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ADXRS646 Datasheet(PDF) 9 Page - Analog Devices |
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ADXRS646 Datasheet(HTML) 9 Page - Analog Devices |
9 / 12 page Data Sheet ADXRS646 Rev. 0 | Page 9 of 12 THEORY OF OPERATION The ADXRS646 operates on the principle of a resonator gyroscope. Figure 19 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 produces the necessary velocity element to produce a Coriolis force when experiencing angular rate. The ADXRS646 is designed to sense a Z-axis (yaw) angular rate. When the sensing structure is exposed to angular rate, the resulting 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, shock, and vibration. The rejection 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 ADXRS646. Figure 19. Simplified Gyroscope Sensing Structure—One Corner The electrostatic resonator requires 21 V for operation. Because only 6 V are typically available in most applications, a charge pump is included on chip. If an external 21 V supply is available, the two capacitors on CP1 to CP4 can be omitted, and this supply can be connected to CP5 (Pin 6D, Pin 7D). CP5 should not be grounded when power is applied to the ADXRS646. No damage occurs, but under certain conditions, the charge pump may fail to start up after the ground is removed without first removing power from the ADXRS646. SETTING BANDWIDTH The combination of an external capacitor (COUT) and the on-chip resistor (ROUT) creates a low-pass filter that limits the bandwidth of the ADXRS646 rate response. The −3 dB frequency set by ROUT and COUT is fOUT = 1/(2 × π × ROUT × COUT) and can be well controlled because ROUT is trimmed during manufacturing to 180 kΩ ± 1%. Any external resistor applied between the RATEOUT pin (1B, 2A) and SUMJ pin (1C, 2C) results in ROUT = (180 kΩ × REXT)/(180 kΩ + REXT) An additional external filter is often added (in either hardware or software) to attenuate high frequency noise arising from demodulation spikes at the 18 kHz resonant frequency of the gyroscope. An RC output filter consisting of a 3.3 kΩ series resistor and 22 nF shunt capacitor (2.2 kHz pole) is recommended. TEMPERATURE OUTPUT AND CALIBRATION It is common practice to temperature-calibrate gyroscopes to improve their overall accuracy. The ADXRS646 has a temperature-dependent voltage output that provides input to such a calibration method. The temperature sensor structure is shown in Figure 20. The temperature output is characteristi- cally nonlinear, and any load resistance connected to the TEMP output results in decreasing the TEMP output and its temperature coefficient. Therefore, buffering the output is recommended. The voltage at TEMP (3F, 3G) is nominally 2.9 V at 25°C, and VRATIO = 6 V. The temperature coefficient is 10 mV/°C (typical) at 25°C; the output response over the full temperature range is shown in Figure 17. Although the TEMP output is highly repeatable, it has only modest absolute accuracy. Figure 20. Temperature Sensor Structure X Y Z VRATIO VTEMP RFIXED RTEMP |
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