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ADXL326 Scheda tecnica(PDF) 11 Page - Analog Devices |
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ADXL326 Scheda tecnica(HTML) 11 Page - Analog Devices |
11 / 16 page ADXL326 Rev. 0 | Page 11 of 16 APPLICATIONS INFORMATION POWER SUPPLY DECOUPLING For most applications, a single 0.1 μF capacitor, CDC, placed close to the ADXL326 supply pins adequately decouples the accelerometer from noise on the power supply. However, in applications where noise is present at the 50 kHz internal clock frequency (or any harmonic thereof), additional care in power supply bypassing is required because this noise can cause errors in acceleration measurement. If additional decoupling is needed, a 100 Ω (or smaller) resistor or ferrite bead can be inserted in the supply line. Additionally, a larger bulk bypass capacitor (1 μF or greater) can be added in parallel to CDC. Ensure that the connection from the ADXL326 ground to the power supply ground is low impedance because noise transmitted through ground has a similar effect as noise transmitted through VS. SETTING THE BANDWIDTH USING CX, CY, AND CZ The ADXL326 has provisions for band limiting the XOUT, YOUT, and ZOUT pins. Capacitors must be added at these pins to implement low-pass filtering for antialiasing and noise reduction. The 3 dB bandwidth equation is f−3 dB = 1/(2π(32 kΩ) × C(X, Y, Z)) or more simply f–3 dB = 5 μF/C(X, Y, Z) The tolerance of the internal resistor (RFILT) typically varies as much as ±15% of its nominal value (32 kΩ), and the bandwidth varies accordingly. A minimum capacitance of 0.0047 μF for CX, CY, and CZ is recommended in all cases. Table 4. Filter Capacitor Selection, CX, CY, and CZ Bandwidth (Hz) Capacitor (μF) 1 4.7 10 0.47 50 0.10 100 0.05 200 0.027 500 0.01 SELF TEST The ST pin controls the self test feature. When this pin is set to VS, an electrostatic force is exerted on the accelerometer beam. The resulting movement of the beam allows the user to test whether the accelerometer is functional. The typical change in output is −1.08 g (corresponding to −62 mV) in the X axis, +1.08 g (+62 mV) on the Y axis, and +1.83 g (+105 mV) on the Z axis. This ST pin can be left open circuit or connected to common (COM) in normal use. Never expose the ST pin to voltages greater than VS + 0.3 V. If this cannot be guaranteed due to the system design (for instance, there are multiple supply voltages), then a low VF clamping diode between ST and VS is recommended. DESIGN TRADE-OFFS FOR SELECTING FILTER CHARACTERISTICS: THE NOISE/BW TRADE-OFF The selected accelerometer bandwidth ultimately determines the measurement resolution (smallest detectable acceleration). Filtering can be used to lower the noise floor to improve the resolution of the accelerometer. Resolution is dependent on the analog filter bandwidth at XOUT, YOUT, and ZOUT. The output of the ADXL326 has a typical bandwidth greater than 500 Hz. The user must filter the signal at this point to limit aliasing errors. The analog bandwidth must be no more than half the analog-to-digital sampling frequency to minimize aliasing. The analog bandwidth can be further decreased to reduce noise and improve resolution. The ADXL326 noise has the characteristics of white Gaussian noise, which contributes equally at all frequencies and is described in terms of μg/√Hz (the noise is proportional to the square root of the accelerometer bandwidth). The user should limit bandwidth to the lowest frequency needed by the application to maximize the resolution and dynamic range of the accelerometer. With the single-pole roll-off characteristic, the typical noise of the ADXL326 is determined by rms Noise = Noise Density × ) 1.6 ( × BW Often, the peak value of the noise is desired. Peak-to-peak noise can only be estimated by statistical methods. Table 5 is useful for estimating the probabilities of exceeding various peak values, given the rms value. Table 5. Estimation of Peak-to-Peak Noise Peak-to-Peak Value % of Time That Noise Exceeds Nominal Peak-to-Peak Value 2 × rms 32 4 × rms 4.6 6 × rms 0.27 8 × rms 0.006 USE WITH OPERATING VOLTAGES OTHER THAN 3 V The ADXL326 is tested and specified at VS = 3 V; however, it can be powered with VS as low as 1.8 V or as high as 3.6 V. Note that some performance parameters change as the supply voltage is varied. The ADXL326 output is ratiometric; therefore, the output sensitivity (or scale factor) varies proportionally to the supply voltage. At VS = 3.6 V, the output sensitivity is typically 68 mV/g. At VS = 2 V, the output sensitivity is typically 38 mV/g. The zero g bias output is also ratiometric; therefore, the zero g output is nominally equal to VS/2 at all supply voltages. The output noise is not ratiometric but is 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. At VS = 3.6 V, the X- and Y- axis noise density is typically 120 μg/√Hz, while at VS = 2 V, the X- and Y-axis noise density is typically 270 μg/√Hz. |
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