MEMSIC MXD2125G/M/N/H Rev.E Page 1 of 7 3/25/2005
Improved, Ultra Low Noise
±3 g Dual Axis Accelerometer with
Digital Outputs
MXD2125G/H
MXD2125M/N
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
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
Internal
Oscillator
Sck
(optional)
CLK
Heater
Control
X axis
Y axis
Factory Adjust
O ffs et & Ga in
LPF
LPF
Temperature
Sensor
Voltage
Reference Vref
Dout X
Vdd VdaGnd
2-AXIS
SENSOR
Dout Y
Tout
Continous
S elf T e s t
A/D
A/D
MXD2125G/H/M/N FUNCTIONAL BLOCK DIAGRAM
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 MXD2125G/H/M/N 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 MXD2125G/H/M/N
measures acceleration with a full-scale range of ±3 g and a
sensitivity of 12.5%/g @5V at 25°C. It can measure both
dynamic acceleration (e.g. vibration) and static acceleration
(e.g. gravity).
The MXD2125G/H/M/N 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.
The MXD2125G/H/M/N 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
MXD2125G/H/M/N 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/N) and 0°C to
70°C(G/H) 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, Inc.
800 Turnpike St., Suite 202, North Andover, MA 01845
Tel: 978.738.0900 Fax: 978.738.0196
www.memsic.com
MEMSIC MXD2125G/M/N/H Rev.E Page 2 of 7 3/25/2005
MXD2125G/H/M/N SPECIFICATIONS (Measurements @ 25°C, Acceleration = 0 g unless otherwise noted; VDD, VDA = 5.0V unless
otherwise specified)
Parameter
Conditions
Min MXD2125G/H
Typ Max
Min MXD2125M/N
Typ
Max
Units
SENSOR INPUT
Measurement Range1
Each Axis
±3.0
±3.0
g
Nonlinearity Best fit straight line 0.5 0.5 % of FS
Alignment Error2 X Sensor to Y Sensor ±1.0 ±1.0 degrees
Transverse Sensitivity3
±2.0 ±2.0 %
SENSITIVITY
Sensitivity, Digital Outputs at
pins
DOUTX and DOUTY
4 Change
Each Axis
11.8
12.5
13.2
11.8
12.5
13.2
% duty
cycle/g
over Temperature -10 +8 -25 +8 %
ZERO g BIAS LEVEL
0 g Offset4
Each Axis
-0.1
0.0
+0.1
-0.1
0.0
+0.1
g
0 g Duty Cycle4 48.7 50 51.3 48.7 50 51.3 % duty
cycle
0 g Offset over Temperature ∆ from 25°C
Based on 12.5%/g
±1.5
±0.02
±1.5
±0.02
mg/°C
%/°C
NOISE PERFORMANCE
Noise Density, rms
0.2
0.4
0.2
0.4
mg/Hz
FREQUENCY RESPONSE
3dB Bandwidth 15 17 9 15 17 19 Hz
TEMPERATURE OUTPUT
Tout Voltage 1.15 1.25 1.35 1.15 1.25 1.35 V
Sensitivity 4.6 5.0 5.4 4.6 5.0 5.4 mV/°K
VOLTAGE REFERENCE
VRef @3.0V-5.25V supply 2.4 2.5 2.65 2.4 2.5 2.65 V
Change over Temperature 0.1 0.1 mV/°C
Current Drive Capability Source 100 100 µA
SELF TEST
Continuous Voltage at DOUTX,
DOUTY under Failure
@5.0V Supply, output
rails to
supply voltage
5.0
5.0
V
Continuous Voltage at DOUTX,
DOUTY under Failure
@3.0V Supply, output
rails to
supply voltage
3.0
3.0
V
DOUTX and DOUTY OUTPUTS
Normal Output Range Output High
Output Low
4.8
0.2
4.8
0.2
V
V
Output Frequency MXD2125G/M
MXD2125H/N
95
380
100
400
105
420
95
380
100
400
105
420
Hz
Hz
Current Source or sink, @
3.0V-5.25V supply
100 100 µA
Rise/Fall Time 3.0 to 5.25V supply 90 100 110 90 100 110 nS
Turn-On Time5 @5.0V Supply
@3.0V Supply
160
300
160
300
mS
mS
POWER SUPPLY
Operating Voltage Range 3.0 5.25 3.0 5.25 V
Supply Current @ 5.0V 2.5 3.1 3.9 2.5 3.1 3.9 mA
Supply Current @ 3.0V 3.0 3.8 4.6 3.0 3.8 4.6 mA
TEMPERATURE RANGE
Operating Range 0 +70 -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 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
level specifications on this page will be met. Please contact the factory for specially
trimmed devices for low supply voltage operation.
5 Output settled to within ±17mg.
MEMSIC MXD2125G/M/N/H Rev.E Page 3 of 7 3/25/2005
ABSOLUTE MAXIMUM RATINGS*
Supply Voltage (VDD, VDA) ………………...-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 TOUT Temperature (Analog Voltage)
2 DOUTY Y-Axis Acceleration Digital Signal
3 Gnd Ground
4 VDA Analog Supply Voltage
5 DOUTX X-Axis Acceleration Digital Signal
6 Vref 2.5V Reference
7 Sck Optional External Clock
8 VDD Digital Supply Voltage
Ordering Guide
Model Package Style Digital
Output Temperature
Range
MXD2125GL LCC8
RoHS compliant 100 Hz 0 to 70°C
MXD2125GF LCC8, Pb-free 100 Hz 0 to 70°C
MXD2125HL LCC8
RoHS compliant 400Hz 0 to 70°C
MXD2125HF LCC8, Pb-free 400Hz 0 to 70°C
MXD2125ML LCC8
RoHS compliant 100 Hz -40 to 105°
MXD2125MF LCC8, Pb-free 100 Hz -40 to 105°
MXD2125NL LCC8
RoHS compliant 400 Hz -40 to 105°
MXD2125NF LCC8, Pb-free 400 Hz -40 to 105°
All parts are shipped in tape and reel packaging.
Caution: ESD (electrostatic discharge) sensitive device.
8
4
1
2
3
7
6
5
Top View
MEMSIC
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 following the right-hand rule. Small
circle indicates pin one(1).
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 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.
MEMSIC MXD2125G/M/N/H Rev.E Page 4 of 7 3/25/2005
MXD2125G/H/M/N PIN DESCRIPTIONS
VDD – 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.
VDA – This is the power supply input for the analog
amplifiers in the accelerometer. VDA should always be
connected to VDD. 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.
DOUTX – 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.
DOUTY – 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 >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.
TOUT – This pin is the buffered output of the temperature
sensor. The analog voltage at TOUT 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
Vref – A reference voltage is available from this pin. It is
set at 2.50V typical and has 100µA of drive capability.
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).
Top View
X
Y
+900
00
gravity
MEMSIC
Figure 2: Accelerometer Position Relative to Gravity
X-Axis Y-Axis
X-Axis
Orientation
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
MEMSIC MXD2125G/M/N/H Rev.E Page 5 of 7 3/25/2005
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 ) * )6.1*)(( HzBandwidth
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 MXD2125G/H/M/N 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 MXD2125G/H/M/N 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 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)/12.5%
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.
T2 (ms)
MEMSIC
Sample
Rate
Counter-
Clock
Rate
(MHz)
Counts
Per T2
Cycle
Counts
per g
Reso-
lution
(mg)
2.5 400 2.0 5000 625 1.6
2.5 400 1.0 2500 312.5 3.2
2.5 400 0.5 1250 156.3 6.4
10.0 100 2.0 20000 2500 0.4
10.0 100 1.0 10000 1250 0.8
10.0 100 0.5 5000 625 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.
1uF
DOUT AOUT
10K
MEMSIC
Accel.
Figure 4: Converting the digital output to an analog voltage
MEMSIC MXD2125G/M/N/H Rev.E Page 6 of 7 3/25/2005
POWER SUPPLY NOISE REJECTION
Two capacitors and a resistor are recommended for best
rejection of power supply noise (reference Figure 5 below).
The capacitors should be located as close as possible to the
device supply pins (VDA, VDD). The capacitor lead length
should be as short as possible, and surface mount capacitors
are preferred. For typical applications, capacitors C1 and
C2 can be ceramic 0.1 µF, and the resistor R can be 10 Ω.
R
MEMSIC
Accelerometer
VDA
C1 C2
VDD
V SUPPLY
Figure 5: Power Supply Noise Rejection
PCB LAYOUT AND FABRICATION SUGGESTIONS
1. The Sck pin should be grounded to minimize noise.
2. Liberal use of ceramic bypass capacitors is
recommended.
3. Robust low inductance ground wiring should be used.
4. 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.
5. 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.
6. Vias can be added symmetrically around the ground
plane. Vias increase thermal isolation of the device
from the rest of the PCB.
MEMSIC MXD2125G/M/N/H Rev.E Page 7 of 7 3/25/2005
LCC-8 PACKAGE DRAWING
Fig 6: Hermetically Sealed Package Outline
