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ADXL103WCEZB-REEL Arkusz danych(PDF) 14 Page - Analog Devices |
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ADXL103WCEZB-REEL Arkusz danych(HTML) 14 Page - Analog Devices |
14 / 16 page ADXL103/ADXL203 Data Sheet Rev. D | Page 14 of 16 APPLICATIONS INFORMATION POWER SUPPLY DECOUPLING For most applications, a single 0.1 μF capacitor, CDC, adequately decouples the accelerometer from noise on the power supply. However, in some cases, particularly where noise is present at the 140 kHz internal clock frequency (or any harmonic thereof), noise on the supply can cause interference on the ADXL103/ ADXL203 output. If additional decoupling is needed, a 100 Ω (or smaller) resistor or ferrite beads can be inserted in the supply line of the ADXL103/ADXL203. Additionally, a larger bulk bypass capacitor (in the 1 μF to 22 μF range) can be added in parallel to CDC. SETTING THE BANDWIDTH USING CX AND CY The ADXL103/ADXL203 has provisions for band limiting the XOUT and YOUT 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–3 dB = 1/(2π(32 kΩ) × C(X, Y)) or more simply, f–3 dB = 5 μF/C(X, Y) The tolerance of the internal resistor (RFILT) can vary typically as much as ±25% of its nominal value (32 kΩ); thus, the bandwidth varies accordingly. A minimum capacitance of 2000 pF for CX and CY is required in all cases. Table 7. Filter Capacitor Selection, CX and CY 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 beam of the accelerometer. The resulting movement of the beam allows the user to test if the accelerometer is functional. The typical change in output is 750 mg (corresponding to 750 mV). This pin can be left open- circuit or connected to common in normal use. Never expose the ST pin to voltages greater than VS + 0.3 V. If the system design is such that this condition cannot be guaranteed (that is, multiple supply voltages are present), a low VF clamping diode between ST and VS is recommended. DESIGN TRADE-OFFS FOR SELECTING FILTER CHARACTERISTICS: THE NOISE/BANDWIDTH TRADE-OFF The accelerometer bandwidth selected ultimately determines the measurement resolution (smallest detectable acceleration). Filtering can be used to lower the noise floor, improving the resolution of the accelerometer. Resolution is dependent on the analog filter bandwidth at XOUT and YOUT. The output of the ADXL103/ADXL203 has a typical bandwidth of 2.5 kHz. 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 ADXL103/ADXL203 noise has the characteristics of white Gaussian noise, which contributes equally at all frequencies and is described in terms of μg/√Hz (that is, 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 noise of the ADXL103/ADXL203 is determined by rmsNoise = (110 μg/√Hz) × ( 6 . 1 BW × ) At 100 Hz, the noise is rmsNoise = (110 μg/√Hz) × ( 6 . 1 100 × ) = 1.4 mg Often, the peak value of the noise is desired. Peak-to-peak noise can only be estimated by statistical methods. Table 8 is useful for estimating the probabilities of exceeding various peak values, given the rms value. Table 8. 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 Peak-to-peak noise values give the best estimate of the uncertainty in a single measurement; peak-to-peak noise is estimated by 6 × rms. Table 9 gives the typical noise output of the ADXL103/ ADXL203 for various CX and CY values. Table 9. Filter Capacitor Selection (CX, CY) Bandwidth (Hz) CX, CY (μF) RMS Noise (mg) Peak-to-Peak Noise Estimate (mg) 10 0.47 0.4 2.6 50 0.1 1.0 6 100 0.047 1.4 8.4 500 0.01 3.1 18.7 |
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