Datasheet ADXL1001, ADXL1002 (Analog Devices) - 9

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Data Sheet ADXL1001/ADXL1002 THEORY OF OPERATION
The ADXL1001/ADXL1002 are high frequency, low noise single-
MECHANICAL DEVICE OPERATION
axis microelectromechanical systems (MEMS) accelerometers The moving component of the sensor is a polysilicon surface- that provide an analog output that is proportional to mechanical micromachined structure built on top of a silicon wafer. vibration. The ADXL1001/ADXL1002 have high g ranges of 100 g Polysilicon springs suspend the structure over the surface of the and 50 g and are suitable for vibration measurements in high wafer and provide a resistance against acceleration forces. bandwidth applications such as vibration analysis systems that monitor and diagnose machine or system health. Deflection of the structure is measured using differential capacitors that consist of independent fixed plates and plates The low noise and high frequency bandwidth allows the attached to the moving mass. Acceleration deflects the structure measurement of vibration patterns caused by small moving and unbalances the differential capacitor, resulting in a sensor parts, such as internal bearings, and the high g range provides output with an amplitude proportional to acceleration. Phase- the dynamic range to be used in high vibration environments sensitive demodulation determines the magnitude and polarity such as heating, ventilation, and air conditioning (HVAC) and of the acceleration. heavy machine equipment. To achieve proper performance, be aware of system noise, mounting, and signal conditioning.
OPERATING MODES
System noise is affected by supply voltage noise. The analog The ADXL1001/ADXL1002 have two operating modes: output of the ADXL1001/ADXL1002 is a ratiometric output; measurement mode and standby mode. Measurement mode therefore, supply voltage modulation affects the output. Use a provides a continuous analog output for active monitoring. properly decoupled stable supply voltage to power the ADXL1001/ Standby mode is a nonoperational, low power mode. ADXL1002 and to provide a reference voltage for the digitizing
Measurement Mode
system. Measurement mode is the normal operating mode of the The output signal is impacted by an overrange stimulus. An ADXL1001/ADXL1002. In this mode, the accelerometer overload indicator output feature is provided to indicate a actively measures acceleration along the axis of sensitivity and condition that is critical for an intelligent measurement system. consumes 1.0 mA (typical) using a 5.0 V supply. For more information about the overrange features, see the
Standby
Overrange section. Placing the ADXL1001/ADXL1002 in standby mode suspends Proper mounting is required to ensure full mechanical transfer the measurement with internal reduction of current consumption of vibration to accurately measure the desired vibration rather to 225 μA (typical for 5.0 V supply). The transition time from than vibration of the measurement system, including the sensor. standby to measurement mode is <50 μs. The transition from A common technique for high frequency mechanical coupling standby to measure mode is shown in Figure 18. is to utilize a sensor stud mount system while considering the mechanical interface of fixing the ADXL1001/ADXL1002 in the
BANDWIDTH
stud. For lower frequencies (below the full capable bandwidth The ADXL1001/ADXL1002 circuitry supports an output signal of the sensor), it is possible to use magnetic or adhesive bandwidth beyond the resonant frequency of the sensor, mounting. Proper mounting technique ensures proper and measuring acceleration over a bandwidth comparable to the repeatable results that are not influenced by measurement resonant frequency of the sensor. The output response is a system mechanical resonances and/or damping at the desired combination of the sensor response and the output amplifier frequency, and represents an efficient and proper mechanical response. Therefore, external band limiting or filtering is transfer to the system being monitored. required; see the Interfacing Analog Output Below 10 kHz Proper application specific signal conditioning is require to section and the Interfacing Analog Output Beyond 10 kHz achieve optimal results. An understanding of measurement section for more information. frequency range and managing overload condition is important When using the ADXL1001/ADXL1002 beyond 10 kHz, to achieve accurate results. The electrical output signal of the consider the nonlinearity due to the resonance frequency of the ADXL1001/ADXL1002 requires some band limiting and proper sensor, the additional noise due to the wideband output of the digitization bandwidth. See the Interfacing Analog Output amplifier, and the discrete frequency spurious tone due to coupling Below 10 kHz section and the Interfacing Analog Output of the internal 200 kHz clock. Aliased interferers in the desired Beyond 10 kHz section for more information. band cannot be removed, and observed performance degrades. A combination of high speed sampling and appropriate band limiting filtering is required for optimal performance. Rev. 0 | Page 9 of 14 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE RECOMMENDED SOLDERING PROFILE ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION MECHANICAL DEVICE OPERATION OPERATING MODES Measurement Mode Standby BANDWIDTH APPLICATIONS INFORMATION APPLICATION CIRCUIT ON DEMAND SELF TEST RATIOMETRIC OUTPUT VOLTAGE INTERFACING ANALOG OUTPUT BELOW 10 kHz INTERFACING ANALOG OUTPUT BEYOND 10 kHz OVERRANGE MECHANICAL CONSIDERATIONS FOR MOUNTING LAYOUT AND DESIGN RECOMMENDATIONS OUTLINE DIMENSIONS ORDERING GUIDE