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ADXL311JE Arkusz danych(PDF) 7 Page - Analog Devices |
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ADXL311JE Arkusz danych(HTML) 7 Page - Analog Devices |
7 / 12 page ADXL311 THEORY OF OPERATION The ADXL311 is a complete, dual-axis acceleration measure- ment system on a single monolithic IC. It contains a polysilicon surface-micromachined sensor and signal conditioning cir- cuitry to implement an open-loop acceleration measurement architecture. The output signals are analog voltage proportional to acceleration. The ADXL311 is capable of measuring both positive and negative accelerations to at least ±2 g. The acceler- ometer can measure static acceleration forces, such as gravity, allowing it to be used as a tilt sensor. The sensor is a surface-micromachined polysilicon structure built on top of the silicon wafer. Polysilicon springs suspend the structure over the surface of the wafer and provide a resistance against acceleration forces. Deflection of the structure is meas- ured using a differential capacitor that consists of independent fixed plates and central plates attached to the moving mass. The fixed plates are driven by 180° out of phase square waves. Accel- eration will deflect the beam and unbalance the differential capacitor, resulting in an output square wave whose amplitude is proportional to acceleration. Phase sensitive demodulation techniques are then used to rectify the signal and determine the direction of the acceleration. The output of the demodulator is amplified and brought off- chip through a 32 kΩ resistor. At this point, the user can set the signal bandwidth of the device by adding a capacitor. This filtering improves measurement resolution and helps prevent aliasing. Applications POWER SUPPLY DECOUPLING For most applications, a single 0.1 µF capacitor, CDC, will ade- quately decouple the accelerometer from noise on the power supply. However, in some cases, particularly where noise is pre- sent at the 100 kHz internal clock frequency (or any harmonic thereof), noise on the supply may cause interference on the ADXL311 output. If additional decoupling is needed, a 100 Ω (or smaller) resistor or ferrite beads may be inserted in the sup- ply line of the ADXL311. Additionally, a larger bulk bypass capacitor (in the 1 µF to 4.7 µF range) may be added in parallel to CDC. SETTING THE BANDWIDTH USING CX AND CY The ADXL311 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 ( ) () ( ) Y X, dB C F × π = kΩ 32 2 / 1 3 – or, more simply () Y X, dB C F / F 5 3 – µ = The tolerance of the internal resistor (RFILT) can vary typically as much as ±15% of its nominal value of 32 kΩ; thus, the band- width will vary accordingly. A minimum capacitance of 1000 pF for CX and CY is required in all cases. Table 4. Filter Capacitor Selection, CX and CY Bandwidth Capacitor (µF) 10 Hz 0.47 50 Hz 0.10 100 Hz 0.05 200 Hz 0.027 500 Hz 0.01 5 kHz 0.001 SELF TEST The ST pin controls the self-test feature. When this pin is set to VDD, an electrostatic force is exerted on the beam of the acceler- ometer. The resulting movement of the beam allows the user to test if the accelerometer is functional. The typical change in output will be 270 m g (corresponding to 45 mV). This pin may be left open circuit or connected to common in normal use. RBIAS SELECTION A bias resistor (RBIAS) must always be used. If no resistor is pre- sent, the ADXL311 may appear to work but will suffer degraded noise performance. The value of the resistor used is not critical. Any value from 50 kΩ to 2 MΩ can be used. Using a 2 MΩ resistor rather than a 50 kΩ will save roughly 25 µA of supply current. Design Trade-Offs for Selecting Filter Characteristics: The Noise/BW Trade-Off The accelerometer bandwidth selected will ultimately determine 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 XOUT and YOUT. The output of the ADXL311 has a typical bandwidth of 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 A/D sampling frequency to minimize aliasing. The analog bandwidth may be further decreased to reduce noise and improve resolution. The ADXL311 noise has the characteristics of white Gaussian noise that contributes equally at all frequencies and is described in terms of µ g/√Hz, i.e., the noise is proportional to the square Rev. A | Page 7 of 12 |
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