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Improved, Ultra Low Noise 2 g Dual Axis Accelerometer with Ratiometric Outputs
MXR2312G/M
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
Sck (optional)
Internal Oscillator
Temperature Sensor
TOUT
CLK
Voltage Reference
Continous Self Test
VREF
Heater Control
X axis
Low Pass Filter
AOUTX
Factory Adjust Offset & Gain
APPLICATIONS
Automotive - Vehicle Security/Vehicle Stability control/ Headlight Angle Control/Tilt Sensing Security - Gas Line/Elevator/Fatigue Sensing/Computer Security Information 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
2-AXIS SENSOR
Y axis
Low Pass Filter
AOUTY
VDD
Gnd
VDA
MXR2312G/M FUNCTIONAL BLOCK DIAGRAM
GENERAL DESCRIPTION The MXR2312G/M 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 MXR2312G/M measures acceleration with a full-scale range of 2 g and a sensitivity of 312mV/g @5V at 25C. It can measure both dynamic acceleration (e.g. vibration) and static acceleration (e.g. gravity). The MXR2312G/M 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 MXR2312G/M provides two ratiometric analog outputs proportional to 50% of the supply voltage at zero g acceleration. 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 (reference Application Note AN-00MX-003). The MXR2312G/M is packaged in a hermetically sealed LCC surface mount package (5 mm x 5 mm x 2 mm height) and is operational over a -40C to 105C(ML) and 0C to 70C(GL) 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 MXR2312G/M
Rev.E
Page 1 of 6
1/19/2005
MXR2312G/M SPECIFICATIONS (Measurements @ 25C, Acceleration = 0 g unless otherwise noted; VDD, VDA = 5.0V unless
otherwise specified) MXR2312G Typ MXR2312M Typ
Parameter SENSOR INPUT Measurement Range1 Nonlinearity Alignment Error2 Transverse Sensitivity3 SENSITIVITY Sensitivity, Analog Outputs at pins AOUTX and AOUTY5 Change over Temperature ZERO g BIAS LEVEL 0 g Offset5 0 g Voltage5 0 g Offset over Temperature NOISE PERFORMANCE Noise Density, rms
Conditions Each Axis Best fit straight line X Sensor to Y Sensor Each Axis
Min 2.0
Max
Min 2.0
Max
Units g
0.5 1.0 2.0
1.0
0.5 1.0 2.0
1.0
% of FS degrees %
296 -10 Each Axis -0.1 2.47 Based on 312 mV/g Without frequency compensation 15
312
328 +8
296 -25 -0.1 2.47
312
328 +8
mV/g % g V mg/C mV/C mg/ Hz Hz Hz V mV/K V mV/C A
0.0 2.50 1.5 0.47 0.2 17 >160 1.25 5.0 2.5 0.1
+0.1 2.53
0.0 2.50 1.5 0.47 0.2
+0.1 2.53
0.4 19 15
0.4 19
FREQUENCY RESPONSE 3dB Bandwidth - uncompensated 3dB Bandwidth - compensated4 TEMPERATURE OUTPUT Tout Voltage Sensitivity VOLTAGE REFERENCE VRef @3.0V-5.25V supply Change over Temperature Current Drive Capability Source SELF TEST Continuous Voltage at AOUTX, @5.0V Supply, output AOUTY under Failure rails to supply voltage Continuous Voltage at AOUTX, @3.0V Supply, output AOUTY under Failure rails to supply voltage AOUTX and AOUTY OUTPUTS Normal Output Range @5.0V Supply Current Source or sink, @ 3.0V-5.25V supply Turn-On Time @5.0V Supply @3.0V Supply POWER SUPPLY Operating Voltage Range Supply Current @ 5.0V Supply Current @ 3.0V TEMPERATURE RANGE Operating Range NOTES
1 2
17 >160 1.25 5.0 2.5 0.1
1.15 4.6 2.4
1.35 5.4 2.65 100
1.15 4.6 2.4
1.35 5.4 2.65 100
5.0 3.0
5.0 3.0
V V
1.0 160 300 3.0 2.7 3.2 0
4.0 100
1.0 160 300
4.0 100
V A mS mS
3.3 4.0
5.25 4.1 4.8 +70
3.0 2.7 3.2 -40
3.3 4.0
5.25 4.1 4.8 +105
V mA mA C
Guaranteed by measurement of initial offset and sensitivity.
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 External circuitry is required to extend the 3dB bandwidth (ref. Application Note: AN-00MX-003)
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.
MEMSIC MXR2312G/M Rev.E
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1/19/2005
ABSOLUTE MAXIMUM RATINGS* Supply Voltage (VDD, VDA) .....................-0.5 to +7.0V 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.
Package Characteristics Package JA LCC-8 110C/W
JC 22C/W
Device Weight < 1 gram
Pin Description: LCC-8 Package Pin Name Description 1 TOUT Temperature (Analog Voltage) 2 AOUTY Y-Axis Acceleration Signal 3 Gnd Ground 4 VDA Analog Supply Voltage 5 AOUTX X-Axis Acceleration Signal 6 Vref 2.5V Reference 7 Sck Optional External Clock 8 VDD Digital Supply Voltage Ordering Guide Model Package Style MXR2312GL LCC8 MXR2312GF LCC8, Pb-free MXR2312ML LCC8 MXR2312MF LCC8, Pb-free Temperature Range 0 to 70C 0 to 70C -40 to 105C -40 to 105C
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.
All parts are shipped in tape and reel packaging.
Caution: ESD (electrostatic discharge) sensitive device.
8 1 2 3 M E M S IC 7 X +g 6 5
4 Y +g Top View
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 MXR2312G/M Rev.E
Page 3 of 6
1/19/2005
MXR2312G/M 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. 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. AOUTX - 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 >100A. 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.
AOUTY - This pin is the output of the y-axis acceleration
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).
X +900 M E M SIC gravity 0
0
Y Top View
sensor. The user should ensure the load impedance is sufficiently high as to not source/sink >100A. 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.
Figure 2: Accelerometer Position Relative to Gravity
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)
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 100A 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.
Change per deg. of tilt (mg)
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)
MEMSIC MXR2312G/M Rev.E
Page 4 of 6
1/19/2005
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 . In many applications it may be equation: f = 1 2RC 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.
AOUTX
R C
AOUTX Filtered Output AOUTY Filtered Output
AOUTY
R C
Figure 4: Low Pass Filter
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 F, and the resistor R can be 10 .
V SUPPLY
AOUTX
C R
AOUTX Filtered Output
C1
R
C2
AOUTY
C R
AOUTY Filtered Output
VDA
VDD
MEMSIC Accelerometer
Figure 5: Power Supply Noise Rejection 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. When designing with MEMSIC ratiometric output accelerometers (MXRxxxx series), it is highly recommended that an external, 20 Hz low pass filter be used to eliminate internally generated periodic noise that is coupled to the output of the accelerometer. The low pass filter shown in Figure 4 has a . For -3dB breakpoint given by the equation: f = 1 2RC the 200 Hz ratiometric output device filter, C=0.2F and R=39k, 5%, 1/8W.
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 MXR2312G/M Rev.E
Page 5 of 6
1/19/2005
PACKAGE DRAWING
Figure 6: Hermetically Sealed Package Outline
MEMSIC MXR2312G/M Rev.E
Page 6 of 6
1/19/2005

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