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ADXL213 Datasheet(PDF) 9 Page - Analog Devices |
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ADXL213 Datasheet(HTML) 9 Page - Analog Devices |
9 / 12 page ADXL213 Rev. A | Page 9 of 12 APPLICATIONS 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 may cause interference on the ADXL213’s output. If additional decoupling is needed, a 100 Ω (or smaller) resistor or ferrite beads may be inserted in the supply line of the ADXL213. Additionally, a larger bulk bypass capacitor (in the range of 1 μF to 22 μF) may be added in parallel to CDC. SETTING THE BANDWIDTH USING CX AND CY The ADXL213 has provisions for bandlimiting 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 band- width varies accordingly. A minimum capacitance of 2000 pF for CX and CY is required in all cases. Table 4. 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 accelero- meter. 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 23%). This pin may be left open circuit, or may be connected to common in normal use. The ST pin should never be exposed to voltages greater than VS + 0.3 V. If the system design is such that this condition cannot be guaranteed (i.e., multiple supply voltages present), a low VF clamping diode between ST and VS is recommended. DESIGN TRADE-OFFS FOR SELECTING FILTER CHARACTERISTICS: THE NOISE/BW TRADE-OFF The accelerometer bandwidth selected ultimately determines the measurement resolution (smallest detectable acceleration). Filtering can be used to lower the noise floor, which improves the resolution of the accelerometer. Resolution is dependent on the analog filter bandwidth at XFILT and YFILT. The output of the ADXL213 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 one-fifth the PWM frequency to minimize aliasing. The analog bandwidth may be further decreased to reduce noise and improve resolution. The ADXL213 noise has the characteristics of white Gaussian noise, which contributes equally at all frequencies and is described in terms of μg/√Hz (i.e., the noise is proportional to the square root of the accelerometer’s bandwidth). The user should limit bandwidth to the lowest frequency needed by the application in order to maximize the resolution and dynamic range of the accelerometer. With the single pole roll-off characteristic, the typical noise of the ADXL213 is determined by ) 6 . 1 ( ) Hz / μ 160 ( × × = BW g rmsNoise At 100 Hz the noise is g g rmsNoise m 2 ) 6 . 1 100 ( ) Hz / μ 160 ( = × × = 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 Will Exceed Nominal Peak-to-Peak Value 2 × RMS 32 4 × RMS 4.6 6 × RMS 0.27 8 × RMS 0.006 |
Número de pieza similar - ADXL213_15 |
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Descripción similar - ADXL213_15 |
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