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Ultra Low Noise, Low offset Drift 1 g Dual Axis Accelerometer with Digital Outputs
MXD2020E/F
FEATURES
Resolution better than 1 mg at 1 Hz Dual axis accelerometer fabricated on a monolithic CMOS IC RoHS compliant
On-chip mixed mode signal processing 50,000 g shock survival rating 17 Hz bandwidth
3.00V to 5.25V single supply operation Small (5mm x 5mm x 2mm) surface mount package Continuous self-test 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/PDAs/Cell Phones Gaming Joystick/RF Interface/Menu Selection/Tilt Sensing Projectors - Leveling and Keystoning White Goods Spin/Vibration Control
GENERAL DESCRIPTION
The MXD2020E/F is an ultra low noise and low cost, dual axis accelerometer built on a standard, submicron CMOS process. The MXD2020E/F measures acceleration with a full-scale range of 1 g and a sensitivity of 20%/g @5V at 25C. It can measure both dynamic acceleration (e.g., vibration) and static acceleration (e.g., gravity). The MXD2020E/F 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 up to 50,000 g, leading to significantly lower failure rates and lower losses due to handling during assembly.
The MXD2020E/F provides two digital outputs. The outputs are digital signals with duty cycles (ratio of pulse width to period) that are proportional to acceleration. The duty cycles outputs can be directly interfaced to a microprocessor.
Sck (optional)
Internal Oscillator
Temperature Sensor
Voltage
CLK
Reference
Heater Control
Continous Self Test
X axis
Low Pass Filter
Factory Adjust Offset & Gain
Y axis
2-AXIS SENSOR
V
DD
Gnd
Low Pass Filter
VDA
TOUT V
REF
D
OUTX
DOUTY
MXD2020E/F FUNCTIONAL BLOCK DIAGRAM
The typical noise floor is 0.2mg / Hz allowing signals below 1mg to be resolved at 1 Hz bandwidth. The MXD2020E/F is available in a LCC surface mount package (5mm x 5mm x 2mm height). It is hermetically sealed and operational over a -40 C to +105 C temperature range.
Due to the standard CMOS structure of the MXD2020E/F, 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
   www.memsic.com
MEMSIC MXD2020E/F Rev H
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2/26/2007
MXD2020E/F SPECIFICATIONS (Measurements @ 25C, Acceleration = 0g unless otherwise noted, VDD=, VDA
5.0V unless otherwise specified)
MXD2020E/F
Parameter
Conditions
Min
Typ
Max
Units
SENSOR INPUT
Each Axis
Measurement Range1
1.0
g
Nonlinearity Alignment Error2 Transverse Sensitivity3
Best fit straight line
0.5
1.0
% of FS
1.0
degrees
2.0
%
SENSITIVITY
Each Axis
DOUTX and DOUTY Change over Temperature (uncompensated)4
@5.0V supply from 25C, at 40C
19.0
20.0
21.0 % Duty
Cycle/g
+120
%
from 25C, at +105C
-55
%
Change over Temperature (compensated) 4 from 25C, 40C to +105C
<3.0
%
ZERO g BIAS LEVEL 0 g Offset5 0 g Duty Cycle5
Each Axis
-0.1
0.00
+0.1
g
48
50
52 % Duty Cycle
0 g Offset over Temperature
from 25C
0.4
mg/C
based on 20%/g
0.008
% / C
PWM output Frequency
For MXD2020EL only
97
100
103
Hz
MXD2020E
95
100
105
Hz
MXD2020F
380
400
420
Hz
NOISE PERFORMANCE Noise Density, rms
0.2
0.4 mg/ Hz
FREQUENCY RESPONSE
3dB Bandwidth
15
17
19
Hz
TEMPERATURE OUTPUT
Tout Voltage Sensitivity
1.15
1.25
1.35
V
4.6
5.0
5.4
mV/K
VOLTAGE REFERENCE OUTPUT
VRef output Change over Temperature
@3.0V-5.25V supply
2.4
2.5
2.65
V
0.1
mV/C
Current Drive Capability
Source
100
A
SELF TEST
Continuous Voltage at DOUTX, DOUTY under @5.0V Supply, output rails to
Failure
supply voltage
5.0
V
Continuous Voltage at DOUTX, DOUTY under @3.0V Supply, output rails to
Failure
supply voltage
3.0
V
DOUTX and DOUTY OUTPUTS
Normal Output Range
Output High
4.8
V
Output Low
0.2
V
Current
Source or sink, @ 3.0V-5.25V supply
100
A
Rise/Fall Time
3.0 to 5.0V Supply
90
100
110
nS
Turn-on Time
@5.0V Supply
160
mS
@3.0V Supply
300
mS
POWER SUPPLY
Operating Voltage Range
3.0
5.25
V
Supply Current
@ 5.0V
2.7
3.3
4.1
mA
Supply Current
@ 3.0V
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 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. 6 Output settled to within 17mg.
MEMSIC MXD2020E/F Rev H
Page 2 of 8
2/26/2007
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.
Package Characteristics
Package
JA
JC
LCC-8 110C/W 22C/W
Device Weight < 1 gram
Ordering Guide
Model
Package Style
MXD2020EL MXD2020EF
MXD2020FL MXD2020FF
LCC8 RoHS compliant LCC8, Pb-free RoHS compliant LCC8 RoHS compliant LCC8, Pb-free RoHS compliant
Digital Output 100 Hz 100Hz
400 Hz 400Hz
All parts are shipped in tape and reel packaging. Caution ESD (electrostatic discharge) sensitive device.
8
1
7
X +g
2
6
3
5
4
Y +g Top View
Note: The MEMSIC logos arrow indicates the +X sensing direction of the device. The +Y sensing direction is rotated 90°away from the +X direction. 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 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.
Pin Description: LCC-8 Package
Pin Name Description
I/O
1 TOUT
Temperature (Analog Voltage) O
2 DOUTY
Y-Axis Acceleration Digital Signal O
3 Gnd
Ground
I
4 VDA
Analog Supply Voltage
I
5 DOUTX
X-Axis Acceleration Digital Signal O
6 Vref
2.5V Reference Output
O
7 Sck
Optional External Clock
I
8 VDD
Digital Supply Voltage
I
MEMSIC MXD2020E/F Rev H
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2/26/2007
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 3.00 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. 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.
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):
Si x Ti 2.90 = Sf x Tf 2.90 where Si is the sensitivity at any initial temperatuire T , and Sf is the sensitivity at any other final temperatufre T with the temperature values in K.
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 >100A 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.
2.5
2.0
1.5
1.0
0.5
0.0
-40
-20
0
20
40
60
Temperature (C)
80
100
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 >100A 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.
TseOnUsTor.TThhisepainnaislotghveobltuafgfeeraetdToOuUTtpiustaonf
the temperature 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 TOUT 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
This
pin
is
the
output
of
a
reference
voltage.
It
is
set
at 2.50V typical and has 100A of drive capability.
Figure 1: Thermal Accelerometer Sensitivity
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 or software. The 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 +50C 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 +105C) 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
MEMSIC MXD2020E/F Rev H
Page 4 of 8
2/26/2007
force of gravity as an input to determine the inclination angle of an object.
resolution of the measurement. With a simple RC low pass filter, the rms noise is calculated as follows:
A MEMSIC accelerometer is most sensitive to changes in position, or tilt, when the accelerometers sensitive axis is perpendicular to the force of gravity, or parallel to the Earths surface. Similarly, when the accelerometers axis is parallel to the force of gravity (perpendicular to the Earths surface), it is least sensitive to changes in tilt.
Table 1 and Figure 2 to 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 +900
00
gravity
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 MXD2020E/F is easily interfaced with low cost microcontroller. For the digital output accelerometer, one digital input port is required to read one accelerometer output. For the analog output accelerometer, many low cost microcontroller 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:
Y Top View
Figure 2: Accelerometer Position Relative to Gravity
X-Axis
Y-Axis
X-Axis
Orientation
Change
Change
To Earths X Output per deg. Y Output per deg.
Surface
(g)
of tilt
(g)
of tilt
(deg.)
(mg)
(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: Accelerometers can be used in a wide variety of low g applications such as tilt and orientation. The device noise floor will vary with the measurement bandwidth. With the reduction of the bandwidth the noise floor drops. This will improve the signal to noise ratio of the measurement and resolution. The output noise scales directly with the square root of the measurement bandwidth. The maximum amplitude of the noise, its peakto- peak value, approximately defines the worst case
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 MXD2020E/F 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
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
T1
A (g)= (T1/T2 - 0.5)/0.2 At 0g T1=T2
T2= 2.5ms or 10ms (factory programmable) Figure 3: Typical output Duty Cycle
MEMSIC MXD2020E/F Rev H
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2/26/2007
For a more detail discussion of temperature compensation
CHOOSING T2 AND COUNTER FREQUENCY
reference MEMSIC application note #AN-00MX-002
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
Ax
MEMSIC Accel
Ay
T
to decode the duty cycle. The faster the counter clock, the
Microcontroller
I/O I/O A/D
higher the resolution of the duty cycle and the shorter the
T2 period can be for a given resolution. Table 2 shows
Figure 4: Zero g Offset Temperature Compensation Circuit
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 accelerometers noise floor may set the lower limit on the resolution.
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
Counter-
MEMSIC Clock Counts
Reso-
Sample
Rate Per T2 Counts lution
T2 (ms) Rate
(MHz) Cycle per g (mg)
10.0
100
1.0
10000 2000
0.5
10.0
100
0.5
5000 1000
1.0
2.5
400
1.0
2500 500
2.0
2.5
400
0.5
1250 250
4.0
Table 2: Trade-Offs Between Microcontroller Counter Rate and
T2 Period.
properties of MEMSIC's mass produced accelerometer are uniform, a digital filter can be used to equally compensate all sensors. The compensating filter does not require adjustment for individual accelerometers. The function of the compensating filter is to apply gain in proportion with the acceleration changes. The faster the acceleration changes occur, the higher the gain that the filter applies. For analog output accelerometers, the compensating filter can be implemented with a circuit involving two op-amps and some resistors and capacitors. For digital output
COMPENSATION FOR ZERO G OFFSET CHANGE
accelerometers, a digital filter is necessary.
OVER TEMPERATURE
The compensation of offset is performed with the following In applications where high frequency accelerations need to
equation: Aoc = A + ( a + b * T + c * T * T)
be measured, a DSP (digital signal processor) may be
where Aoc is the offset compensated acceleration, A is the necessary to implement the digital filter. DSP ICs and
uncompensated acceleration, T is temperature and a, b, c development tools are readily available from major IC
are constants characteristic to each accelerometer. manufacturers.
Computer programs are used to determine these constants. The constants can be read from and written to the MCU However, if the bandwidth requirement is relatively low EEPROM via the RS-232. The constants a,b,c are normally (i.e. 100Hz), it is possible to implement a digital frequency stored in the MCU EEPROM. To determine the values of compensating filter with an 8 bit microcontroller. The the constants, each accelerometer is taken to three different microcontroller will likely have to be capable of operating temperatures, preferably evenly spread across the desired at relatively high clock frequencies (20MHz).
temperature span. The zero g bias (A0, A1 and A2) and the temperatures (T0, T1 and T2) are recorded at each temperature. The data collected (A0, T0, A1, T1, A2, T2) is used in a quadratic interpolation (or LaGrange polynomial) to determine a, b and c as follows:
r0 = A0 / ( (T0-T1)*(T0-T2) ) r1 = A1 / ( (T1-T0)*(T1-T2) ) r2 = A2 / ( (T2-T0)*(T2-T1) ) a = r0 * T1 * T2 + r1 * T0 * T2 + r2 * T0 * T1
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 5 for an example.
b = - r0 * (T1+T2) r1 * (T0+T2) r2 *(T0+T1)
c = r0 + r1 + r2
10K DOUT
AOUT
In many cases a computer is used to control the
temperature, communicate with the MCU, and to calculate MEMSIC
1uF
the constants. After calculating the constants, the computer Accel.
downloads the constants to EEPROM.
Figure 5: Converting the digital output to an analog voltage
MEMSIC MXD2020E/F Rev H
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2/26/2007
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 6.
TOUT
8.2K
Filtered TOUT
Power Supply
C1 0.1uF
VDA VDD
MEMSIC Accelerometer
MEMSIC Accel.
0.1uF
Figure 7: Power Supply Noise Rejection
Figure 6: Temperature Output Noise Reduction
PCB LAYOUT AND FABRICATION SUGGESTIONS 1. The Sck pin should be grounded to minimize noise.
POWER SUPPLY NOISE REJECTION
2.
One capacitor is recommended for best rejection of power
supply noise (reference Figure 7 below). The capacitor 3.
should be located as close as possible to the device supply 4.
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. 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 ground plane should be similar to the MEMSIC devices 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 MXD2020E/F Rev H
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PACKAGE DRAWING Fig 8: Hermetically Sealed Package Outline
MEMSIC MXD2020E/F Rev H
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2/26/2007