ETC1 MXD7202NL Low cost, low noise â±2 g dual axis accelerometer with digital output Datasheet

Low Cost, Low Noise ±2 g Dual Axis
Accelerometer with Digital Outputs
MXD7202GL/HL/ML/NL
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; No loose particle issues
>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
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 MXD7202GL/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
signal processing. The MXD7202GL/HL/ML/NL measures
acceleration with a full-scale range of ±2 g and a sensitivity
of 12.5%/g @5V. It can measure both dynamic acceleration
(e.g. vibration) and static acceleration (e.g. gravity). The
MXD7202GL/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 greater than
50,000 g, leading to significantly lower failure rate and lower
loss due to handling during assembly and at customer field
application.
MXD7202GL/HL/ML/NL FUNCTIONAL BLOCK DIAGRAM
The MXD7202GL/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 microprocessor.
The typical noise floor is 0.3 mg/ Hz allowing signals
below 1 milli-g to be resolved at 1 Hz bandwidth. The
MXD7202GL/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 0°C to 70°C(GL/HL) or a 40°C to +85°C(ML/NL) temperature range.
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 MXD7202GL/HL/ML/NL
Page 1 of 10
MEMSIC, Inc.
800 Turnpike St., Suite 202, North Andover, MA 01845
Tel: 978.738.0900
Fax: 978.738.0196
www.memsic.co
2003.08.04
MXD7202GL/HL/ML/NL SPECIFICATIONS (Measurements @ 25°C, Acceleration = 0 g unless otherwise noted; VDD = 5.0V
unless otherwise specified)
MXD7202ML/N
MXD7202GL/H
Parameter
Conditions
Min
Max
L
Min.
SENSOR INPUT
L
Max.
Units
Typ.
Typ.
Each Axis
1
±2.0
Measurement Range
Nonlinearity
±2.0
Best fit straight line
2
Alignment Error
Alignment Error
Cross Axis Sensitivity
X Sensor to Y Sensor
3
SENSITIVITY
g
0.5
0.5
% of FS
±1.0
±1.0
degrees
0.01
0.01
Degrees
±0.5
±0.5
%
Each Axis
Duty cycle per g
VDD=5.0V
11.5
12.5
13.5
11.5
12.5
13.5
%/g
Duty cycle per g
VDD=3.0V
11.3
12.5
13.7
11.3
12.5
13.7
%/g
15
%
Sensitivity Change over
Temperature4
10
Delta from 25°C
Each Axis
ZERO g BIAS LEVEL
0 g Duty cycle
VDD=5.0V
47
50
53
47
50
53
%
0 g Duty cycle
VDD=5.0V
-0.24
0.00
0.24
-0.24
0.00
0.24
g
0 g Duty cycle
VDD=3.0V
46
50
54
46
50
54
%
VDD=3.0V
-0.32
0.00
0.32
-0.32
0.00
0.32
g
0 g Duty cycle
4
0 g Offset vs. Temperature
Delta from 25°C
1.5
@25°C
0.3
1.5
mg/°C
NOISE PERFORMANCE
Noise Density, rms
0.8
0.3
0.8
mg/
Hz
FREQUENCY RESPONSE
3dB Bandwidth
19
19
Hz
DUTY CYCLE OUTPUT STAGE
Output Low Voltage
Current
V
Vs-0.2V
Vs-0.2V
Output High Voltage
Source or sink @
0.2
0.2
V
250
250
uA
-600
ppm/°C
3.0V- 5.0V Supply
T25 Drift vs. Temperature
-900
-750
-600
-900
200
200
Rise/Fall Time
-750
ns
POWER SUPPLY
Operating Voltage Range
2.7
5.25
2.7
5.25
V
Quiescent Supply Current
@5.0V
3.0
3.8
3.0
3.8
mA
Quiescent Supply Current
@3.0V
3.4
4.3
3.4
4.3
mA
Level (0g)
100
Turn-On Time
100
mS
TEMPERATURE RANGE
Operating Range
0
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.
MEMSIC MXD7202GL/HL/ML/NL
+70
-40
°C
+85
3
Cross axis sensitivity is the algebraic sum of the alignment and the inherent
sensitivity errors.
4
Defined as the output change from ambient to maximum temperature or ambient to
minimum temperature.
5
Duty cycle period
Page 2 of 10
2003.08.04
ABSOLUTE MAXIMUM RATINGS*
Supply Voltage (VDD)
………………...-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
NC
Do Not Connect
2
TP
Connected to ground
3
COM
Common
4
Yout
Y Channel Duty Cycle Output
5
Xout
X Channel Duty Cycle Output
6
NC
Do Not Connect
7
NC
Do Not Connect
8
VDD
2.7V to 5.25V
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.
Ordering Guide
PWM
Frequency
Temperature
Range
Device
Weight
MXD7202GL
100Hz
0 to 70°C
<1.0 gram
MXD7202HL
400Hz
0 to 70°C
<1.0 gram
MXD7202ML
100Hz
-40 to +85°C
<1.0 gram
MXD7202NL
400Hz
-40 to +85°C
<1.0 gram
Model
All parts are shipped in tape and reel packaging.
Caution: ESD (electrostatic discharge) sensitive device.
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.
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 MXD7202GL/HL/ML/NL
Page 3 of 10
2003.08.04
be achieved with the MEMSIC device (reference
application note AN-00MX-007).
MEMSIC
MXD7202GL/HL/ML/NL PIN DESCRIPTIONS
VDD – 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.
COM– This is the ground pin for the accelerometer.
TP– This pin should be connected to ground.
Figure 2: Accelerometer Position Relative to Gravity
Xout – This pin is the digital output of the X-axis
acceleration sensor. It is factory programmable to 100Hz or
400Hz. 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.
X-Axis
X-Axis
Orientation
To Earth’s
Surface
(deg.)
90
85
80
70
60
45
30
20
10
5
0
Yout – This pin is the digital output of the Y-axis
acceleration sensor. It is factory programmable to 100Hz or
400Hz. 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.
X Output
(g)
Change
per deg.
of tilt
(mg)
Y-Axis
Change
per deg.
of tilt
(mg)
Y Output
(g)
-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
17.45
17.37
17.16
16.35
15.04
12.23
8.59
5.86
2.88
1.37
0.15
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%).
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
MEMSIC MXD7202GL/HL/ML/NL
Page 4 of 10
2003.08.04
DIGITAL INTERFACE
The MXD7202GL/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 MXD7202GL/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 12.5% 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.
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.
T2 (ms)
2.5
2.5
2.5
10.0
10.0
10.0
CounterClock
Rate
(MHz)
2.0
1.0
0.5
2.0
1.0
0.5
Counts
Per T2
Cycle
5000
2500
1250
20000
10000
5000
Counts
per g
625
313
156
2500
1250
625
Resolution
(mg)
1.6
3.2
6.4
0.4
0.8
1.6
Table 2: Trade-Offs Between Microcontroller Counter Rate and
T2 Period.
CONVERTING THE DIGITAL OUTPUT TO AN
ANALOG OUTPUT
The PWM output can be easily converted into an analog
output by integration. A simple RC filter can do the
conversion. Note that that the impedance of the circuit
following the integrator must be much higher than the
impedance of the RC filter. Reference figure 4 for an
example.
DOUT
MEMSIC
Accel.
T2
T1
MEMSIC
Sample
Rate
400
400
400
100
100
100
10K
AOUT
1uF
Figure 4: Converting the digital output to an analog
voltage
A (g)= (T1/T2 - 0.5)/0.125
0g = 50% Duty Cycle
T2= 2.5ms or 10ms (factory programmable)
Figure 3: Typical output Duty C ycle
MEMSIC MXD7202GL/HL/ML/NL
Page 5 of 10
2003.08.04
PCB LAYOUT AND FABRICATION SUGGESTIONS
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 (VDD). 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.
1.
2.
3.
4.
5.
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.
Figure 5: Power Supply Noise Rejection
MEMSIC MXD7202GL/HL/ML/NL
Page 6 of 10
2003.08.04
MXD7202GL/HL/ML/NL TYPICAL PERFORMANCE CHARACTERISTICS ( @ 25°C, unless otherwise specified)
VDD = 3V
VDD = 5V
45%
70%
PERCENT OF PARTS
40%
60%
PERCENT OF PARTS
35%
30%
25%
20%
15%
10%
5%
0%
50%
40%
30%
20%
10%
0%
46
47
48
49
50
51
52
53
47
54
%
X-axis Zero g Bias Distribution at Xout, VDD=3V
50
51
52
53
60%
45%
50%
PERCENT OF PARTS
40%
PERCENT OF PARTS
49
%
X-axis Zero g Bias Distribution at Xout, VDD=5V
50%
35%
30%
25%
20%
15%
10%
5%
40%
30%
20%
10%
0%
0%
46
47
48
49
50
51
52
53
54
47
%
Y-axis Zero g Bias Distribution at Xout, VDD=3V
48
49
50
51
52
53
%
Y-axis Zero g Bias Distribution at Xout, VDD=5V
35%
25%
30%
20%
PERCENT OF PARTS
PERCENT OF PARTS
48
15%
10%
5%
0%
11.3
11.9
12.5
13.1
13.7
25%
20%
15%
10%
5%
0%
11.5
%/g
X-axis Sensitivity Distribution at Xout, VDD=3V
MEMSIC MXD7202GL/HL/ML/NL
12.0
12.5
13.0
13.5
%/g
X-axis Sensitivity Distribution at Xout, VDD=5V
Page 7 of 10
2003.08.04
45%
30%
40%
PERCENT OF PARTS
PERCENT OF PARTS
25%
20%
15%
10%
5%
0%
11.3
11.9
12.5
13.1
35%
30%
25%
20%
15%
10%
5%
0%
11.5
13.7
12.0
12.5
13.0
13.5
%/g
Y-axis Sensitivity Distribution at Yout, VDD=5V
%/g
Y-axis Sensitivity Distribution at Yout, VDD=3V
1.10
1.08
1.06
1.04
1.02
1.00
0.98
0.96
0.94
0.92
0.90
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
70
80
90
T(°C)
Normalized (with 25°C) X-axis Sensitivity vs. Temperature, VDD=5V
1.10
1.08
1.06
1.04
1.02
1.00
0.98
0.96
0.94
0.92
0.90
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
T(°C)
Normalized (with 25°C) Y-axis Sensitivity vs. Temperature, VDD=5V
MEMSIC MXD7202GL/HL/ML/NL
Page 8 of 10
2003.08.04
Offset(%)
52.5
52.0
51.5
51.0
50.5
50.0
49.5
49.0
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
50
60
70
80
90
T(°C)
X-axis Zero g Offset vs. Temperature, VDD=5V
Offset(%)
52.5
52.0
51.5
51.0
50.5
50.0
49.5
49.0
-50
-40
-30
-20
-10
0
10
20
30
40
T(°C)
Y-axis Zero g Offset vs. Temperature, VDD=5V
MEMSIC MXD7202GL/HL/ML/NL
Page 9 of 10
2003.08.04
LCC-8 PACKAGE DRAWING
Fig 6: Hermetically Sealed Package Outline
MEMSIC MXD7202GL/HL/ML/NL
Page 10 of 10
2003.08.04
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