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ADXL213EB Datasheet(PDF) 8 Page - Analog Devices |
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ADXL213EB Datasheet(HTML) 8 Page - Analog Devices |
8 / 12 page ADXL213 Rev. 0 | Page 8 of 12 THEORY OF OPERATION EARTH'S SURFACE TOP VIEW (Not to Scale) PIN 8 XOUT = 50% YOUT = 80% XOUT = 50% YOUT = 50% PIN 8 XOUT = 50% YOUT = 20% PIN 8 XOUT = 80% YOUT = 50% PIN 8 XOUT = 20% YOUT = 50% Figure 21. Output Response vs. Orientation The ADXL213 is a complete dual axis acceleration measure- ment system on a single monolithic IC. It contains a polysilicon surface-micromachined sensor and signal conditioning circuitry to implement an open-loop acceleration measurement architecture. The output signals are duty cycle modulated digital signals proportional to acceleration. The ADXL213 is capable of measuring both positive and negative accelerations to ±1.2 g. The accelerometer can measure static acceleration forces such as gravity, allowing the ADXL213 to be used as a tilt sensor. The sensor is a surface-micromachined polysilicon structure built on top of the silicon wafer. Polysilicon springs suspend the structure over the surface of the wafer and provide a resistance against acceleration forces. Deflection of the structure is mea- sured using a differential capacitor that consists of independent fixed plates and plates attached to the moving mass. The fixed plates are driven by 180° out-of-phase square waves. Accelera- tion deflects the beam and unbalances the differential capacitor, resulting in an output square wave whose amplitude is propor- tional to acceleration. Phase sensitive demodulation techniques are then used to rectify the signal and determine the direction of the acceleration. The output of the demodulator is amplified and brought off- chip through a 32 kΩ resistor. At this point, the user can set the signal bandwidth of the device by adding a capacitor. This filtering improves measurement resolution and helps prevent aliasing. After being low-pass filtered, the duty cycle modulator converts the analog signals to duty cycle modulated outputs that can be read by a counter. A single resistor (RSET) sets the period for a complete cycle. A 0 g acceleration produces a 50% nominal duty cycle. The acceleration can be determined by measuring the length of the positive pulse width (t1) and the period (t2). The nominal transfer function of the ADXL213 is Acceleration = ((t1/t2) – Zero g Bias)/Sensitivity Where in the case of the ADXL213 Zero g Bias = 50% nominal Sensitivity = 30%/g nominal t2 = RSET/125 MΩ PERFORMANCE Rather than using additional temperature compensation circuitry, innovative design techniques have been used to ensure that high performance is built in. As a result, there is essentially no quantization error or nonmonotonic behavior, and temperature hysteresis is very low (typically less than 10 mg over the –40°C to +85°C temperature range). Figure 9 shows the zero g output performance of eight parts (X and Y axis) over a –40°C to +85°C temperature range. Figure 12 demonstrates the typical sensitivity shift over temperature for VS = 5 V. Sensitivity stability is optimized for VS = 5 V, but is still very good over the specified range; it is typically better than ±2% over temperature at VS = 3 V. |
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