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Low Cost, Low Noise 2 g Dual Axis Accelerometer with Ratiometric Analog Outputs
MXR7311GL
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; >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
Tilt and motion sensing in cost-sensitive applications Smart handheld devices Computer security Input devices Pedometers and activity monitors Gaming controllers Toys and entertainment products GENERAL DESCRIPTION The MXR7311GL 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 MXR7311GL measures acceleration with a full-scale range of 2 g and a sensitivity of 167mV/g @3V. It can measure both dynamic acceleration (e.g. vibration) and static acceleration (e.g. gravity). The MXR7311GL 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.
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.
MXR7311GL FUNCTIONAL BLOCK DIAGRAM
The MXR7311GL provides two ratiometric analog outputs that are proportional to 50% of the supply at zero g acceleration. The typical noise floor is 0.3 mg/ Hz allowing signals below 1 milli-g to be resolved at 1 Hz bandwidth. The MXR7311GL is packaged in a hermetically sealed LCC surface mount package (5 mm x 5 mm x 2 mm height) and is operational over a 0C to 70C temperature range.
MEMSIC, Inc. 800 Turnpike St., Suite 202, North Andover, MA 01845 Tel: 978.738.0900 Fax: 978.738.0196 www.memsic.com
MEMSIC MXR7311GL Rev.B
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2004.03.15
specified)
MXR7311GL SPECIFICATIONS (Measurements @ 25C, Acceleration = 0 g unless otherwise noted; VDD = 3.0V unless otherwise
Parameter SENSOR INPUT Measurement Range1 Nonlinearity Alignment Error2 Alignment Error Cross Axis Sensitivity3 SENSITIVITY Sensitivity, Sensitivity Change over Temperature4
Conditions Each Axis Best fit straight line X Sensor to Y Sensor Each Axis VDD = 3.0V Delta from 25C
(From 0C to 70C) (From 10C to 40C)
5
Min
MXR7311GL Typ 2.0 0.5 1.0 0.01 0.5
Max
Units g % of FS degrees degrees %
153
167
181 10 3
mV/g % % V g mg/C mg/ Hz Hz
ZERO g BIAS LEVEL 0 g Offset 0 g Offset 0 g Offset vs. Temperature4 NOISE PERFORMANCE Noise Density, rms FREQUENCY RESPONSE 3dB Bandwidth POWER SUPPLY Operating Voltage Range Quiescent Supply Current AoutX and AoutY OUTPUTS Output High Voltage Output Low Voltage Current Turn-On Time TEMPERATURE RANGE Operating Range NOTES
1 2
Each Axis VDD = 3.0V Delta from 25C @25C
1.46 -0.24
1.50 0.00 1.5 0.3 19
1.54 0.24
2.7 @3.0V supply @3.0V supply Source or sink @ 3.0V5.0V Supply Level (0g), @3.0V supply 0 Vs-0.25 4.2
5.25 5.1 0.25 250
V mA V V uA mS C
140
+70
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 Defined as the output change from ambient to maximum temperature or ambient to minimum temperature. 5
A simple RC filter should be added at Aoutput to eliminate noise effect.
MEMSIC MXR7311GL Rev.D
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ABSOLUTE MAXIMUM RATINGS* .....................-0.5 to +7.0V Supply Voltage (VDD) Storage Temperature ......................-65C 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.
Pin Description: LCC-8 Package Pin Name Description 1 NC Do Not Connect 2 TP Connect to ground 3 COM Common 4 NC Do Not Connect 5 NC Do Not Connect 6 Yout Y Channel Output 7 Xout X Channel Output 8 VDD 2.7V to 5.25V Ordering Guide Model Package Style LCC - 8 MXR7311GL
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. Device Weight <1.0 gram 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 0 to 70C
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 MXR7311GL Rev.D
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COM- This is the ground pin for the accelerometer. TP- This pin should be connected to the ground. Xout - 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 >250A 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.
Yout - This pin is the output of the y-axis acceleration Figure 2: Accelerometer Position Relative to Gravity
MEMSIC
MXR7311GL 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.
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)
Change per deg. of tilt (mg)
sensor. The user should ensure the load impedance is sufficiently high as to not source/sink >250A 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. 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).
-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
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%).
MEMSIC MXR7311GL Rev.D
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2004.03.15
USING THE MXR7311GL WITH OPERATING VOLTAGES OTHER THAN 3V The MXR7311GL is tested and specified at VDD=3V; however, it can be powered with VDD as low as 2.7V or as high as 5.25V. Some performance parameters will change as the supply voltage is varied. The MXR7311GL output sensitivity will be linearly proportional to supply voltage. At 5V the output sensitivity is typically 278mV/g. The zero g bias output is also ratiometric, so the zero g is normally equal to VDD/2 at all supply voltages. 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 zero g offset variations from part to part and zero g offset change over temperature can be eliminated. Figure 3 is a HFP (high pass filter) with a -3dB breakpoint given by the . In many applications it may be equation: f = 1 2RC desirable to have the HFP -3dB point at a very low frequency in order to detect very low frequency accelerations. Sometimes the implementation of this HFP may result in unreasonably large capacitors, and the designer may turn to digital implementations of HFPs where very low frequency -3dB breakpoints can be achieved.
AOUTX
R C
AOUTX F iltered O u tp u t
AOUTY
R C
AO UTY F iltered O u tp u t
Figure 4: Low Pass Filter
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 F.
A OUTX
C
R C
A O UTX Filtered Output
Figure 5: Power Supply Noise Rejection
A OUTY A O UTY Filtered Output
PCB LAYOUT AND FABRICATION SUGGESTIONS 1. 2. 3. 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.
R
Figure 3: High Pass Filter
Low Pass Filter: An external low pass filter would be 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 . For the 200 Hz ratiometric output equation: f = 1 2RC device filter, C=0.1F and R=8k, 5%, 1/8W.
4.
5.
MEMSIC MXR7311GL Rev.D
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TYPICAL PERFORMANCE CHARACTERISTICS ( @ 25C,
45%
50% 45% 40% 35% 30% 25% 20% 15% 10% 5% 0% 1.46
VDD = 3.0V unless otherwise specified)
PERCENT OF PARTS
40% 35% 30% 25% 20% 15% 10% 5% 0% 1.46 1.48 1.50 1.52 1.54
PERCENT OF PARTS
1.48
1.50
1.52
1.54
V X-axis zero g BIAS Output Distribution
30% 35%
V Y-axis zero g BIAS Output Distribution
PERCENT OF PARTS
PERCENT OF PARTS
25% 20% 15% 10% 5% 0% 153
30% 25% 20% 15% 10% 5% 0% 153 160 167 174 181
160
167
174
181
mV/g X-axis Output Sensitivity Distribution at Xout
mV/g Y-axis Output Sensitivity Distribution at Yout
MEMSIC MXR7311GL Rev.D
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LCC-8 PACKAGE DRAWING
Fig 6: Hermetically Sealed Package Outline
MEMSIC MXR7311GL Rev.D
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2004.03.15

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