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| Ultra Low Cost, 1.0 g Dual Axis Accelerometer with Absolute Outputs MXA2500J/K FEATURES RoHS Compliant Dual axis accelerometer Monolithic CMOS construction On-chip mixed mode signal processing Resolution better than 2 mg 30Hz bandwidth 2.70V to 5.25V single supply operation Low height surface mount package Sck (optional) Internal Oscillator Temperature Sensor Voltage Reference Continous Self Test Tout CLK Vref Heater Control X axis Low Pass Filter Aout X APPLICATIONS Consumer Electronics * Cell phones, PDAs, MP3 Players, Gaming consoles * Screen and image orientation * Tilt and motion input * Menu navigation * Auto power on/off * Active HDD protection * Pedometer Security * Tamper detection * Catastrophic event detection * Black box event recorders Office Equipment * Computer Peripherals * Mouse input GENERAL DESCRIPTION The MXA2500J/K is an ultra low cost, dual axis accelerometer fabricated on a standard, submicron CMOS process. The MXA2500J/K measures acceleration with a full-scale range of 1.0 g and a sensitivity of 500mV/g @3V power supply at 25C. It can measure both dynamic acceleration (e.g., vibration) and static acceleration (e.g., gravity). 2-AXIS SENSOR Factory Adjust Offset & Gain Low Pass Filter Y axis Aout Y Vdd Gnd Vda MXA2500J/K FUNCTIONAL BLOCK DIAGRAM The MXA2500J/K design is based on heat convection and requires no solid proof mass. This eliminates stiction and particle problems, leading to significantly lower failure rates and lower loss due to handling during assembly. The MXA2500J/K provides two absolute analog outputs. The typical noise floor is 1.0 mg/ Hz allowing signals below 2mg to be resolved at 1Hz bandwidth. The MXA2500J/K has an inherent low pass frequency response with a 30Hz 3dB cutoff frequency, which eliminates unwanted higher frequency vibrations from obscuring the measurement. The MXA2500J/K is available in a LCC surface mount package (5.5mm x 5.5mm x 1.40mm height, with maximum height of 1.50mm). It is operational over a 0C to +70C (J) and -40C to +85C (K) 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. (c)MEMSIC, Inc. 800 Turnpike Street, Suite 202 , North Andover, MA 01845 Tel: 978.738.0900 Fax: 978.738.0196 www.memsic.com MEMSIC MXA2500J/K Rev.A Page 1 of 8 2/17/2006 MXA2500J/K SPECIFICATIONS (Measurements @ 25C, Acceleration = 0 g unless otherwise noted; VDD, VDA = 3.0V unless otherwise specified) MXA2500J Parameter SENSOR INPUT Measurement Range1 Nonlinearity Alignment Error2 Transverse Sensitivity3 SENSITIVITY Sensitivity, Analog Outputs at pins AOUTX and AOUTY Change over Temperature ZERO g BIAS LEVEL 0 g Offset 0 g Voltage 0 g Offset over Temperature Each Axis @3.0V supply from 25C Each Axis from 25C from 25C, based on 500mV/g Without frequency compensation 25 450 500 550 10 +0.3 1.40 450 500 550 20 +0.3 1.40 mV/g % g V mg/C mV/C mg/ Conditions Each Axis 1.0 Best fit straight line 1.0 1.0 2.0 2.0 1.0 1.0 1.0 2.0 2.0 g % of FS degree s % Min Typ Max Min MXA2500K Typ Max Units -0.3 1.10 0.00 1.25 3.0 1.5 -0.3 1.10 0.00 1.25 3.0 1.5 NOISE PERFORMANCE Noise Density, rms FREQUENCY RESPONSE 3dB Bandwidth uncompensated TEMPERATURE OUTPUT Tout Voltage Sensitivity VOLTAGE REFERENCE VRef Change over Temperature Current Drive Capability SELF TEST Continuous Voltage at AOUTX, AOUTY under Failure AOUTX and AOUTY OUTPUTS Normal Output Range Current Turn-On Time4 POWER SUPPLY Operating Voltage Range Supply Current TEMPERATURE RANGE Operating Range NOTES 1.0 1.0 Hz 30 35 25 30 35 Hz 1.15 4.6 @2.7V-5.25V supply Source @3.0V Supply, output rails to supply voltage @3.0V Supply Source or sink, @ 2.7V5.25V supply @3.0V Supply 2.7 @ 3.0V 0 0.1 2.4 1.25 5.0 2.5 0.1 1.35 5.4 2.65 100 1.15 4.6 2.4 1.25 5.0 2.5 0.1 1.35 5.4 2.65 100 V mV/K V mV/C A V 3.0 3.0 2.9 100 300 5.25 4.8 +70 0.1 2.9 100 300 V A mS 2.7 4.8 -40 5.25 V mA C 105 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 Output settled to within +/-17mg. 2 1 MEMSIC MXA2500J/K Rev.A Page 2 of 8 2/17/2006 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 JC LCC8 110C/W 22C/W Device Weight < 1 gram Ordering Guide Model MXA2500JV MXA2500KV Temperature Range 0~70C -40~85C Package Style LCC8, RoHS compliant LCC8, RoHS compliant Pin Description: Pin Name 1 TOUT 2 AOUTY 3 Gnd 4 VDA 5 AOUTX 6 Vref 7 Sck 8 VDD LCC8 Package Description Temperature (Analog Voltage) Y-Axis Acceleration Signal Ground Analog Supply Voltage X-Axis Acceleration Signal 2.5V Reference Connect to Ground Digital Supply Voltage *LCC parts are shipped in tape and reel packaging. Caution ESD (electrostatic discharge) sensitive device. 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/poly-silicon 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 xaxis and one to measure acceleration in the y-axis. 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. MEMSIC MXA2500J/K Rev.A Page 3 of 8 2/17/2006 TYPICAL CHARACTERISTICS, % OF UNITS (@ 25C, VDD = 3V) OffsetX Distribution 14% 12% 10% 8% 6% 4% 2% 0% 1.126 1.159 1.192 1.225 1.258 1.291 1.324 1.357 1.390 14% 12% 10% 8% 6% 4% 2% 0% OffsetY Distribution 1.126 1.159 1.192 1.225 1.258 1.291 1.324 1.357 1.390 SenX Distribution 60% 60% SenY Distribution 50% 50% 40% 40% 30% 30% 20% 20% 10% 10% 0% 465 474 484 493 502 512 521 531 540 0% 465 474 484 493 502 512 521 531 540 MEMSIC MXA2500J/K Rev.A Page 4 of 8 2/17/2006 OVER TEMPERATURE CHARACTERISTICS Normalized OffsetX vs. Temp 1.20 1.15 1.10 1.05 1.00 0.95 0.90 0.85 0.80 0 10 20 30 40 50 60 T(C) 70 1.20 1.15 1.10 1.05 1.00 0.95 0.90 0.85 0.80 0 10 Normalized OffsetY vs. Temp T(C) 20 30 40 50 60 70 Normalized SenX vs. Temp 1.06 1.06 Normalized SenY vs. Temp 1.04 1.04 1.02 1.02 1.00 1.00 0.98 0.98 0.96 T(C) 0 10 20 30 40 50 60 70 0.96 T(C) 0.94 0 10 20 30 40 50 60 70 0.94 MEMSIC MXA2500J/K Rev.A Page 5 of 8 2/17/2006 M E M SIC 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 2.70 and 5.25 volts. VDA - This is the power supply input for the analog amplifiers in the accelerometer. The DC voltage should be between 2.70 and 5.25 volts 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. AOUTY - This pin is the output of the y-axis acceleration X +900 gravity 0 0 Y Top View 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 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) 1.000 0.996 0.985 0.940 0.866 0.707 0.500 0.342 0.174 0.087 0.000 Change per deg. of tilt (mg) 0.15 1.37 2.88 5.86 8.59 12.23 15.04 16.35 17.16 17.37 17.45 Y Output (g) 0.000 0.087 0.174 0.342 0.500 0.707 0.866 0.940 0.985 0.996 1.000 sensor. The user should ensure the load impedance is sufficiently high as to not source/sink >100A. 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 - This pin should be grounded. 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 MINIMUM 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. Following table and figure 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-00MX007). MEMSIC MXA2500J/K Rev.A Changes in Tilt for X- and Y-Axes Minimum 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. The peak-to-peak noise is approximately equal to 6.6 times the rms value (with an average uncertainty of .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 following figure. 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. Following figure is a HPF (high pass filter) with a -3dB . In many breakpoint given by the equation: f = 1 2RC 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 2/17/2006 Page 6 of 8 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. Power supply A OUTX C R C A O UTX Filtered Output C1 0.1uF VDD VDA MEMSIC Accelerometer A OUTY R A O UTY Filtered Output Power Supply Noise Rejection 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 (MXR2xxx series), it is highly recommended that an external, 200 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 following figure has a -3dB . For the breakpoint given by the equation: f = 1 2RC 200 Hz ratiometric output device filter, C=0.1F and R=8k, 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 ground plane should be similar to the MEMSIC device's footprint and 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. A O UTX R C R C A OUTX Filtered Output A O UTY A OUTY Filtered Output Low Pass Filter POWER SUPPLY NOISE REJECTION A capacitor is recommended for best rejection of power supply noise (reference following figure). The capacitor should be located as close as possible to the device supply pins VDA. The capacitor lead length should be as short as possible, and surface mount capacitors are preferred. For typical applications, capacitors C1 can be ceramic 0.1 F. MEMSIC MXA2500J/K Rev.A Page 7 of 8 2/17/2006 Package Drawing CERAMIC (BLACK) Package Outline MEMSIC MXA2500J/K Rev.A Page 8 of 8 2/17/2006 |
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