Datasheet ADXL213 (Analog Devices) - 10

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ADXL213 APPLICATIONS POWER SUPPLY DECOUPLING DESIGN TRADE-OFFS FOR SELECTING FILTER
For most applications, a single 0.1 μF capacitor, C
CHARACTERISTICS: THE NOISE/BW TRADE-OFF
DC, adequately decouples the accelerometer from noise on the power supply. The accelerometer bandwidth selected ultimately determines However, in some cases, particularly where noise is present at the measurement resolution (smallest detectable acceleration). the 140 kHz internal clock frequency (or any harmonic Filtering can be used to lower the noise floor, which improves thereof), noise on the supply may cause interference on the the resolution of the accelerometer. Resolution is dependent on ADXL213’s output. If additional decoupling is needed, a 100 Ω the analog filter bandwidth at XFILT and YFILT. (or smaller) resistor or ferrite beads may be inserted in the supply line of the ADXL213. Additionally, a larger bulk bypass The output of the ADXL213 has a typical bandwidth of 2.5 kHz. capacitor (in the range of 1 μF to 22 μF) may be added in The user must filter the signal at this point to limit aliasing parallel to C errors. The analog bandwidth must be no more than one-fifth DC. the PWM frequency to minimize aliasing. The analog
SETTING THE BANDWIDTH USING CX AND CY
bandwidth may be further decreased to reduce noise and The ADXL213 has provisions for bandlimiting the X improve resolution. OUT and YOUT pins. Capacitors must be added at these pins to implement The ADXL213 noise has the characteristics of white Gaussian low-pass filtering for antialiasing and noise reduction. The noise, which contributes equally at all frequencies and is equation for the –3 dB bandwidth is described in terms of μg/√Hz (i.e., the noise is proportional to F the square root of the accelerometer’s bandwidth). The user –3 dB = 1/(2π(32 kΩ) × C(X, Y)) should limit bandwidth to the lowest frequency needed by the or more simply, application in order to maximize the resolution and dynamic range of the accelerometer. F–3 dB = 5 μF/C(X, Y) With the single pole roll-off characteristic, the typical noise of The tolerance of the internal resistor (RFILT) can vary typically as the ADXL213 is determined by much as ±25% of its nominal value (32 kΩ); thus, the band- width varies accordingly. A minimum capacitance of 2000 pF rmsNoise = 160 ( μg / Hz )×( BW ×1.6 ) for CX and CY is required in all cases. At 100 Hz the noise is
Table 4. Filter Capacitor Selection, CX and CY Bandwidth (Hz) Capacitor (μF)
rmsNoise = 160 ( μg / Hz )×( 100×1.6) = 2 g m 1 4.7 10 0.47 Often, the peak value of the noise is desired. Peak-to-peak noise 50 0.10 can only be estimated by statistical methods. Table 5 is useful 100 0.05 for estimating the probabilities of exceeding various peak 200 0.027 values, given the rms value. 500 0.01
Table 5. Estimation of Peak-to-Peak Noise SELF TEST % of Time that Noise Will Exceed Peak-to-Peak Value Nominal Peak-to-Peak Value
The ST pin controls the self-test feature. When this pin is set to 2 × RMS 32 VS, an electrostatic force is exerted on the beam of the accelero- 4 × RMS 4.6 meter. The resulting movement of the beam allows the user to 6 × RMS 0.27 test if the accelerometer is functional. The typical change in 8 × RMS 0.006 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. Rev. A | Page 9 of 12 Document Outline REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION PERFORMANCE APPLICATIONS POWER SUPPLY DECOUPLING SETTING THE BANDWIDTH USING CX AND CY SELF TEST DESIGN TRADE-OFFS FOR SELECTING FILTER CHARACTERISTICS: THE NOISE/BW TRADE-OFF USING THE ADXL213 WITH OPERATING VOLTAGES OTHER THAN 5 V USING THE ADXL213 AS A DUAL-AXIS TILT SENSOR Dual-Axis Tilt Sensor: Converting Acceleration to Tilt PIN CONFIGURATIONS AND FUNCTIONAL DESCRIPTIONS OUTLINE DIMENSIONS ESD CAUTION ORDERING GUIDE AUTOMOTIVE PRODUCTS