Low Cost,
±2
g
Dual Axis
Accelerometer with Analog Outputs
MXA2312A
FEATURES
Dual axis accelerometer fabricated on a monolithic CMOS IC
On-chip mixed mode signal processing
Resolution better than 2 milli-g
50,000
g
shock survival rating
30 Hz bandwidth expandable to >160 Hz
2.70V to 5.25V single supply operation
Small (5mm x 5mm x 2mm) surface mount package
Continuous self test
Independent axis programmability (special order)
Sck
(optional)
Internal
Oscillator
Temperature
Sensor
Voltage
Reference
Continous
Self Test
Tout
CLK
Vref
Heater
Control
X axis
Low Pass
Filter
Aout X
APPLICATIONS
Automotive –
Vehicle Security/Active Suspension/ABS
HED Angle Control/Tilt Sensing
Security
– Gas Line/Elevator/Fatigue Sensing
Office Equipment
– Computer Peripherals/PDA’s/Mouse
Smart Pens/Cell Phones
Gaming
– Joystick/RF Interface/Menu Selection/Tilt Sensing
White Goods
– Spin/Vibration Control
2-AXIS
SENSOR
Factory Adjust
Offset & Gain
Low Pass
Filter
Y axis
Aout Y
Vdd
Gnd
Vda
MXA2312A FUNCTIONAL BLOCK DIAGRAM
GENERAL DESCRIPTION
The MXA2312A is a very low cost, dual axis accelerometer
fabricated on a standard, submicron CMOS process. The
MXA2312A measures acceleration with a full-scale range
of
±2
g
and a sensitivity of 312mV/g at 25°C. (The
MEMSIC accelerometer product line extends from
±1
g
to
±10
g
with custom versions available above
±10
g.)
It can
measure both dynamic acceleration (e.g., vibration) and
static acceleration (e.g., gravity). The MXA2312A design
is based on heat convection and requires no solid proof
mass. This eliminates stiction and particle problems
associated with competitive devices and provides shock
survival up to 50,000
g,
leading to significantly lower
failure rates and lower loss due to handling during
assembly.
The MXA2312A provides an absolute analog output (ref.
other MEMSIC data sheets for ratiometric, analog or digital
outputs). The typical noise floor is 0.75 mg/
Hz
allowing
Information furnished by MEMSIC is believed to be accurate and reliable.
However, no responsibility is assumed by MEMSIC for its use, nor for any
infringements of patents or other rights of third parties which may result from
its use. No license is granted by implication or otherwise under any patent or
patent rights of MEMSIC.
signals below 2 milli-g to be resolved at 1 Hz bandwidth.
The 3dB rolloff of the device occurs at 30 Hz but is
expandable to >160 Hz (refernce Application Note AN-
00MX-003). The MXA2312A is available in a low profile
LCC surface mount package (5mm x 5mm x 2mm height).
It is hermetically sealed and are operational over a -40°C to
+105°C temperature range.
Due to the standard CMOS structure of the MXA2312A,
additional circuitry can easily be incorporated into custom
versions for high volume applications. Contact the factory
for more information.
MEMSIC,
Inc.
100 Burtt Road, Andover, MA 01810
Tel: 978.623.8188
Fax: 978.623.9945
www.memsic.com
MXA2312A SPECIFICATIONS
(Measurements @ 25°C, Acceleration = 0
g
unless otherwise noted; V
DD
, V
DA
= 5.0V
unless otherwise specified)
Parameter
SENSOR INPUT
Measurement Range
1
Nonlinearity
Alignment Error
2
Transverse Sensitivity
3
SENSITIVITY
Sensitivity, Analog Outputs at pins
A
OUTX
and A
OUTY6
Change over Temperature (uncompensated)
4
Change over Temperature (compensated)
ZERO
g
BIAS LEVEL
0
g
Offset
6
0
g
Voltage
6
0
g
Offset over Temperature
4
Conditions
Each Axis
Best fit straight line
Min
±2.0
MXA2312A
Typ
Max
Units
g
% of FS
degrees
%
1.0
±1.0
±2.0
2.0
Each Axis
@5.0V supply
∆
from 25°C, at –40°C
∆
from 25°C, at +105°C
∆
from 25°C, –40°C to +105°C
Each Axis
∆
from 25°C
∆
from 25°C, based on 312mV/g
Without frequency compensation
280
-47
<3.0
-0.5
1.09
0.00
1.25
±2.0
±0.62
0.75
30
>160
1.15
4.6
@2.7V-5.0V supply
Source
@5.0V Supply, output rails to
supply voltage
@2.7V Supply, output rails to
supply voltage
@5.0V Supply
@2.7V Supply
Source or sink, @ 2.7V-5.0V supply
@5.0V Supply
@2.7V Supply
0.1
0.1
100
100
40
2.7
3.0
3.0
-40
6
312
342
+93
mV/g
%
%
%
g
V
mg/°C
mV/°C
mg/
Hz
Hz
Hz
+0.5
1.41
NOISE PERFORMANCE
Noise Density, rms
FREQUENCY RESPONSE
3dB Bandwidth - uncompensated
3dB Bandwidth - compensated
5
TEMPERATURE OUTPUT
T
out
Voltage
Sensitivity
VOLTAGE REFERENCE
V
Ref
Change over Temperature
Current Drive Capability
SELF TEST
Continuous Voltage at A
OUTX
, A
OUTY
under
Failure
Continuous Voltage at A
OUTX
, A
OUTY
under
Failure
A
OUTX
and A
OUTY
OUTPUTS
Normal Output Range
Current
Turn-On Time
POWER SUPPLY
Operating Voltage Range
Supply Current
Supply Current
6,7
TEMPERATURE RANGE
Operating Range
NOTES
1.0
1.25
5.0
2.5
0.1
1.35
5.4
2.65
100
V
mV/°K
V
mV/°C
µA
2.4
5.0
2.7
4.9
2.6
V
V
V
V
µA
mS
mS
V
mA
mA
°C
@ 5.0V
@ 2.7V
3.9
5.4
5.25
4.6
6.3
+105
Guaranteed by measurement of initial offset and sensitivity.
Alignment error is specified as the angle between the true and indicated
axis of sensitivity.
3
Transverse sensitivity is the algebraic sum of the alignment and the
inherent sensitivity errors.
4
The sensitivity change over temperature for thermal accelerometers is
based on variations in heat transfer that are governed by the laws of
physics and it is highly consistent from device to device. Please refer to
the section in this data sheet titled “Compensation for the Change of
Sensitivity over Temperature” for more information.
5
External circuitry is required to extend the 3dB bandwidth.
2
1
The device operates over a 2.7V to 5.25V supply range. Please note that
sensitivity and zero
g
bias level will be slightly different at 2.7V operation.
For devices to be operated at 2.7V/3.0V in production, they can be
trimmed at the factory specifically for this lower supply voltage operation,
in which case the sensitivity and zero
g
bias level specifications on this
page will be met. Please contact the factory for specially trimmed devices
for low supply voltage operation.
7
Note that the accelerometer has a constant heater power control circuit
thereby requiring higher supply current at lower operating voltage.
MEMSIC MXA2312A Rev .E
Page 2 of
8
5/15/2004
ABSOLUTE MAXIMUM RATINGS*
Supply Voltage (V
DD
, V
DA
) ………………...-0.5 to +7.0V
Storage Temperature ……….…………-65°C to +150°C
Acceleration ……………………………………..50,000
g
*Stresses above those listed under Absolute Maximum Ratings may cause permanent
damage to the device. This is a stress rating only; the functional operation of the
device at these or any other conditions above those indicated in the operational
sections of this specification is not implied. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.
Package Characteristics
Package
θ
JA
θ
JC
LCC-8
110°C/W
22°C/W
Ordering Guide
Model
A2312AL
Device Weight
< 1 gram
Pin Description: LCC-8 Package
Pin
Name
Description
1
T
OUT
Temperature (Analog Voltage)
2
A
OUTY
Y-Axis Acceleration Signal
3
Gnd
Ground
4
V
DA
Analog Supply Voltage
5
A
OUTX
X-Axis Acceleration Signal
6
V
ref
2.5V Reference
7
Sck
Optional External Clock
8
V
DD
Digital Supply Voltage
Package Style
LCC-8 SMD*
*LCC parts are shipped in tape and reel packaging.
Caution
ESD (electrostatic discharge) sensitive device.
8
1
2
3
M E M S IC
7
X +g
6
5
4
Y +g
Top View
Note:
The MEMSIC logo’s arrow indicates the +X sensing
direction of the device. The +Y sensing direction is rotated 90°
away from the +X direction. Small circle indicates pin one(1).
MEMSIC MXA2312A Rev .E
Page 3 of
8
5/15/2004
THEORY OF OPERATION
The MEMSIC device is a complete dual-axis acceleration
measurement system fabricated on a monolithic CMOS IC
process. The device operation is based on heat transfer by
natural convection and operates like other accelerometers
having a proof mass. The stationary element, or ‘proof
mass’, in the MEMSIC sensor is a gas.
A single heat source, centered in the silicon chip is
suspended across a cavity. Equally spaced
aluminum/polysilicon thermopiles (groups of
thermocouples) are located equidistantly on all four sides of
the heat source (dual axis). Under zero acceleration, a
temperature gradient is symmetrical about the heat source,
so that the temperature is the same at all four thermopiles,
causing them to output the same voltage.
Acceleration in any direction will disturb the temperature
profile, due to free convection heat transfer, causing it to be
asymmetrical. The temperature, and hence voltage output
of the four thermopiles will then be different. The
differential voltage at the thermopile outputs is directly
proportional to the acceleration. There are two identical
acceleration signal paths on the accelerometer, one to
measure acceleration in the x-axis and one to measure
acceleration in the y-axis. Please visit the MEMSIC
website at www.memsic.com for a picture/graphic
description of the free convection heat transfer principle.
PIN DESCRIPTIONS
V
DD
– This is the supply input for the digital circuits and
the sensor heater in the accelerometer. The DC voltage
should be between 2.70 and 5.25 volts. Refer to the section
on PCB layout and fabrication suggestions for guidance on
external parts and connections recommended.
V
DA
– This is the power supply input for the analog
amplifiers in the accelerometer. Refer to the section on
PCB layout and fabrication suggestions for guidance on
external parts and connections recommended.
Gnd
– This is the ground pin for the accelerometer.
accelerometer can be programmed for non-equal
sensitivities on the x- and y-axes. Contact the factory for
additional information on this feature.
T
OUT
– This pin is the buffered output of the temperature
sensor. The analog voltage at T
OUT
is an indication of the
die temperature. This voltage is useful as a differential
measurement of temperature from ambient and not as an
absolute measurement of temperature. After correlating the
voltage at T
OUT
to 25°C ambient, the change in this voltage
due to changes in the ambient temperature can be used to
compensate for the change over temperature of the
accelerometer offset and sensitivity. Please refer to the
section on Compensation for the Change in Sensitivity
Over Temperature for more information.
Sck
– The standard product is delivered with an internal
clock option (800kHz).
This pin should be grounded
when operating with the internal clock.
An external
clock option can be special ordered from the factory
allowing the user to input a clock signal between 400kHz
and 1.6MHz.
V
ref
– A reference voltage is available from this pin. It is
set at 2.50V typical and has 100µA of drive capability.
COMPENSATION FOR THE CHANGE IN
SENSITIVITY OVER TEMPERATURE
All thermal accelerometers display the same sensitivity
change with temperature. The sensitivity change depends
on variations in heat transfer that are governed by the laws
of physics. Manufacturing variations do not influence the
sensitivity change, so there are no unit to unit differences in
sensitivity change. The sensitivity change is governed by
the following equation (and shown in Figure 1 in
°C):
S
i
x T
i2.67
= S
f
x T
f2.67
where S
i
is the sensitivity at any initial temperature T
i
, and
S
f
is the sensitivity at any other final temperature T
f
with
the temperature values in
°K.
2.0
A
OUTX
– This pin is the output of the x-axis acceleration
sensor. The user should ensure the load impedance is
sufficiently high as to not source/sink >100µA. While the
sensitivity of this axis has been programmed at the factory
to be the same as the sensitivity for the y-axis, the
accelerometer can be programmed for non-equal
sensitivities on the x- and y-axes. Contact the factory for
additional information on this feature.
A
OUTY
–
This pin is the output of the y-axis acceleration
Sensitivity (normalized)
1.5
1.0
0.5
0.0
-40
-20
0
20
40
60
80
100
Temperature (C)
sensor. The user should ensure the load impedance is
sufficiently high as to not source/sink >100µA. While the
sensitivity of this axis has been programmed at the factory
to be the same as the sensitivity for the x-axis, the
Page 4 of
8
Figure 1: Thermal Accelerometer Sensitivity
In gaming applications where the game or controller is
typically used in a constant temperature environment,
sensitivity might not need to be compensated in hardware
5/15/2004
MEMSIC MXA2312A Rev .E
or software. Any compensation for this effect could be
done instinctively by the game player.
For applications where sensitivity changes of a few percent
are acceptable, the above equation can be approximated
with a linear function. Using a linear approximation, an
external circuit that provides a gain adjustment of –0.9%/°C
would keep the sensitivity within 10% of its room
temperature value over a 0°C to +50°C range.
For applications that demand high performance, a low cost
micro-controller can be used to implement the above
equation. A reference design using a Microchip MCU (p/n
16F873/04-SO) and MEMSIC developed firmware is
available by contacting the factory. With this reference
design, the sensitivity variation over the full temperature
range (-40°C to +105°C) can be kept below 3%. Please
visit the MEMSIC web site at
www.memsic.com
for
reference design information on circuits and programs
including look up tables for easily incorporating sensitivity
compensation.
DISCUSSION OF TILT APPLICATIONS AND
RESOLUTION
Tilt Applications:
One of the most popular applications of
the MEMSIC accelerometer product line is in
tilt/inclination measurement. An accelerometer uses the
force of gravity as an input to determine the inclination
angle of an object.
A MEMSIC accelerometer is most sensitive to changes in
position, or tilt, when the accelerometer’s sensitive axis is
perpendicular to the force of gravity, or parallel to the
Earth’s surface. Similarly, when the accelerometer’s axis is
parallel to the force of gravity (perpendicular to the Earth’s
surface), it is least sensitive to changes in tilt.
Table 1 and Figure 2 help illustrate the output changes in
the X- and Y-axes as the unit is tilted from +90° to 0°.
Notice that when one axis has a small change in output per
degree of tilt (in mg), the second axis has a large change in
output per degree of tilt. The complementary nature of
these two signals permits low cost accurate tilt sensing to
be achieved with the MEMSIC device (reference
application note AN-00MX-007).
X
+90
0
M E M SIC
X-Axis
X-Axis
Orientation
To Earth’s
Surface
(deg.)
90
85
80
70
60
45
30
20
10
5
0
Y-Axis
Y Output
(g)
Change
per deg.
of tilt
(mg)
17.45
17.37
17.16
16.35
15.04
12.23
8.59
5.86
2.88
1.37
0.15
X Output
(g)
Change
per deg.
of tilt
(mg)
1.000
0.15
0.000
0.996
1.37
0.087
0.985
2.88
0.174
0.940
5.86
0.342
0.866
8.59
0.500
0.707
12.23
0.707
0.500
15.04
0.866
0.342
16.35
0.940
0.174
17.16
0.985
0.087
17.37
0.996
0.000
17.45
1.000
Table 1: Changes in Tilt for X- and Y-Axes
Minimum Resolution:
The accelerometer resolution is
limited by noise. The output noise will vary with the
measurement bandwidth. With the reduction of the
bandwidth, by applying an external low pass filter, the
output noise drops. Reduction of bandwidth will improve
the signal to noise ratio and the resolution. The output noise
scales directly with the square root of the measurement
bandwidth. The maximum amplitude of the noise, its peak-
to- peak value, approximately defines the worst case
resolution of the measurement. The peak-to-peak noise is
approximately equal to 6.6 times the rms value (with an
average uncertainty of .1%). The maximum noise for 1.0Hz
bandwidth will be
1
mg Hz
. For example, if the
bandwidth is increased to 10 Hz, then 3.162 mg is the
maximum rms noise and 20.87mg is the maximum peak -
to-peak noise.
EXTERNAL FILTERS
AC Coupling:
For applications where only dynamic
accelerations (vibration) are to be measured, it is
recommended to ac couple the accelerometer output as
shown in Figure 3. The advantage of ac coupling is that
variations from part to part of zero
g
offset and zero
g
offset versus temperature can be eliminated. Figure 3 is a
HPF (high pass filter) with a –3dB breakpoint given by the
. In many applications it may be
equation:
f
=
1
2
π
RC
desirable to have the HPF –3dB point at a very low
frequency in order to detect very low frequency
accelerations. Sometimes the implementation of this HPF
may result in unreasonably large capacitors, and the
designer must turn to digital implementations of HPFs
where very low frequency –3dB breakpoints can be
achieved.
gravity
0
0
Y
Top View
Figure 2: Accelerometer Position Relative to Gravity
MEMSIC MXA2312A Rev .E
Page 5 of
8
5/15/2004
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