MEMSIC MXD2020U/W Rev A Page 1 of 8 05/02
Ultra Low Noise,
1 g Dual Axis
Accelerometer with Digital Outputs
MXD2020U/W
FEATURES
Resolution better than 1 milli-g
Dual axis accelerometer fabricated on a monolithic CMOS IC
On-chip mixed mode signal processing
50,000 g shock survival rating
25 Hz bandwidth
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
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
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
V
REF
D
OUTX
V
DD
V
DA
Gnd
2-AXIS
SENSOR
D
OUTY
T
OUT
Continous
Self Test
MXD2020U/W FUNCTIONAL BLOCK DIAGRAM
GENERAL DESCRIPTION
The MXD2020U/W is an ultra low noise and low cost, dual
axis accelerometer built on a standard, submicron CMOS
process. The MXD2020U/W measures acceleration with a
full-scale range of
1 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 MXD2020U/W 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 MXD2020U/W provides a digital output. The outputs
are digital signals with duty cycles (ratio of pulse width to
period) that are proportional to acceleration. The duty
cycles 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 0.2mg / Hz allowing signals
below 1 milli-g to be resolved at 1 Hz bandwidth. The
MXD2020U/W 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 +105C
temperature range.
Due to the standard CMOS structure of the MXD2020U/W,
additional circuitry can easily be incorporated into custom
versions for high volume applications. Contact the factory
for more information.
MEMSIC, Inc.
800 Turnpike St., Suite 202, North Andover, MA 01845
Tel: 978.738.0900
Fax: 978.738.0196
www.memsic.com
MEMSIC MXD2020U/W Rev A Page 2 of 8 05/02
MXD2020U/W SPECIFICATIONS
(Measurements @ 25
C, Acceleration = 0 unless otherwise noted, V
DD
, V
DA
=
5.0V unless otherwise specified)
Parameter
Conditions
Min
MXD2020U/W
Typ
Max
Units
SENSOR INPUT
Measurement Range
1
Each Axis
1.0
g
Nonlinearity
Best fit straight line
0.5
1.0
% of FS
Alignment Error
2
1.0
degrees
Transverse Sensitivity
3
2.0
%
SENSITIVITY
D
OUTX
and D
OUTY
Each Axis
@5.0V supply
19.00
20.00
21.00
% Duty
Cycle/g
Change over Temperature (uncompensated)
4
from 25C, at 40C
+100
%
from 25C, at +105C
-50 %
Change over Temperature (compensated)
4
from 25C, 40C to +105C
<3.0 %
ZERO g BIAS LEVEL
0 g Offset
5
Each Axis
-0.1
0.00
+0.1
g
0 g Duty Cycle
5
48
50
52
% Duty Cycle
0 g Offset over Temperature
from 25C
from 25C, based on 20%/g
0.75
0.015
mg/
C
% /
C
NOISE PERFORMANCE
Noise Density, rms
0.2
0.4
mg/
Hz
FREQUENCY RESPONSE
3dB Bandwidth
25
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
@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 D
OUTX
, D
OUTY
under
Failure
@5.0V Supply, output rails to
supply voltage
5.0
V
Continuous Voltage at D
OUTX
, D
OUTY
under
Failure
@2.7V Supply, output rails to
supply voltage
2.7
V
D
OUTX
and D
OUTY
OUTPUTS
Digital Signal of 100 Hz or 400 Hz
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
Rise/Fall Time
2.7 to 5.0V Supply
90
100
110
nS
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
3.6
4.2
mA
Supply Current
5,6
@
2.7V
4.0
4.9
5.8
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
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.
The device operates over a 2.7V to 5.0V 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
6
Note that the accelerometer has a constant heater power control circuit thereby
displaying higher supply current at lower operating voltage.
MEMSIC MXD2020U/W Rev A Page 3 of 8 05/02
ABSOLUTE MAXIMUM RATINGS*
Supply Voltage (V
DD
, V
DA
) .....................-0.5 to +7.0V
Storage Temperature ......................-65
C to +150C
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 22C//W
< 1 gram
Ordering Guide
Model Package
Style
Digital
Output
D2020UL
LCC-8 SMD*
100 Hz
D2020WL LCC-8 SMD*
400 Hz
*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 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
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
MEMSIC MXD2020U/W Rev A Page 4 of 8 05/02
sufficiently high as to not source/sink >100
A 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
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 >100
A 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 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
2.81
= S
f
x T
f
2.81
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. 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 +50C 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 +105C) 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 to 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
MEMSIC MXD2020U/W Rev A Page 5 of 8 05/02
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
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: 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. With a simple RC low pass
filter, the rms noise is calculated as follows:
Noise (mg rms) = Noise(mg/ Hz ) *
)
6
.
1
*
)
(
(
Hz
Bandwidth
The peak-to-peak noise is approximately equal to 6.6 times
the rms value (for an average uncertainty of 0.1%).
DIGITAL INTERFACE
The MXD2020U/W 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
DUTY CYCLE DEFINITION
The MXD2020U/W 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 20% 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
Length of the "on" portion of the cycle.
T2 (Period)
Length of the total cycle.
Duty Cycle
Ratio of the "0n" time (T1) of the cycle to
the total cycle (T2). Defined as T1/T2.
Pulse width
Time period of the "on" pulse. Defined as
T1.
T2
T1
A (g)= (T1/T2 - 0.5)/20%
0g = 50% Duty Cycle
T2= 2.5ms or 10ms (factory programmable)
Figure 4: Typical output Duty Cycle
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.
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