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ADXL206HDZ Arkusz danych(PDF) 10 Page - Analog Devices |
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ADXL206HDZ Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 12 page ADXL206 Rev. 0 | Page 10 of 12 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. In some cases, however, 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 ADXL206 output. If additional decoupling is needed, a 100 Ω (or smaller) resistor or ferrite bead can be inserted in the supply line of the ADXL206. 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 ADXL206 has provisions for band-limiting the XOUT and YOUT pins. A capacitor must be added to the pin 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) × Cx) or more simply, f−3 dB = 5 μF/Cx The tolerance of the internal resistor (RFILT) can vary typically as much as ±25% of its nominal value (32 kΩ); thus, the band- width varies accordingly. A minimum capacitance of 2000 pF for CX and CY is required in all cases. Table 6. 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 acceler- ometer. The resulting movement of the beam allows the user to test whether the accelerometer is functional. The typical change in output is 800 mg (corresponding to 250 mV). This pin can be left open-circuit or connected to common in normal use. The ST pin should never be exposed to voltage 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), it is recommended that a clamping diode with low forward voltage be connected between ST and VS. DESIGN TRADE-OFFS FOR SELECTING FILTER CHARACTERISTICS: 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. The output of the ADXL206 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 ADXL206 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). 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 ADXL206 is determined by ( ) ( )6 . 1 Hz / 110 BW g Noise rms × × μ = At 100 Hz, the noise is ( ) ( ) g g Noise rms m 4 . 1 6 . 1 100 Hz / 110 = × × μ = Often, the peak value of the noise is desired. Peak-to-peak noise can only be estimated by statistical methods. Table 7 is useful for estimating the probability of exceeding various peak values, given the rms value. Table 7. 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 uncer- tainty in a single measurement; peak-to-peak noise is estimated by 6 × rms. Table 8 gives the typical noise output of the ADXL206 for various CX and CY values. Table 8. Typical Noise Output for Various Capacitor Values 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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