Low Cost,
1 g Dual Axis
Accelerometer with Analog Outputs
MXA2500A
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)
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
Internal
Oscillator
Sck
(optional)
CLK
Heater
Control
X axis
Y axis
Factory Adjust
Offset & Gain
Low Pass
Filter
Low Pass
Filter
Temperature
Sensor
Voltage
Reference
Vref
Aout X
Vdd
Vda
Gnd
2-AXIS
SENSOR
Aout Y
Tout
Continous
Self Test
MXA2500A FUNCTIONAL BLOCK DIAGRAM
GENERAL DESCRIPTION
The MXA2500A is a low cost, dual axis accelerometer
fabricated on a standard, submicron CMOS process. The
MXA2500A measures acceleration with a full-scale range of
1 g and a sensitivity of 500mV/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 MXA2500A 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 losses due to
handling during assembly.
The MXA2500A 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
signals below 2 milli-g to be resolved at 1 Hz bandwidth. The
3dB rolloff of the device occurs at above 30 Hz but is
expandable to >160. The MXA2500A is available in a low
profile LCC surface mount package (5mm x 5mm x 2mm
height). Both are hermetically sealed and are operational over a
-40
C to +105
C temperature range.
Due to the standard CMOS structure of the MXA2500A,
additional circuitry can easily be incorporated into custom
versions for high volume applications. Contact the factory for
more information.
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.
MEMSIC, Inc.
100 Burtt Road, Andover, MA 01810
Tel: 978.623.8188
Fax: 978.623.9945
www.memsic.com
MEMSIC MXA2500A Rev 01 Page 2 of 8 04/02
MXA2500A SPECIFICATIONS
(Measurements @ 25
C, Acceleration = 0 g unless otherwise noted; V
DD
, V
DA
= 5.0V unless
otherwise specified)
Parameter
Conditions
Min
MXA2500A
Typ
Max
Units
SENSOR INPUT
Measurement Range
1
Each Axis
1.0
g
Nonlinearity
Best fit straight line
1.0
2.0
% of FS
Alignment Error
2
1.0
degrees
Transverse Sensitivity
3
2.0
%
SENSITIVITY
Sensitivity, Analog Outputs at pins
A
OUTX
and A
OUTY
6
Each Axis
@5.0V supply
450
500
550
mV/g
Change over Temperature (uncompensated)
4
from 25
C, at 40
C
+93
%
from 25
C, at +105
C
-47 %
Change over Temperature (compensated)
4
from 25
C, 40
C to +105
C
<3.0 %
ZERO g BIAS LEVEL
0 g Offset
6
Each Axis
-0.50
0.00
+0.50
g
0 g Voltage
6
1.00
1.25
1.50
V
0 g Offset over Temperature
from 25
C
from 25
C, based on 500mV/g
2.0
1.00
mg/
C
mV/
C
NOISE PERFORMANCE
Noise Density, rms
Without frequency compensation
0.75
1.0
mg/
Hz
FREQUENCY RESPONSE
3dB Bandwidth - uncompensated
30
Hz
3dB Bandwidth - compensated
5
>160
Hz
TEMPERATURE OUTPUT
T
out
Voltage
1.21
1.25
1.29
V
Sensitivity
4.6
5.0
5.4
mV/
K
VOLTAGE REFERENCE
V
Ref
@2.7V-5.0V
supply
2.4
2.5
2.65
V
Change over Temperature
0.1
mV/
C
Current Drive Capability
Source
100
A
SELF TEST
Continuous Voltage at A
OUTX
, A
OUTY
under
Failure
@5.0V Supply, output rails to
supply voltage
5.0
V
Continuous Voltage at A
OUTX
, A
OUTY
under
Failure
@2.7V Supply, output rails to
supply voltage
2.7
V
A
OUTX
and A
OUTY
OUTPUTS
Normal Output Range
@5.0V Supply
@2.7V Supply
0.1
0.1
4.9
2.6
V
V
Current
Source or sink, @ 2.7V-5.0V supply
100
A
Turn-On Time
@5.0V Supply
@2.7V Supply
100
40
mS
mS
POWER SUPPLY
Operating Voltage Range
2.7
5.25
V
Supply Current
@ 5.0V
3.0
3.9
4.6
mA
Supply Current
6,7
@
2.7V
3.0
5.4
6.3
mA
TEMPERATURE RANGE
Operating Range
-40
+105
C
NOTES
1
Guaranteed by measurement of initial offset and sensitivity.
2
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.
6
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 MXA2500A Rev 01 Page 3 of 8 04/02
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
JA
JC
Device
Weight
LCC-8
110
C/W 22
C/W
< 1 gram
Ordering Guide
Model Package
Style
A2500AL LCC-8
SMD*
*LCC parts are shipped in tape and reel packaging.
Caution
ESD (electrostatic discharge) sensitive device.
8
4
1
2
3
7
6
5
Top View
ME
MS
I
C
X +g
Y +g
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.
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
MEMSIC MXA2500A Rev 01 Page 4 of 8 04/02
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. V
DA
should always be connected to V
DD
.
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.
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 >100
A. 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 sensor.
The user should ensure the load impedance is sufficiently high
as to not source/sink >100
A. While the sensitivity of this axis
has been programmed at the factory to be the same as the
sensitivity for the x-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.
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 100
A 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
i
2.67
= S
f
x T
f
2.67
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.
0.0
0.5
1.0
1.5
2.0
-40
-20
0
20
40
60
80
100
Temperature (C)
Sensitivity (normalized)
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 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.
MEMSIC MXA2500A Rev 01 Page 5 of 8 04/02
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).
Top View
X
Y
+90
0
0
0
gravity
MEMS
IC
Figure 2: Accelerometer Position Relative to Gravity
X-Axis
Y-Axis
X-Axis
Orientation
To Earth's
Surface
(deg.)
X Output
(g)
Change
per deg.
of tilt
(mg)
Y Output
(g)
Change
per deg.
of tilt
(mg)
90
1.000
0.15 0.000
17.45
85
0.996
1.37 0.087
17.37
80
0.985
2.88 0.174
17.16
70
0.940
5.86 0.342
16.35
60
0.866
8.59 0.500
15.04
45
0.707
12.23 0.707
12.23
30
0.500
15.04 0.866
8.59
20
0.342
16.35 0.940
5.86
10
0.174
17.16 0.985
2.88
5
0.087
17.37 0.996
1.37
0
0.000
17.45 1.000
0.15
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
Hz
mg
1
. 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 equation:
RC
f
2
1
=
. In many
applications it may be 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.
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