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ADXRS649BBGZ-RL Scheda tecnica(PDF) 9 Page - Analog Devices |
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ADXRS649BBGZ-RL Scheda tecnica(HTML) 9 Page - Analog Devices |
9 / 12 page Data Sheet ADXRS649 Rev. B | Page 9 of 12 THEORY OF OPERATION The ADXRS649 operates on the principle of a resonator gyro. Figure 18 shows a simplified version of one of four polysilicon sensing structures. Each sensing structure contains a dither frame that is electrostatically driven to resonance. This pro- duces the necessary velocity element to produce a Coriolis force when experiencing angular rate. The ADXRS649 is designed to sense a z-axis (yaw) angular rate. When the sensing structure is exposed to angular rate, the result- ing Coriolis force couples into an outer sense frame, which contains movable fingers that are placed between fixed pickoff fingers. This forms a capacitive pickoff structure that senses Coriolis motion. The resulting signal is fed to a series of gain and demodulation stages that produce the electrical rate signal output. The quad sensor design rejects linear and angular acceleration, including external g-forces and vibration. This is achieved by mechanically coupling the four sensing structures such that external g-forces appear as common-mode signals that can be removed by the fully differential architecture implemented in the ADXRS649. The electrostatic resonator requires 13 V to 15 V for operation. Because only 5 V are typically available in most applications, a charge pump is included on chip. If an external 13 V to 15 V supply is available, the two capacitors on CP1 to CP4 can be omitted, and this supply can be connected to CP5 (Pin D6, Pin D7). CP5 should not be grounded when power is applied to the ADXRS649. No damage occurs, but under certain condi- tions, the charge pump may fail to start up after the ground is removed without first removing power from the ADXRS649. SETTING THE BANDWIDTH External Capacitor COUT is used in combination with the on- chip ROUT resistor to create a low-pass filter to limit the bandwidth of the ADXRS649 rate response. The −3 dB frequency set by ROUT and COUT is fOUT = 1/(2 × π × ROUT × COUT) fOUT can be well controlled because ROUT has been trimmed during manufacturing to be 180 kΩ ± 1%. Any external resistor applied between the RATEOUT pin (B1, A2) and the SUMJ pin (C1, C2) results in ROUT = (180 kΩ × REXT)/(180 kΩ + REXT) In general, an additional filter (in either hardware or software) is added to attenuate high frequency noise arising from demodu- lation spikes at the 18 kHz resonant frequency of the gyro. An RC output filter consisting of a 3.3 kΩ series resistor and 22 nF shunt capacitor (2.2 kHz pole) is recommended. X Y Z Figure 18. Simplified Gyro Sensing Structure—One Corner |
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