Low Profile, Low Consumption,
±2
g
Dual Axis Accelerometer with
Digital Outputs
MXD6235M
FEATURES
Low power consumption: typically 2mA @ 3.0V
RoHS compliant
Resolution better than 1 mg
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
Low profile LCC package: 5mm
X
5mm
X
1.55mm
2.7V to 3.6V single supply continuous operation
No adjustment needed outside
APPLICATIONS
Security
– Gas Line/Elevator/Fatigue Sensing
I
nformation 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 MXD6235M 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 MXD6235M measures acceleration with a
full-scale range of
±2
g
and a sensitivity of 12.5%/g @3.0V
at 25
°C
. It can measure both dynamic acceleration (e.g.
vibration) and static acceleration (e.g. gravity). The
MXD6235M 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.
The MXD6235M 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 maximum noise floor is 1 mg/
Hz
allowing signals
below 1mg to be resolved at 1 Hz bandwidth. The
MXD6235M is packaged in a hermetically sealed, low
profile LCC surface mount package (5 mm x 5 mm x 1.55
mm height).
It is operational over a -40°C to 85°C temperature
range.
MXD6235M FUNCTIONAL BLOCK DIAGRAM
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,
Inc.
One Technology Drive Suite 325,Andover MA01810,USA
Tel: +1 978 738 0900
Fax: +1 978 738 0196
www.memsic.com
MEMSIC MXD6235M Rev. A
Page 1 of 6
4/7/2010
specified)
MXD6235M SPECIFICATIONS (
Measurements @ 25°C, Acceleration = 0
g
unless otherwise noted; V
DD
= 3.0V unless otherwise
MXD6235M
Parameter
SENSOR INPUT
Measurement Range
1
Nonlinearity
Alignment Error
2
Transverse Sensitivity
3
SENSITIVITY
At outputs
Change over Temperature
ZERO
g
BIAS LEVEL
0
g
Offset
0
g
Duty Cycle
0
g
Offset over Temperature
Based on 20%/g
PWM Frequency
NOISE PERFORMANCE
Noise Density, rms
FREQUENCY RESPONSE
3dB Bandwidth
SELF TEST
Continuous Voltage at outputs under
failure
OUTPUTS PERFORMANCE
Normal Output Range
Current
Rise/Fall Time
Turn-On Time
4
POWER SUPPLY
Operating Voltage Range
Supply Current
TEMPERATURE RANGE
Operating Range
95
380
Conditions
Min
Each Axis
±2.0
Best fit straight line
0.5
±1.0
±2.0
11.8
12.5
13.2
20
Each Axis
-0.1
48.7
0.0
50
1.5
0.0225
100
400
0.4
15
@3.0V Supply, output rails to
supply voltage
Output High
Output Low
Source or sink, @ 2.7V-3.6V
supply
2.7 to 3.6V supply
@3.0V Supply
2.8
0.2
100
90
100
75
3.0
2.0
110
17
3.0
+0.1
51.3
g
% duty
cycle
mg/°C
%/°C
Hz
Hz
mg/
g
% of FS
degrees
%
% duty
cycle/g
%
Typ
Max
Units
Each Axis
@3.0V supply
105
420
1.0
19
Hz
Hz
V
V
V
μA
nS
mS
V
mA
°C
2.7
@ 3.0 V
-40
3.6
+85
NOTES
1
2
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.
4
Output settled to within
±17mg.
MEMSIC MXD6235M Rev.A
Page 2 of 6
4/7/2010
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
PD
Power down pin
2
TP
Connected to ground
3
GND
Connected to ground
4
NC
Do Not Connect
5
NC
Do Not Connect
6
Yout
Y Channel Duty Cycle Output
7
Xout
X Channel Duty Cycle Output
8
V
DD
2.7V to 3.6 V
Ordering Guide
Model
MXD6235MP
Package Type
PWM
Frequency
I/O
I
I
I
NC
NC
O
O
I
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 except it is a gas in the MEMSIC
sensor.
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
-40 to 85°C
LCC8
RoHS compliant
100Hz
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 MXD6235M Rev.A
Page 3 of 6
4/7/2010
GND–
This is the ground pin for the
accelerometer
.
TP–
This pin should be connected to ground.
Xout
– This pin is the digital output of the X-axis
acceleration sensor. The user should ensure the load
impedance is sufficiently high as to not source/sink
>100μA typical.
Yout
–
This pin is the digital output of the Y-axis
Accelerometer Position Relative to Gravity
MEMSIC
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 3.6 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)
acceleration sensor. The user should ensure the load
impedance is sufficiently high as to not source/sink
>100μA typical.
PD
–
Pin1 is the power down control pin. Pull this pin
Change
per deg.
of tilt
(mg)
HIGH will put the accelerometer into power down mode.
When the part does into power down mode, the total
current will be smaller than 0.1uA at 3V.
In normal operation mode, this pin should be
connected to Ground.
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.
Following table and figure 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
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%).
DIGITAL INTERFACE
The MXD6235M is easily interfaced with low cost
microcontroller. For the digital output accelerometer, one
digital input port is required to read one accelerometer
output.
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:
MEMSIC MXD6235M Rev.A
Page 4 of 6
4/7/2010
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
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.
DUTY CYCLE DEFINITION
The MXD6235M 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
10ms period (100 Hz)).
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
Power Supply Noise Rejection
PCB LAYOUT AND FABRICATION SUGGESTIONS
1.
2.
3.
It is best to solder a 0.1uF capacitor directly across V
DD
and COM pin.
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.
T1
A (g)= (T1/T2 - 0.5)/0.125
At 0g T1=T2
T2= 10ms
Typical output Duty Cycle
4.
5.
POWER SUPPLY NOISE REJECTION
One capacitor is recommended for best rejection of power
supply noise (reference figure below). The capacitor should
MEMSIC MXD6235M Rev.A
Page 5 of 6
4/7/2010
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