Low Offset Drift, 2g Dual Axis
Accelerometer with Ratiometric Output
MXR2312EL
FEATURES
Better than 1 milli-g resolution
Dual axis accelerometer fabricated on a monolithic CMOS IC
On-chip mixed mode signal processing
No moving parts
50,000 g shock survival rating
0.4mg/C offset drift over temperature
3V to 5.25V single supply continuous operation
Small (5mm x 5mm x 2mm) surface mount package
Continuous self test
Custom programmable specifications
Independently programmable axes (special order)
APPLICATIONS
Automotive Vehicle Security/Active Suspension/ABS
Headlight 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
MXR2312EL FUNCTIONAL BLOCK DIAGRAM
GENERAL DESCRIPTION
The MXR2312EL is an low noise and low cost, dual axis
accelerometer fabricated on a standard, submicron CMOS
process. It is a complete sensing system with on-chip
mixed mode signal processing. The MXR2312E 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
0.5g to
250g with custom
versions available above
10 g.) It can measure both
dynamic acceleration (e.g., vibration) and static
acceleration (e.g., gravity). The MXR2312EL design is
based on heat transfer 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 MXR2312EL provides a ratiometric analog output.
The typical noise floor is 0.2 mg/ Hz allowing signals
below 1 milli-g to be resolved at 1 Hz bandwidth. The 3dB
rolloff of the device occurs at 17 Hz. The offset drift over
temperature is 0.4mg/C. The MXR2312EL is available
in a low profile LCC surface mount package (5mm x 5mm
x 2mm). It is hermetically sealed and is operational over a
-40
C to +85
C temperature range. It also contains an on-
chip temperature sensor and a bandgap voltage reference.
Due to the standard CMOS structure of the MXR2312EL,
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.
800 Turnpike Street, Suite 202, North Andover, MA 01845 USA
Tel: +1-978.738.0900
Fax: +1-978.738.0196
www.memsic.com
MEMSIC MXR2312EL
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MEMSIC MXR2312EL
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MXR2312EL SPECIFICATIONS
(Measurements @ 25
C, Acceleration = 0 g unless otherwise noted; V
DD
, V
DA
=
5.0V unless otherwise specified)
Parameter
Conditions
Min
MXR2312EL
Typ
Max
Units
SENSOR INPUT
Measurement Range
1
Each Axis
2.0
g
Nonlinearity
Best fit straight line
0.5
1.0
% of FS
Alignment Error
2
1.0
degrees
Transverse Sensitivity
3
2.0
%
SENSITIVITY
Sensitivity, Analog Outputs at pins
A
OUTX
and A
OUTY
5
Each Axis
296
312
328
mV/g
Change over Temperature (uncompensated)
4
from 25
C, at 40
C
+100
%
from 25
C, at +105
C
-50 %
Change over Temperature (compensated)
4
from 25
C, 40
C to +105
C
<3.0 %
ZERO g BIAS LEVEL
0 g Offset
5
Each Axis
-0.1
0.00
+0.1
g
0 g Voltage
5
2.47
2.50
2.53
V
0 g Offset over Temperature
from 25
C
from 25
C, based on 312mV/g
0.4
0.12
mg/
C
mV/
C
NOISE PERFORMANCE
Noise Density, rms
Without frequency compensation
0.2
0.4
mg/
Hz
FREQUENCY RESPONSE
3dB Bandwidth - uncompensated
17
Hz
TEMPERATURE OUTPUT
T
out
Voltage
1.23
1.25
1.27
V
Sensitivity
4.6
5.0
5.4
mV/
K
VOLTAGE REFERENCE
V
Ref
@3V-5.25V
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
@3V Supply, output rails to
supply voltage
3.0
V
A
OUTX
and A
OUTY
OUTPUTS
Normal Output Range
@5.0V Supply
@3V Supply
0.1
0.1
4.9
2.9
V
V
Current
Source or sink, @ 3V-5.0V supply
100
A
Turn-On Time
@5.0V Supply
@3V Supply
100
40
mS
mS
POWER SUPPLY
Operating Voltage Range
3.0
5.25
V
Supply Current
@ 5.0V
2.7
3.3
4.1
mA
Supply Current
5,6
@
3V
3.2
4.0
4.8
mA
TEMPERATURE RANGE
Operating Range
-40
+85
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
The device operates over a 3.0V to 5.25V supply range.
6
Note that the accelerometer has a constant heater power control circuit
thereby requiring higher supply current at lower operating voltage.
MEMSIC MXR2312EL
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05/30/2003
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
MXR2312EL 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
M
S
I
C
X +g
Y +g
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
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 following the right-hand rule.
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 3.0 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
.
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.
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.
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
-2.90
= S
f
x T
f
-2.90
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)
Se
n
s
it
iv
it
y
(
n
o
r
ma
lize
d
)
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
MEMSIC MXR2312EL
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MEMSIC MXR2312EL
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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 +85
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
MINIMUM 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
1g
ME
M
S
I
C
Figure 2: Accelerometer Position Relative to Gravity
X-Axis
Y-Axis
X-Axis
Orientatio
n
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
Resolution: The accelerometers resolution is limited by
noise. The output noise floor 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 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%). With a simple RC low pass
filter, the rms noise is calculated as follows:
Noise (mg rms ) = Noise(mg/ Hz )*
)
6
.
1
*
)
(
(
Hz
Bandwidth
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
zero g offset variations from part to part and zero g offset
change over temperature can be eliminated. Figure 3 is a
HFP (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 HFP 3dB point at a very low
frequency in order to detect very low frequency
accelerations. Sometimes the implementation of this HFP
may result in unreasonably large capacitors, and the
designer must turn to digital implementations of HFPs
where very low frequency 3dB breakpoints can be
achieved.
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