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ADXL316WBCSZ-RL7 데이터시트(PDF) 11 Page - Analog Devices |
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ADXL316WBCSZ-RL7 데이터시트(HTML) 11 Page - Analog Devices |
11 / 14 page Data Sheet ADXL316 Rev. 0 | Page 11 of 14 APPLICATIONS INFORMATION POWER SUPPLY DECOUPLING For most applications, a single 0.1 µF capacitor, CDC, placed close to the ADXL316 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 a 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 ADXL316 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 ADXL316 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 equation for the −3 dB bandwidth is f−3dB = 1/(2π(32 kΩ) × C(X,Y, Z)) or more simply f–3dB = 5 µF/C(X,Y, Z) The tolerance of the internal resistor (RFILT) can vary by 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 connected to VS, an electrostatic force is exerted on the accelerometer beam. The resulting movement of the beam allows the user to test if the accelerometer is functional. The typical change in output is −0.88 g (corresponding to −50 mV) on the x-axis, 0.88 g (or +50 mV) on the y-axis, and 1.58 g (or +90 mV) on the z-axis. The ST pin may be left open circuit or connected to the common pin (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, if there are multiple supply voltages), 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 (the smallest detectable acceleration). Filtering can 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 ADXL316 has a typical bandwidth of 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 decrease further to reduce noise and improve resolution. The ADXL316 has 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). 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 rms noise of the ADXL316 is determined by ) 1.6 ( × × = BW Density Noise Noise RMS Often, the peak value of the noise is desired. Statistical methods can only estimate peak-to-peak noise. 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 |
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