Low Cost,
±10
g
Dual Axis
Accelerometer with Digital Outputs
MXD2002A/B
FEATURES
Sck
(optional)
Internal
O scillator
Tem perature
Sensor
Voltage
R eference
Tout
Dual axis accelerometer fabricated on a monolithic CMOS IC
On-chip mixed mode signal processing
Resolution better than 6 milli-g
50,000
g
shock survival rating
30 Hz bandwidth
2.70V to 5.25V single supply operation
Small (5mm x 5mm x 2mm) surface mount package
Continuous self test
Independently programmable axis (factory special)
C LK
Vref
H eater
Control
C ontinous
Self Test
X axis
LPF
A/D
D out X
Factory Adjust
O ffset & G ain
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
Y axis
LPF
A/D
D out Y
2-AX IS
SENSO R
Vdd
Gnd
Vda
MXD2002A/B FUNCTIONAL BLOCK DIAGRAM
The MXD2002A/B is a very low cost, dual axis
accelerometer fabricated on a standard, submicron CMOS
process. The MXD2002A/B measures acceleration with a
full-scale range of
±10
g.
(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 MXD2002A/B 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 MXD2002A/B provides a digital output (ref. other
MEMSIC data sheets for analog or ratiometric analog
outputs). The outputs are digital signals with duty cycles
(ratio of pulsewidth to period) that are proportional to
acceleration. The duty cycle outputs can be directly
interfaced to a micro-processor.
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.
The typical noise floor is .0015% duty cycle
/
Hz
allowing
signals below 6 milli-g to be resolved at 1 Hz bandwidth.
The MXD2002A/B is available in a low profile LCC
surface mount package (5mm x 5mm x 2mm height). It is
hermetically sealed and operational over a -40°C to +105°C
temperature range.
Due to the standard CMOS structure of the MXD2002A/B,
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
MEMSIC MXD2002A/B Rev.E
Page 1 of
8
5/15/2004
MXD2002A/B SPECIFICATIONS
(Measurements @ 25°C, Acceleration = 0 unless otherwise noted, V
DD
, V
DA
=
5.0V unless otherwise specified)
MXD2002A/B
Parameter
SENSOR INPUT
Measurement Range
1
Nonlinearity
Alignment Error
2
Transverse Sensitivity
3
SENSITIVITY
D
OUTX
and D
OUTY
Change over Temperature (uncompensated)
4
Change over Temperature (compensated)
ZERO
g
BIAS LEVEL
0
g
Offset
5
0
g
Duty Cycle
5
0
g
Offset over Temperature
NOISE PERFORMANCE
Noise Density, % Duty Cycle
FREQUENCY RESPONSE
3dB Bandwidth
TEMPERATURE OUTPUT
T
out
Voltage
Sensitivity
VOLTAGE REFERENCE
V
Ref
Change over Temperature
Current Drive Capability
SELF TEST
Continuous Voltage at D
OUTX
, D
OUTY
under
Failure
Continuous Voltage at D
OUTX
, D
OUTY
under
Failure
D
OUTX
and D
OUTY
OUTPUTS
Normal Output Range
Current
Rise/Fall Time
POWER SUPPLY
Operating Voltage Range
Supply Current
Supply Current
6,7
TEMPERATURE RANGE
Operating Range
NOTES
4
Conditions
Each Axis
Best fit straight line
Min
±10
Typ
Max
Units
g
% of FS
degrees
%
1.0
±1.0
±2.0
1.80
-47
<3.0
-0.70
48.6
0.00
50
±2.0
±004
.0015
2.00
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 2%/g
2.20 % Duty
Cycle/g
+93
%
%
%
+0.70
g
51.4
% Duty Cycle
mg/°C
% /
°C
0.005 % Duty
Cycle/
Hz
30
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
Digital Signal of 100 Hz or 400Hz
@5.0V Supply
@2.7V Supply
Source or sink, @ 2.7V-5.0V supply
2.7 to 5.0V Supply
2.4
1.25
5.0
2.5
0.1
1.35
5.4
2.65
100
Hz
V
mV/°K
V
mV/°C
µA
5.0
2.7
0.1
0.1
90
100
100
4.9
2.6
110
V
V
V
V
µA
nSec
@ 5.0V
@ 2.7V
2.7
3.0
3.0
-40
5
4.2
5.4
5.25
4.9
6
+105
V
mA
mA
°C
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.
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.
6
Note that the accelerometer has a constant heater power control circuit
thereby displaying higher supply current at lower operating voltage.
MEMSIC MXD2002A/B 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.
Caution
ESD (electrostatic discharge) sensitive device.
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
.
D
OUTX
– This pin is the digital output of the x-axis
acceleration sensor. It is factory programmable to 100 Hz
or 400 Hz. The user should ensure the load impedance is
sufficiently high as to not source/sink >100µA 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.
D
OUTY
–
This pin is the digital output of the y-axis
Package Characteristics
Package
θ
JA
θ
JC
LCC-8
110°C/W
22°C/W
8
1
2
3
M E M S IC
7
Device Weight
< 1 gram
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).
Pin Description: LCC-8 Package
Pin
Name
Description
1
T
OUT
Temperature (Analog Voltage)
2
D
OUTY
Y-Axis Acceleration Digital Signal
3
Gnd
Ground
4
V
DA
Analog Supply Voltage
5
D
OUTX
X-Axis Acceleration Digital Signal
6
V
ref
2.5V Reference
7
Sck
Optional External Clock
8
V
DD
Digital Supply Voltage
Ordering Guide
Model
Package Style
D2002AL
D2002BL
LCC-8 SMD*
LCC-8 SMD*
Digital
Output
100 Hz
400 Hz
*LCC parts are shipped in tape and reel packaging.
acceleration sensor. It is factory programmable to 100 Hz
5/15/2004
MEMSIC MXD2002A/B Rev.E
Page 3 of
8
or 400 Hz. The user should ensure the load impedance is
sufficiently high as to not source/sink >100µA 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.
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
and the sensitivity expressed
in % duty cycle/g.
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. The 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
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 to 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
2.0
Sensitivity (normalized)
1.5
1.0
0.5
0.0
-40
-20
0
20
40
60
80
100
Temperature (C)
gravity
0
0
Y
Top View
Figure 1: Thermal Accelerometer Sensitivity
Figure 2: Accelerometer Position Relative to Gravity
MEMSIC MXD2002A/B Rev.E
Page 4 of
8
5/15/2004
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
DUTY CYCLE DEFINITION
The MXD2002A/B has two PWM duty cycle outputs (x,y).
The acceleration is proportional to the ratio T1/T2. The
zero
g
output is set to 50% duty cycle and the sensitivity
scale factor is set to 2% duty cycle change per
g.
These
nominal values are affected by the initial tolerance of the
device including zero
g
offset error and sensitivity error.
This device is offered from the factory programmed to
either a 10ms period (100 Hz) or a 2.5ms period (400Hz).
T1
T2 (Period)
Duty Cycle
Pulse width
Length of the “on” portion of the cycle.
Length of the total cycle.
Ratio of the “0n” time (T1) of the cycle to
the total cycle (T2). Defined as T1/T2.
Time period of the “on” pulse. Defined as
T1.
T2
Minimum Resolution:
Accelerometers can be used in a
wide variety of low g applications such as tilt and
orientation. The device noise floor will vary with the
measurement bandwidth. With the reduction of the
bandwidth the noise floor drops. This will improve the
signal to noise ratio of the measurement 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 as the rms
value (with an average uncertainty of .1%). The maximum
noise for 1.0Hz bandwidth will be 1 mg
/
Hz
. If the
bandwidth is increased to 10 Hz for example: 3.162 mg is
the maximum rms noise and 20.87mg is the maximum
peak -to-peak noise.
DIGITAL INTERFACE
The MXD2002A/B is easily interfaced with low cost
microcontrollers. For the digital output accelerometer, one
digital input port is required to read one accelerometer
output. For the analog output accelerometer, many low cost
microcontrollers are available today that feature integrated
a/d (analog to digital converters) with resolutions ranging
from 8 to 12 bits.
In many applications the microcontroller provides an
effective approach for the temperature compensation of the
sensitivity and the zero
g
offset. Specific code set, reference
designs, and applications notes are available from the
factory. The following parameters must be considered in a
digital interface:
Resolution:
smallest detectable change in input acceleration
Bandwidth:
detectable accelerations in a given period of
time
Acquisition Time:
the duration of the measurement of the
acceleration signal
MEMSIC MXD2002A/B Rev.E
Page 5 of
8
T1
A (g)= (T1/T2 - 0.5)/2%
0g = 50% Duty Cycle
T2= 2.5ms or 10ms (factory programmable)
Figure 4: Typical output Duty C ycle
CHOOSING T2 AND COUNTER FREQUENCY
DESIGN TRADE-OFFS
The noise level is one determinant of accelerometer
resolution. The second relates to the measurement
resolution of the counter when decoding the duty cycle
output. The actual resolution of the acceleration signal is
limited by the time resolution of the counting devices used
to decode the duty cycle. The faster the counter clock, the
higher the resolution of the duty cycle and the shorter the
T2 period can be for a given resolution. Table 2 shows
some of the trade-offs. It is important to note that this is the
resolution due to the microprocessors’ counter. It is
probable that the accelerometer’s noise floor may set the
lower limit on the resolution.
Counter-
Reso-
Counts
Clock
MEMSIC
lution
Counts
Per T2
Rate
Sample
(mg)
per
g
Cycle
(MHz)
Rate
T2 (ms)
2.5
400
2.0
5000
100
10
2.5
400
1.0
2500
50
20
2.5
400
0.5
1250
25
40
10.0
100
2.0
20000
400
2.5
10.0
100
1.0
10000
200
5.0
10.0
100
0.5
5000
100
10
Table 2: Trade-Offs Between Microcontroller Counter Rate and
T2 Period.
5/15/2004
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