Datasheet ADXL1004 (Analog Devices) - 9

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Data Sheet ADXL1004 THEORY OF OPERATION
The ADXL1004 is a low noise, single-axis, MEMS accelerometer,
MECHANICAL DEVICE OPERATION
with a 45 kHz resonant frequency that provides an analog output The moving component of the sensor is a polysilicon surface- proportional to mechanical vibration. The ADXL1004 has a high micromachined structure built on top of a silicon wafer. Polysilicon g range of ±500 g, suitable for vibration measurements in high springs suspend the structure over the surface of the wafer and bandwidth applications. Such applications include vibration provide a resistance against acceleration forces. analysis systems for monitoring and diagnosing machines or system health. Differential capacitors that consist of independent fixed plates and plates attached to the moving mass measure the deflection The low noise and high frequency bandwidth al ows the of the structure. Acceleration deflects the structure and unbalances measurement of vibration patterns caused by small moving the differential capacitor, resulting in a sensor output with amp- components, such as internal bearings. The high g range litude proportional to acceleration. Phase sensitive demodulation provides the dynamic range necessary for in high vibration determines the magnitude and polarity of the acceleration. environments such as heating, ventilation, and air conditioning (HVAC) and heavy machine equipment. To achieve proper
OPERATING MODES
performance, be aware of system noise, mounting, and signal The ADXL1004 has two operating modes: measurement mode conditioning. and standby mode. Measurement mode provides a continuous System noise is affected by supply voltage noise. The analog analog output for active monitoring. Standby mode is a output of the ADXL1004 is a ratiometric output; therefore, nonoperational, low power mode. supply voltage modulation affects the output. Use a properly
Measurement Mode
decoupled, stable supply voltage to power the ADXL1004 and to Measurement mode is the normal operating mode of the provide a reference voltage for the digitizing system. ADXL1004. In this mode, the accelerometer actively measures The output signal is impacted by an overrange stimulus. An acceleration along the axis of sensitivity and consumes 1.0 mA overload indicator output feature indicates a condition that is (typical) using a 5.0 V supply. critical for an intel igent measurement system. For more infor-
Standby Mode
mation about the overrange features, see the Overrange section. Placing the ADXL1004 in standby mode suspends the measure- Proper mounting ensures ful mechanical transfer of vibration ment with internal reduction of current consumption to 225 µA to accurately measure the desired vibration rather than vibration (typical for 5.0 V supply). The transition time from standby to of the measurement system, including the sensor. A common measurement mode is <50 µs. Figure 16 shows the transition technique for high frequency mechanical coupling is to use a from standby to measure mode. sensor stud mount system while considering the mechanical interface of fixing the ADXL1004 in the stud. For lower frequencies
BANDWIDTH
(below the ful capable bandwidth of the sensor), it may be possible The ADXL1004 circuitry supports an output signal bandwidth to use magnetic or adhesive mounting. Proper mounting technique beyond the resonant frequency of the sensor, measuring accel- ensures proper and repeatable results that are not influenced by eration over a bandwidth comparable to the resonant frequency measurement system mechanical resonances and/or damping at of the sensor. The output response is a combination of the sensor the desired frequency, and represents an efficient and proper response and the output amplifier response. Therefore, external mechanical transfer to the system being monitored. band limiting or filtering is required; see the Interfacing Analog Proper application specific signal conditioning is required to Output Below 10 kHz section and the Interfacing Analog achieve optimal results. Understanding the measurement Output Beyond 10 kHz section for more information. frequency range and managing overload conditions is When using the ADXL1004 beyond 10 kHz, consider the important to achieve accurate results. The electrical output nonlinearity due to the resonance frequency of the sensor, the signal of the ADXL1004 requires some band limiting and a additional noise due to the wideband output of the amplifier, proper digitization bandwidth. See the Interfacing Analog and the discrete frequency spurious tone due to coupling of the Output Below 10 kHz section and the Interfacing Analog internal 200 kHz clock. Aliased interferers in the desired band Output Beyond 10 kHz section for more information. 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 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 Mode 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 Untitled