Datasheet ADXL377 (Analog Devices) - 9

ADXL377 Data Sheet THEORY OF OPERATION
The ADXL377 is a complete 3-axis acceleration measurement The demodulator output is amplified and brought off chip through system with a typical measurement range of ±200 g. The ADXL377 a 32 kΩ resistor. The user then sets the signal bandwidth of the contains a polysilicon, surface-micromachined sensor and signal device by adding a capacitor. This filtering improves measurement conditioning circuitry to implement an open-loop acceleration resolution and helps prevent aliasing. measurement architecture. The output signals are analog voltages
MECHANICAL SENSOR
that are proportional to acceleration. The accelerometer can mea- sure the static acceleration of gravity in tilt-sensing applications, The ADXL377 uses a single structure for sensing the accelera- as well as dynamic acceleration resulting from motion, shock, tion in the x-axis, y-axis, and z-axis. As a result, the three sense or vibration. directions are highly orthogonal with little cross-axis sensitivity. Mechanical misalignment of the sensor die to the package or The sensor is a polysilicon, surface-micromachined structure misalignment of the package to the PCB is the chief source of built on top of a silicon wafer. Polysilicon springs suspend the cross-axis sensitivity. Mechanical misalignment can be calibrated structure over the surface of the wafer and provide resistance at the system level. against acceleration forces. Deflection of the structure is mea- sured using a differential capacitor that consists of independent
PERFORMANCE
fixed plates and plates attached to the moving mass. The fixed Rather than using additional temperature compensation circuitry, plates are driven by 180° out-of-phase square waves. Acceleration the ADXL377 uses innovative design techniques to ensure high deflects the moving mass and unbalances the differential capacitor, performance. As a result, there is neither quantization error nor resulting in a sensor output whose amplitude is proportional to nonmonotonic behavior, and temperature hysteresis is very low. acceleration. Phase-sensitive demodulation techniques are then used to determine the magnitude and direction of the acceleration. Rev. 0 | Page 8 of 12 Document Outline Features Applications General Description Functional Block Diagram Table of Contents Revision History Specifications Absolute Maximum Ratings ESD Caution Pin Configuration and Function Descriptions Typical Performance Characteristics Theory of Operation Mechanical Sensor Performance Applications Information Power Supply Decoupling Setting the Bandwidth Using CX, CY, and CZ Self-Test Selecting Filter Characteristics: Noise/Bandwidth Trade-Off Axes of Acceleration Sensitivity Layout and Design Recommendations Outline Dimensions Ordering Guide