performance
Electrical Saturation Recovery Time
(11)
Full Scale Output Range (I
OUT
= 200
μA)
Capacitive Load Drive
(12)
Output Impedance
Mechanical Characteristics
Transverse Sensitivity
(13)
Symbol
V
DD
I
DD
T
A
g
FS
V
OFF
V
OFF
S
S
f
-3dB
NL
OUT
n
RMS
n
SD
ΔV
ST
V
IL
V
IH
I
IN
t
ST
V
OL
V
OH
t
DELAY
V
FSO
C
L
Z
O
V
XZ,YZ
Min
4.75
1.1
–40
—
2.25
0.4 V
DD
380
74
40.0
–1.0
—
—
0.2
V
SS
0.7 V
DD
–300
—
—
V
DD
– 0.8
—
V
SS
+ 0.25
—
—
—
Typ
5.00
2.1
—
5
2.5
0.5 V
DD
400
80
50
—
2.0
700
0.25
—
—
–125
2.0
—
—
—
—
—
50
—
Max
5.25
3.0
+105
—
2.75
0.6 V
DD
420
86
60.0
+1.0
6.0
—
0.3
0.3 V
DD
V
DD
–50
25
0.4
—
2.0
V
DD
– 0.25
100
—
5.0
Unit
V
mA
°C
g
V
V
mV/g
mV/g/V
Hz
% FSO
mVrms
μg/
Hz
V/V
V
V
μA
ms
V
V
ms
V
pF
Ω
% FSO
1. For a loaded output the measurements are observed after an RC filter consisting of a 1 kΩ resistor and a 0.1
μF
capacitor to ground.
2. These limits define the range of operation for which the part will meet specification.
3. Within the supply range of 4.75 and 5.25 volts, 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. The device can measure both + and – acceleration. With -1g acceleration the output is at mid-supply. For positive acceleration the output
will increase above V
DD
/2 and for negative acceleration the output will decrease below V
DD
/2.
5. The device is calibrated at 3g. Sensitivity limits apply to 0 Hz acceleration.
6. At clock frequency
≅70
kHz.
7. The digital input pin has an internal pull-down current source to prevent inadvertent self test initiation due to external board level leakages.
8. Time for the output to reach 90% of its final value after a self-test is initiated.
9. The Status pin output is not valid following power-up until at least one rising edge has been applied to the self-test pin. The Status pin is high
whenever the self-test input is high, as a means to check the connectivity of the self-test and Status pins in the application.
10. The Status pin output latches high if a Low Voltage Detection or Clock Frequency failure occurs, or the EPROM parity changes to odd. The
Status pin can be reset low if the self-test pin is pulsed with a high input for at least 100
μs,
unless a fault condition continues to exist.
11. Time for amplifiers to recover after an acceleration signal causes them to saturate.
12. Preserves phase margin (60°) to guarantee output amplifier stability.
13. A measure of the device's ability to reject an acceleration applied 90° from the true axis of sensitivity.
MMA1251
Sensors
Freescale Semiconductor, Inc.
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 CMOS 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 two
stationary plates with a moveable plate in-between. The
center plate can be deflected from its rest position by
subjecting the system to an acceleration (Figure
3).
When the center plate deflects, the distance from it to one
fixed plate will increase by the same amount that the distance
to the other plate 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 CMOS 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
The Freescale accelerometers contain an onboard 2-pole
switched capacitor filter. A Bessel implementation is used
because it provides a maximally flat delay response (linear
phase) thus preserving pulse shape integrity. Because the
filter is realized using switched capacitor techniques, there is
no requirement for external passive components (resistors
and capacitors) to set the cutoff frequency.
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 automotive airbag
systems where system integrity must be ensured over the life
of the vehicle. A fourth “plate'' is used in the g-cell as a self-
test plate. When the user applies a logic high input to the self-
test pin, a calibrated potential is applied across the self-test
plate and the moveable plate. The resulting electrostatic
2
⎛
1
V
⎞
-
force
⎜
Fe
=
--
εA
------
⎟
causes the center plate to deflect.
2
d
2
⎠
⎝
The resultant deflection is measured by the accelerometer's
control ASIC and a proportional output voltage results. This
procedure assures that both the mechanical (g-cell) and
electronic sections of the accelerometer are functioning.
Status
Freescale accelerometers include fault detection circuitry
and a fault latch. The Status pin is an output from the fault
latch, OR'd with self-test, and is set high whenever the
following event occurs:
• Parity of the EPROM bits becomes odd in number.
The fault latch can be reset by a rising edge on the self-test
input pin, unless one (or more) of the fault conditions
continues to exist.
Acceleration
Figure 3. Transducer
Physical Model
Figure 4. Equivalent
Circuit Model
MMA1251
4
Sensors
Freescale Semiconductor, Inc.
BASIC CONNECTIONS
Pinout Description
PCB Layout
STATUS
V
SS
V
SS
V
SS
V
OUT
STATUS
V
DD
V
SS
ST
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
N/C
N/C
N/C
N/C
N/C
N/C
N/C
N/C
Accelerometer
ST
V
OUT
V
SS
V
DD
R
1 kΩ
C 0.1
μF
V
RH
C 0.1
μF
C
0.1
μF
P1
Microcontroller
P0
A/D In
V
SS
C 0.1
μF
V
DD
Table 3. Pin Descriptions
Pin No.
1 thru 3
4
5
6
7
8
9 thru 13
14 thru 16
Pin Name
V
SS
V
OUT
STATUS
V
DD
V
SS
ST
Trim pins
—
Description
Redundant connections to the internal
V
SS
and may be left unconnected.
Output voltage of the accelerometer.
Logic output pin to indicate fault.
The power supply input.
The power supply ground.
Logic input pin used to initiate self-test.
Used for factory trim. Leave
unconnected.
No internal connection. Leave
unconnected.
Power Supply
Figure 6. Recommended PCB Layout for Interfacing
Accelerometer to Microcontroller
NOTES:
1. Use a 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 an RC filter of 1 kΩ and 0.1
μF
on the output 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.
V
DD
LOGIC
INPUT
C1
0.1
μF
1
2
3
8
6
MMA1251
ST
V
DD
V
SS
V
SS
V
SS
7 V
SS
5
R1
1 kΩ
C2
0.1
μF
STATUS
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. This will prevent aliasing errors.
V
OUT
4
OUTPUT
SIGNAL
Figure 5. SOIC Accelerometer with Recommended
Connection Diagram
MMA1251
Sensors
Freescale Semiconductor, Inc.
5
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