Datasheet ADXL212 (Analog Devices) - 10

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ADXL212 THEORY OF OPERATION PIN 8 XOUT = 62.5% YOUT = 50% PIN 8 TOP VIEW PIN 8 XOUT = 50% (Not to Scale) XOUT = 50% Y Y OUT = 37.5% OUT = 62.5% XOUT = 50% YOUT = 50% PIN 8 XOUT = 37.5% YOUT = 50% EARTH'S SURFACE
20 0 4- 80 09 Figure 20. Output Response vs. Orientation The ADXL212 is a complete dual axis acceleration measure- A single resistor (RSET) sets the period for a complete cycle (t2) ment system on a single monolithic IC. It contains a polysilicon according to the following equation: surface-micromachined sensor and signal conditioning circuitry t2 (nominal) = RSET/125 MΩ to implement an open-loop acceleration measurement archi- tecture. The output signals are duty cycle modulated digital A 0 g acceleration produces a 50% nominal duty cycle. The signals proportional to the acceleration. The ADXL212 is capable acceleration can be determined by measuring the length of the of measuring both positive and negative accelerations to ±2 g. positive pulse width (t1) and the period (t2). The nominal The accelerometer can measure static acceleration forces such transfer function of the ADXL212 is as gravity, allowing the ADXL212 to be used as a tilt sensor. Acceleration = ((t1/t2) − Zero g Bias)/Sensitivity The sensor is a surface-micromachined polysilicon structure where: built on top of a silicon wafer. Polysilicon springs suspend the Zero g Bias = 50% nominal. structure over the surface of the wafer and provide a resistance Sensitivity = 12.5%/g nominal. against acceleration forces. Deflection of the structure is measured
PERFORMANCE
using a differential capacitor that consists of independent fixed High performance is built into the device through innovative plates and plates attached to the moving mass. The fixed plates design techniques rather than by using additional temperature are driven by 180° out-of-phase square waves. Acceleration compensation circuitry. As a result, there is essentially no quantiza- deflects the beam and unbalances the differential capacitor, tion error or nonmonotonic behavior, and temperature hysteresis resulting in an output square wave with an amplitude that is is very low (typically less than 10 mg over the −40°C to +85°C proportional to acceleration. Phase sensitive demodulation tech- temperature range). niques are used to rectify the signal and determine the direction of the acceleration. Figure 10 shows the zero g output performance of eight parts (x-axis and y-axis) over a –40°C to +85°C temperature range. The output of the demodulator is amplified and brought off chip through a 32 kΩ resistor, at which point the user can set Figure 13 demonstrates the typical sensitivity shift over temper- the signal bandwidth of the device by adding a capacitor. This ature for VS = 5 V. Sensitivity stability is optimized for VS = 5 V filtering improves measurement resolution and helps prevent but remains very good over the specified range; it is typically aliasing. better than ±2% over temperature at VS = 3 V. After being low-pass filtered, the analog signals are converted to duty cycle modulated outputs that can be read by a counter. Rev. 0 | Page 9 of 12 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION PERFORMANCE APPLICATIONS INFORMATION POWER SUPPLY DECOUPLING SETTING THE BANDWIDTH USING CX AND CY SELF TEST DESIGN TRADE-OFFS FOR SELECTING FILTER CHARACTERISTICS: NOISE vs. BANDWIDTH USING THE ADXL212 WITH OPERATING VOLTAGES OTHER THAN 5 V USING THE ADXL212 AS A DUAL AXIS TILT SENSOR Dual Axis Tilt Sensor: Converting Acceleration to Tilt OUTLINE DIMENSIONS ORDERING GUIDE