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ADXL311_15 Datasheet(Arkusz danych) 9 Page  Analog Devices 

9 page ADXL311 Rev. B  Page 9 of 12 The output of the ADXL311 has a typical bandwidth of 3 kHz. The user must filter the signal at this point to limit aliasing errors. The analog bandwidth must be no more than half the A/D sampling frequency to minimize aliasing. The analog bandwidth can be further decreased to reduce noise and improve resolution. The ADXL311 noise has the characteristics of white Gaussian noise that contribute equally at all frequencies and are described in terms of µg/√Hz, i.e., the noise is proportional to the square root of the bandwidth of the accelerometer. It is recommended that the user limits the bandwidth to the lowest frequency needed by the application to maximize the resolution and dynamic range of the accelerometer. With the singlepole rolloff characteristic, the typical noise of the ADXL311 is determined by ( ) ( )6 . 1 Hz / 300 × × µ = BW g Noise RMS At 100 Hz the noise will be ( ) ( ) g g Noise RMS m 8 . 3 6 . 1 100 Hz / 300 = × × µ = Often the peak value of the noise is desired. Peaktopeak noise can only be estimated by statistical methods. Table 6 shows the probabilities of exceeding various peak values, given the rms value. Table 6. Estimation of PeaktoPeak Noise PeaktoPeak Value % of Time That Noise Exceeds Nominal PeaktoPeak Value 2 × rms 32 4 × rms 4.6 6 × rms 0.27 8 × rms 0.006 The peaktopeak noise value gives the best estimate of the uncertainty in a single measurement. Table 7 gives the typical noise output of the ADXL311 for various CX and CY values. Table 7. Filter Capacitor Selection, CX and CY Bandwidth (Hz) CX, CY (µF) RMS Noise (mg) PeaktoPeak Noise Estimate (mg) 10 0.47 1.2 7.2 50 0.1 2.7 16.2 100 0.047 3.8 22.8 500 0.01 8.5 51 USING THE ADXL311 WITH OPERATING VOLTAGES OTHER THAN 3 V The ADXL311 is tested and specified at VDD = 3 V; however, it can be powered with VDD as low as 2.4 V, or as high as 5.25 V. Some performance parameters change as the supply voltage varies. The ADXL311 output is ratiometric, so the output sensitivity (or scale factor) varies proportionally to the supply voltage. At VDD = 5 V, the output sensitivity is typically 312 mV/g. The 0 g bias output is also ratiometric, so the 0 g output is nominally equal to VDD/2 at all supply voltages. The output noise is not ratiometric, but absolute in volts; therefore, the noise density decreases as the supply voltage increases. This is because the scale factor (mV/g) increases while the noise voltage remains constant. The selftest response is roughly proportional to the square of the supply voltage. At VDD = 5 V, the selftest response is approximately equivalent to 750 mg (typical). The supply current increases as the supply voltage increases. Typical current consumption at VDD = 5 V is 750 µA. USING THE ADXL311 AS A DUALAXIS TILT SENSOR One of the most popular applications of the ADXL311 is tilt measurement. An accelerometer uses the force of gravity as an input vector to determine the orientation of an object in space. An accelerometer is most sensitive to tilt when its sensitive axis is perpendicular to the force of gravity, i.e., parallel to the earth’s surface. When the accelerometer is oriented parallel to the gravity vector, i.e., near its +1 g or –1 g reading, the change in output acceleration per degree of tilt is negligible. When the acceler ometer is perpendicular to gravity, its output changes nearly 17.5 mg per degree of tilt, but at 45° degrees, it changes only 12.2 mg per degree, and resolution declines. DualAxis Tilt Sensor: Converting Acceleration to Tilt When the accelerometer is oriented so both its Xaxis and Yaxis are parallel to the earth’s surface, it can be used as a twoaxis tilt sensor with a roll axis and a pitch axis. Once the output signal from the accelerometer has been converted to an acceleration that varies between –1 g and +1 g, the output tilt in degrees is calculated as follows: ( )g A SIN Pitch X 1 / A = ( )g A SIN Roll Y 1 / A = Be sure to account for overranges. It is possible for the accelerometers to output a signal greater than ±1 g due to vibration, shock, or other accelerations. 
