Ultra Low Noise, Offset Drift
1 g Dual Axis
Accelerometer with Analog Outputs
MXA2500E
FEATURES
Better than 1 milli-g resolution
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
3V to 5.25V single supply continuous operation
Small (5mm x 5mm x 2mm) surface mount package
Continuous self test
Custom programmable specifications
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
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
A
OUTX
V
DD
V
DA
Gnd
2-AXIS
SENSOR
A
OUTY
T
OUT
Continous
Self Test
MXA2500E FUNCTIONAL BLOCK DIAGRAM
Gaming Joystick/RF Interface/Menu Selection/Tilt Sensing
White Goods Spin/Vibration Control
GENERAL DESCRIPTION
The MXA2500E is an ultra low noise and 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 MXA2500E
measures acceleration with a full-scale range of
1 g and a
sensitivity of 500mV/g at 25
C. (The MEMSIC
accelerometer product line extends from
0.5g to 250g
with custom versions available above
10 g.) It can
measure both dynamic acceleration (e.g., vibration) and
static acceleration (e.g., gravity). The MXA2500E 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
rates and lower loss due to handling during assembly.
The MXA2500E provides an absolute analog output The
typical noise floor is 0.2 mg/ Hz allowing signals below
1 milli-g to be resolved at 1 Hz bandwidth. The 3dB
rolloff of the device occurs at 17 Hz but is expandable to
>160 Hz (ref. Application Note AN-00MX-003). The
MXA2500E is available in a low profile LCC surface
mount package (5 mm x 5 mm x 2 mm). It is hermetically
sealed and is operational over a -40
C to +105C
temperature range. It also contains an on-chip temperature
sensor and a bandgap voltage reference.
Due to the standard CMOS structure of the MXA2500E,
additional circuitry can easily be incorporated into custom
versions for high volume applications. Contact the factory
for more information.
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 Street, Suite 202 , North Andover, MA 01845
Tel: 978.738.0900
Fax: 978.738.0196
www.memsic.com
MEMSIC MXA2500E Rev C Page 1 of 9 29/7/2003
MEMSIC MXA2500E Rev C Page 2 of 9 29/7/2003
MXA2500E SPECIFICATIONS
(Measurements @ 25
C, Acceleration = 0 g unless otherwise noted; V
DD
, V
DA
= 5.0V
unless otherwise specified)
Parameter
Conditions
Min
MXA2500E
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
degree
Transverse Sensitivity
3
2.0
%
SENSITIVITY
Sensitivity, Analog Outputs at pins
A
OUTX
and A
OUTY
6
Each Axis
475
500
525
mV/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
6
Each Axis
-0.1
0.00
+0.1
g
0 g Voltage
6
1.20
1.25
1.30
V
0 g Offset over Temperature
from 25C
from 25C, based on 500mV/g
0.4
0.2
mg/
C
mV/
C
NOISE PERFORMANCE
Noise Density, rms
Without frequency compensation
0.2
0.4
mg/
Hz
FREQUENCY RESPONSE
3dB Bandwidth - uncompensated
17
Hz
3dB Bandwidth - compensated
5
>160
Hz
TEMPERATURE OUTPUT
T
out
Voltage
1.23
1.25
1.27
V
Sensitivity
4.6
5.0
5.4
mV/
C
VOLTAGE REFERENCE
V
Ref
@3V-5.25V
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 A
OUTX
, A
OUTY
under
Failure
@5.0V Supply, output rails to
supply voltage
5.0
V
Continuous Voltage at A
OUTX
, A
OUTY
under
Failure
@3V Supply, output rails to
supply voltage
3.0
V
A
OUTX
and A
OUTY
OUTPUTS
Normal Output Range
@5.0V Supply
@3V Supply
0.1
0.1
4.9
2.9
V
V
Current
Source or sink, @ 3V-5.0V supply
100
A
Turn-On Time
@5.0V Supply
@3V Supply
100
40
mS
mS
POWER SUPPLY
Operating Voltage Range
3.0
5.25
V
Supply Current
@ 5.0V
2.7
3.3
4.1
mA
Supply Current
6,7
@
3V
3.2
4.0
4.8
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
External circuitry is required to extend the 3dB bandwidth. (ref.
Application Note: AN-00MX-003).
6
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.
7
Note that the accelerometer has a constant heater power control circuit
thereby requiring higher supply current at lower operating voltage.
MEMSIC MXA2500E Rev C Page 3 of 9 29/7/2003
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
Pin Description: LCC-8 Package
Pin Name Description
1 T
OUT
Temperature (Analog Voltage)
2 A
OUTY
Y-Axis Acceleration Signal
3 Gnd Ground
4 V
DA
Analog Supply Voltage
5 A
OUTX
X-Axis Acceleration Signal
6 V
ref
2.5V
Reference
7
Sck
Optional External Clock
8 V
DD
Digital Supply Voltage
Ordering Guide
Model Package
Style
MXA2500EL LCC-8
SMD*
*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
M
S
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 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 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 3.0volts 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.
A
OUTX
This pin is the output of the x-axis acceleration
sensor. The user should ensure the load impedance is
sufficiently high as to not source/sink >100
A. 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 on this feature.
A
OUTY
This pin is the output of the y-axis acceleration
sensor. The user should ensure the load impedance is
sufficiently high as to not source/sink >100
A. 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 on this feature.
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
-2.90
= S
f
x T
f
-2.90
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)
S
e
n
s
itiv
ity
(n
o
rma
lize
d
)
Figure 1: Thermal Accelerometer Sensitivity
MEMSIC MXA2500E Rev C Page 4 of 9 29/7/2003
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
MEMSIC MXA2500E Rev C Page 5 of 9 29/7/2003
or software. Any 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 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
+90
0
0
0
gravity
ME
M
S
I
C
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
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
*
)
(
(
Hz
Bandwidth
The peak-to-peak noise is approximately equal to 6.6 times
the rms value (for an average uncertainty of 0.1%).
EXTERNAL FILTERS
AC Coupling: For applications where only dynamic
accelerations (vibration) are to be measured, it is
recommended to ac couple the accelerometer output as
shown in Figure 3. The advantage of ac coupling is that
variations from part to part of zero g offset and zero g
offset versus temperature can be eliminated. Figure 3 is a
HPF (high pass filter) with a 3dB breakpoint given by the
equation:
RC
f
2
1
=
. In many applications it may be
desirable to have the HPF 3dB point at a very low
frequency in order to detect very low frequency
accelerations. Sometimes the implementation of this HPF
may result in unreasonably large capacitors, and the
designer must turn to digital implementations of HPFs
where very low frequency 3dB breakpoints can be
achieved.
29/7/2003
A
OUTX
R
C
A
OUTY
R
C
A
OUTX
Filtered
Output
A
OUTY
Filtered
Output
Figure 3: High Pass Filter
Low Pass Filter: An external low pass filter is useful in
low frequency applications such as tilt or inclination. The
low pass filter limits the noise floor and improves the
resolution of the accelerometer. The low pass filter shown
in Figure 4 has a 3dB breakpoint given by the equation:
RC
f
2
1
=
. For the 200 Hz ratiometric output device
filter, C=0.1
F and R=8k, 5%, 1/8W.
MEMSIC MXA2500E Rev C Page 6 of 9
A
OUTX
R
C
A
OUTY
R
C
A
OUTX
Filtered
Output
A
OUTY
Filtered
Output
Figure 4: Low Pass Filter
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 3V
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.
COMPENSATION FOR EXTENDING THE
FREQUENCY RESPONSE
The response of the thermal accelerometer is a function of
the internal gas physical properties, the natural convection
mechanism and the sensor electronics. Since the gas
properties of MEMSIC's mass produced accelerometer are
uniform, a simple circuit can be used to equally compensate
all sensors. For most applications, the compensating circuit
does not require adjustment for individual units.
A simple compensating network comprising two
operational amplifiers and a few resistors and capacitors
provides increasing gain with increasing frequency (see
Figure 5). The circuit shown is for an absolute output
accelerometer operating at 5 V supply. It provides a DC
gain of X2, so the offset at the output is 2.5V and the
sensitivity is doubled. The 14.3 K
and the 5.9K
resistors along with the non-polarized 0.82
F capacitors
tune the gain of the network to compensate for the output
attenuation at the higher frequencies. The resistors and the
capacitors provide noise reduction and stability.
Figure 5: Frequency Response Extension Circuit
The accelerometer response (bottom trace), the network
response (top trace) and the compensated response (middle
trace) are shown in Figure 6. The amplitude remains above
3db beyond 100 Hz, and there is useable signal well
after this frequency.
8.06K
5.9K
160K
1.5uF
-
+
UA
0.01uF
8.06K
14.3K
5.9K
0.01uF
0.047uF
1.5uF
-
+
UB
0.047uF
14.3K
0.0022uF
Aout X or Y
Freq. Comp. Output
MEMSIC MXA2500E Rev C Page 7 of 9 29/7/2003
-6 0
-4 5
-3 0
-1 5
0
1 5
3 0
4 5
6 0
1 0
1 0 0
1 0 0 0
Fre que n c y - H z
A
m
plitude - dB
Figure 6: Amplitude Frequency Response
COMPENSATION FOR ZERO G OFFSET CHANGE
OVER TEMPERATURE
In applications where a stable zero g offset is required, and
where the AC coupling external filter described earlier can
not be used, analog or digital temperature compensation
can be applied. The compensation requires individual
calibration because the magnitude of the zero g offset
change over temperature is different for each unit. To
compensate the change, a calibrated temperature dependent
signal equal in magnitude but with opposite polarity is
added to the accelerometer output. The circuit in Figure 7
shows a circuit example applying an analog linear
compensation technique. In this circuit the accelerometer
temperature sensor output is added to or subtracted from
the accelerometer output. The calibration sequence is: start
at room temperature with the 100K pot set so that its wiper
is at V
ref
. Next, soak the accelerometer at the expected
extreme temperature and observe the direction of the
change. Then set the switch to the non-inverting input if the
change is negative or vice versa. Finally, adjust the 100K
pot while monitoring the circuit output, until the zero g
offset change is removed.
+5V
-
+
100K
Aoutx or y
zero g drift
compensated
SW SPDT
100K
Aoutx or y
100K
100K
100K
10K
100K
10K
Tout
Vref
10K
Figure 7: Zero g Offset Temperature Compensation Circuit
Various digital compensation techniques can be applied
using a similar concept. Digital techniques can provide
better compensation because they can compensate for non-
linear zero g offset vs. temperature. A micro-controller or
micro-processor would perform the compensation. The
acceleration signal and the temperature signal would be
digitized using an analog to digital converter. Like in the
analog compensation, the first step is to test and
characterize the zero g change. The purpose of the
characterization is to create a look up table or to estimate a
mathematical representation of the change. For example,
the change could be characterized by an equation of the
form:
Change = a * Temperature
2
+ b * Temperature + c
where
a,b,c
are unique constants for each accelerometer.
In normal operation the processor calculates the output:
Compensated Output = Acceleration Change.
For a more detail discussion of temperature compensation
reference MEMSIC application note #AN-00MX-002.
TEMPERATURE OUTPUT NOISE REDUCTION
It is recommended that a simple RC low pass filter is used
when measuring the temperature output. Temperature
output is typically a very slow changing signal, so a very
low frequency filter eliminates erroneous readings that may
result from the presence of higher frequency noise. A
simple filter is shown in Figure 8.
Filtered TOUT
8.2K
0.1uF
MEMSIC
Accel.
TOUT
Figure 8: Temperature Output Noise Reduction
MEMSIC MXA2500E Rev C Page 8 of 9 29/7/2003
POWER SUPPLY NOISE REJECTION
Two capacitors and a resistor are recommended for best
rejection of power supply noise (reference Figure 9 below).
The capacitors should be located as close as possible to the
device supply pins (V
DA
, V
DD
). 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 F, and the resistor R can be 10 .
In 5V applications where power consumption is not a
concern, maximum supply noise rejection can be obtained
by significantly increasing the values of C1, C2 and R. For
example, C1 = C2 = 0.47 F and R = 270 will virtually
eliminate power supply noise effects.
R
MEMSIC
Accelerometer
VDA
C1
C2
VDD
V SUPPLY
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 ground 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.
Figure 9: Power Supply Noise Rejection
MEMSIC MXA2500E Rev C Page 9 of 9 29/7/2003
PACKAGE DRAWING
Fig 10: Hermetically Sealed Package Outline
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