Improved, Ultra Low Noise
±1
g
Dual Axis Accelerometer with
Digital Outputs
MXD2020GL/HL
MXD2020ML/NL
FEATURES
Resolution better than 1 milli-g
Dual axis accelerometer fabricated on a monolithic CMOS IC
On chip mixed mode signal processing
No moving parts
50,000
g
shock survival rating
17 Hz bandwidth expandable to >160 Hz
3.0V to 5.25V single supply continuous operation
Continuous self test
Independent axis programmability (special order)
Compensated for Sensitivity over temperature
Ultra low initial Zero-g Offset
Sck
(optional)
Internal
Oscillator
Tem perature
Sensor
Voltage
R eference
C ontinous
Self Test
Tout
C LK
Vref
H eater
C ontrol
X axis
LPF
A/D
Dout X
Factory Adjust
Offset & Gain
Y axis
LPF
A/D
Dout Y
APPLICATIONS
Automotive –
Vehicle Security/Vehicle stability control/
Headlight Angle Control/Tilt Sensing
Security
– Gas Line/Elevator/Fatigue Sensing
Information Appliances
– Computer Peripherals/PDA’s/Mouse
Smart Pens/Cell Phones
Gaming
– Joystick/RF Interface/Menu Selection/Tilt Sensing
GPS
–
electronic Compass tilt Correction
Consumer
–
LCD projectors, pedometers, blood pressure
Monitor, digital cameras
GENERAL DESCRIPTION
The MXD2020GL/HL/ML/NL is a 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
MXD2020GL/HL/ML/NL measures acceleration with a
full-scale range of
±1
g
and a sensitivity of 20%/g. It can
measure both dynamic acceleration (e.g. vibration) and
static acceleration (e.g. gravity).
The MXD2020GL/HL/ML/NL 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 of 50,000
g,
leading to significantly lower failure rate and lower loss
due to handling during assembly.
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.
2-AXIS
SEN SOR
Vdd
G nd
Vda
MXD2020GL/HL/ML/NL FUNCTIONAL BLOCK
DIAGRAM
The MXD2020GL/HL/ML/NL provides two digital outputs
that are set to 50% duty cycle at zero g acceleration. The
outputs are digital with duty cycles (ratio of pulse width to
period) that are proportional to acceleration. The duty
cycle outputs can be directly interfaced to a micro-
processor.
The typical noise floor is 0.2 mg/
Hz
allowing signals
below 1 milli-g to be resolved at 1 Hz bandwidth.
The
MXD2020GL/HL/ML/NL is packaged in a hermetically
sealed LCC surface mount package (5 mm x 5 mm x 2 mm
height) and is operational over a -40
°C
to 105
°C(M/NL)
and
0°C to 70°C(G/HL) 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 MXD2020GL/ML
Page 1 of 7
2002.08.29.1
unless otherwise specified)
Parameter
SENSOR INPUT
Measurement Range
1
MXD2020GL/HL/ML/NL SPECIFICATIONS
(Measurements @ 25°C, Acceleration = 0
g
unless otherwise noted; V
DD
, V
DA
= 5.0V
MXD2020G/HL
Typ
MXD2020M/NL
Typ
Conditions
Each Axis
Best fit straight line
X Sensor to Y Sensor
Each Axis
Min
±1.0
Max
Min
±1.0
Max
Units
g
Nonlinearity
Alignment Error
2
Transverse Sensitivity
3
SENSITIVITY
Sensitivity, Digital Outputs at
pins
D
OUTX
and D
OUTY4
Change
over Temperature
ZERO
g
BIAS LEVEL
0
g
Offset
4
0
g
Duty Cycle
4
0
g
Offset over Temperature
0.5
±1.0
±2.0
19
-10
20
21
+8
0.0
50
±1.5
±0.03
0.2
12
1.21
4.6
17
1.25
5.0
2.5
0.1
1.29
5.4
2.65
100
0.4
12
1.21
4.6
2.4
+0.1
52
19
-25
-0.1
48
0.5
±1.0
±2.0
20
21
+8
0.0
50
±1.5
±0.03
0.2
17
1.25
5.0
2.5
0.1
1.29
5.4
2.65
100
0.4
+0.1
52
% of FS
degrees
%
% duty
cycle/g
%
g
% duty
cycle
mg/°C
%/°C
mg/
Hz
Hz
V
mV/°K
V
mV/°C
µA
Each Axis
-0.1
48
Based on 20%/g
NOISE PERFORMANCE
Noise Density, rms
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
Turn-On Time
POWER SUPPLY
Operating Voltage Range
Supply Current
Supply Current
4
TEMPERATURE RANGE
Operating Range
NOTES
1
2
@3.0V-5.0V supply
Source
@5.0V Supply, output
rails to
supply voltage
@3.0V Supply, output
rails to
supply voltage
@5.0V Supply
@3.0V Supply
Source or sink, @
3.0V-5.0V supply
3.0 to 5.0V supply
@5.0V Supply
@3.0V Supply
2.4
5.0
3.0
5.0
3.0
V
V
0.1
0.1
100
90
100
100
40
4.9
2.9
110
0.1
0.1
100
90
100
100
40
4.9
2.9
110
V
V
µA
nS
mS
mS
V
mA
mA
°C
@ 5.0V
@ 3.0V
3.0
2.7
3.2
0
3.8
4.7
5.25
4.4
5.4
+70
3.0
2.7
3.2
-40
3.8
4.7
5.25
4.4
5.4
+105
Guaranteed by measurement of initial offset and sensitivity.
level specifications on this page will be met. Please contact the factory for specially
trimmed devices for low supply voltage operation.
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.
The device operates over a 3.0V to 5.25V supply range. Please note that sensitivity
and zero
g
bias level will be slightly different at 3.0V operation. For devices to be
operated at 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
4
MEMSIC MXD2020GL/ML/NL/HL
Page 2 of 7
2002.08.29.1
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.
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
LCC - 8
MXD2020GL
MXD2020HL
MXD2020ML
MXD2020NL
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.
Digital
Output
100 Hz
400Hz
100 Hz
400 Hz
Temperature
Range
0 to 70°C
0 to 70°C
-40 to 105°
-40 to 105°
LCC - 8
LCC - 8
LCC - 8
All parts are shipped in tape and reel packaging.
Caution:
ESD (electrostatic discharge) sensitive device.
8
1
2
3
7
X +g
6
5
Y +g
Top V iew
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.
MEMSIC MXD2020GL/ML/NL/HL
M EM SIC
4
Page 3 of 7
2002.08.29.1
MXD2020GL/HL/ML/NL 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
. 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
.
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
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).
X
+90
0
C
I
S
M
E
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 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.
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.
gravity
0
0
Y
Top View
Figure 2: Accelerometer Position Relative to Gravity
M
X-Axis
X-Axis
Orientatio
n
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
MEMSIC MXD2020GL/ML/NL/HL
Page 4 of 7
2002.08.29.1
Pulse width
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%).
DIGITAL INTERFACE
The MXD2020GL/HL/ML/NL 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 MXD2020GL/HL/ML/NL 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
T2 (Period)
Duty Cycle
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.
Page 5 of 7
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 3: 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.
MEMSIC
Sample
Rate
400
400
400
100
100
100
Counter-
Clock
Rate
(MHz)
2.0
1.0
0.5
2.0
1.0
0.5
Counts
Per T2
Cycle
5000
2500
1250
20000
10000
5000
Reso-
lution
(mg)
1.0
2.0
4.0
0.25
0.5
1.0
T2 (ms)
2.5
2.5
2.5
10.0
10.0
10.0
Counts
per
g
1000
500
250
4000
2000
1000
Table 2: Trade-Offs Between Microcontroller Counter Rate and
T2 Period.
USING THE ACCELEROMETER IN VERY LOW
POWER APPLICATIONS (BATTERY OPERATION)
In applications with power limitations, power cycling can
be used to extend the battery operating life. One important
consideration when power cycling is that the accelerometer
turn on time limits the frequency bandwidth of the
accelerations to be measured. For example, operating at
3.0V the turn on time is 40mS. To double the operating
time, a particular application may cycle power ON for
40mS, then OFF for 40mS, resulting in a measurement
period of 80mS, or a frequency of 12.5Hz. With a
frequency of measurements of 12.5Hz, accelerations
changes as high as 6.25Hz can be detected. Power cycling
can be used effectively in many inclinometry applications,
where inclination changes can be slow and infrequent.
MEMSIC MXD2020GL/ML/NL/HL
2002.08.29.1
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