Low Cost, Low Noise
±2
g
Dual
Axis Accelerometer with
Ratiometric Analog Outputs
MXR7311GL
FEATURES
Low cost
Resolution better than 1 milli-g
Dual axis accelerometer fabricated on a monolithic
CMOS IC
On chip mixed signal processing
No moving parts; >50,000
g
shock survival rating
5mm X 5mm X 2mm LCC package
2.7V to 5.25V single supply continuous operation
Compensated for Sensitivity over temperature
Ultra low initial Zero-g Offset
No adjustment needed outside
APPLICATIONS
Tilt and motion sensing in cost-sensitive applications
Smart handheld devices
Computer security
Input devices
Pedometers and activity monitors
Gaming controllers
Toys and entertainment products
GENERAL DESCRIPTION
The MXR7311GL is a low cost, dual axis accelerometer
fabricated on a standard, submicron CMOS process. It is a
complete sensing system with on-chip mixed signal
processing. The MXR7311GL measures acceleration with a
full-scale range of
±2
g
and a sensitivity of 167mV/g @3V.
It can measure both dynamic acceleration (e.g. vibration) and
static acceleration (e.g. gravity). The MXR7311GL 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 greater than
50,000
g,
leading to significantly lower failure rate and lower
loss due to handling during assembly and at customer field
application.
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.
MXR7311GL FUNCTIONAL BLOCK DIAGRAM
The MXR7311GL provides two ratiometric analog outputs
that are proportional to 50% of the supply at zero g
acceleration.
The typical noise floor is 0.3 mg/
Hz
allowing signals
below 1 milli-g to be resolved at 1 Hz bandwidth.
The
MXR7311GL is packaged in a hermetically sealed LCC
surface mount package (5 mm x 5 mm x 2 mm height) and is
operational over a
0°C to 70°C temperature range.
MEMSIC,
Inc.
800 Turnpike St., Suite 202, North Andover, MA 01845
Tel: 978.738.0900
Fax: 978.738.0196
www.memsic.com
MEMSIC MXR7311GL Rev.B
Page 1 of 1
2004.03.15
specified)
MXR7311GL SPECIFICATIONS
(Measurements @ 25°C, Acceleration = 0
g
unless otherwise noted; V
DD
= 3.0V unless otherwise
Parameter
SENSOR INPUT
Measurement Range
1
Nonlinearity
Alignment Error
2
Alignment Error
Cross Axis Sensitivity
3
SENSITIVITY
Sensitivity,
Sensitivity Change over
Temperature
4
Conditions
Each Axis
Best fit straight line
X Sensor to Y Sensor
Each Axis
V
DD
= 3.0V
Delta from 25°C
(From 0°C to 70°C)
(From 10°C to 40°C)
5
Min
MXR7311GL
Typ
±2.0
0.5
±1.0
0.01
±0.5
Max
Units
g
% of FS
degrees
degrees
%
153
167
181
10
3
mV/g
%
%
V
g
mg/°C
mg/
Hz
Hz
ZERO
g
BIAS LEVEL
0
g
Offset
0 g Offset
0
g
Offset vs. Temperature
4
NOISE PERFORMANCE
Noise Density, rms
FREQUENCY RESPONSE
3dB Bandwidth
POWER SUPPLY
Operating Voltage Range
Quiescent Supply Current
AoutX and AoutY OUTPUTS
Output High Voltage
Output Low Voltage
Current
Turn-On Time
TEMPERATURE RANGE
Operating Range
NOTES
1
2
Each Axis
VDD = 3.0V
Delta from 25°C
@25°C
1.46
-0.24
1.50
0.00
1.5
0.3
19
1.54
0.24
2.7
@3.0V supply
@3.0V supply
Source or sink @ 3.0V-
5.0V Supply
Level (0g), @3.0V
supply
0
Vs-0.25
4.2
5.25
5.1
0.25
250
V
mA
V
V
uA
mS
°C
140
+70
Guaranteed by measurement of initial offset and sensitivity.
Alignment error is specified as the angle between the true and indicated axis of
sensitivity.
3
Cross axis sensitivity is the algebraic sum of the alignment and the inherent
sensitivity errors.
Defined as the output change from ambient to maximum temperature or ambient to
minimum temperature.
5
4
A simple RC filter should be added at Aoutput to eliminate noise effect.
MEMSIC MXR7311GL Rev.D
Page 2 of 2
2004.03.15
ABSOLUTE MAXIMUM RATINGS*
………………...-0.5 to +7.0V
Supply Voltage (V
DD
)
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.
Pin Description: LCC-8 Package
Pin
Name
Description
1
NC
Do Not Connect
2
TP
Connect to ground
3
COM
Common
4
NC
Do Not Connect
5
NC
Do Not Connect
6
Yout
Y Channel Output
7
Xout
X Channel Output
8
V
DD
2.7V to 5.25V
Ordering Guide
Model
Package
Style
LCC - 8
MXR7311GL
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.
Device
Weight
<1.0 gram
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.
Temperature
Range
0 to 70°C
All parts are shipped in tape and reel packaging.
Caution:
ESD (electrostatic discharge) sensitive device.
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. Small
circle indicates pin one(1).
MEMSIC MXR7311GL Rev.D
Page 3 of 3
2004.03.15
COM–
This is the ground pin for the
accelerometer
.
TP-
This pin should be connected to the ground.
Xout
– 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 >250µA typical.
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.
Yout
–
This pin is the output of the y-axis acceleration
Figure 2: Accelerometer Position Relative to Gravity
MEMSIC
MXR7311GL PIN DESCRIPTIONS
V
DD
– This is the supply input for the circuits and the
sensor heater in the
accelerometer
. The DC voltage should
be between 2.7 and 5.25 volts. Refer to the section on PCB
layout and fabrication suggestions for guidance on external
parts and connections recommended.
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)
sensor. The user should ensure the load impedance is
sufficiently high as to not source/sink >250µA typical.
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.
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).
-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
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. With a simple RC low pass filter, the rms
noise is calculated as follows:
Noise (mg rms) = Noise(mg/
Hz
) *
(
Bandwidth
(
Hz
) *1.6)
The peak-to-peak noise is approximately equal to 6.6 times
the rms value (for an average uncertainty of 0.1%).
MEMSIC MXR7311GL Rev.D
Page 4 of 4
2004.03.15
USING THE MXR7311GL WITH OPERATING
VOLTAGES OTHER THAN 3V
The MXR7311GL is tested and specified at V
DD
=3V;
however, it can be powered with V
DD
as low as 2.7V or as
high as 5.25V. Some performance parameters will change
as the supply voltage is varied. The MXR7311GL output
sensitivity will be linearly proportional to supply voltage.
At 5V the output sensitivity is typically 278mV/g. The zero
g bias output is also ratiometric, so the zero g is normally
equal to V
DD
/2 at all supply voltages.
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
. In many applications it may be
equation:
f
=
1
2
π
RC
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 may turn to digital implementations of HFPs
where very low frequency –3dB breakpoints can be
achieved.
A
OUTX
R
C
A
OUTX
F iltered
O u tp u t
A
OUTY
R
C
A
O UTY
F iltered
O u tp u t
Figure 4: Low Pass Filter
POWER SUPPLY NOISE REJECTION
One capacitor is recommended for best rejection of power
supply noise (reference Figure 5 below). The capacitor
should be located as close as possible to the device supply
pin (V
DD
). The capacitor lead length should be as short as
possible, and surface mount capacitor is preferred. For
typical applications, the capacitor can be ceramic 0.1 µF.
A
OUTX
C
R
C
A
O UTX
Filtered
Output
Figure 5: Power Supply Noise Rejection
A
OUTY
A
O UTY
Filtered
Output
PCB LAYOUT AND FABRICATION SUGGESTIONS
1.
2.
3.
Liberal use of ceramic bypass capacitors is
recommended.
Robust low inductance ground wiring should be used.
Care should be taken to ensure there is “thermal
symmetry” on the PCB immediately surrounding the
MEMSIC device and that there is no significant heat
source nearby.
A metal ground plane should be added directly beneath
the MEMSIC device. The size of the plane should be
similar to the MEMSIC device’s footprint and be as
thick as possible.
Vias can be added symmetrically around the ground
plane. Vias increase thermal isolation of the device
from the rest of the PCB.
R
Figure 3: High Pass Filter
Low Pass Filter:
An external low pass filter would be
useful in low frequency applications such as tilt or
inclination. The low pass filter limits the noise floor and
improves the resolution of the accelerometer. The low pass
filter shown in Figure 4 has a –3dB breakpoint given by the
. For the 200 Hz ratiometric output
equation:
f
=
1
2
π
RC
device filter, C=0.1µF and R=8kΩ,
±5%,
1/8W.
4.
5.
MEMSIC MXR7311GL Rev.D
Page 5 of 5
2004.03.15
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