Datasheet ADXRS649 (Analog Devices) - 10

link to page 10
Data Sheet ADXRS649 THEORY OF OPERATION
The ADXRS649 operates on the principle of a resonator gyro. Pin D7). CP5 should not be grounded when power is applied Figure 18 shows a simplified version of one of four polysilicon to the ADXRS649. No damage occurs, but under certain condi- sensing structures. Each sensing structure contains a dither tions, the charge pump may fail to start up after the ground is frame that is electrostatical y driven to resonance. This pro- removed without first removing power from the ADXRS649. duces the necessary velocity element to produce a Coriolis force
SETTING THE BANDWIDTH
when experiencing angular rate. The ADXRS649 is designed to sense a z-axis (yaw) angular rate. External Capacitor COUT is used in combination with the on- chip R When the sensing structure is exposed to angular rate, the result- OUT resistor to create a low-pass filter to limit the bandwidth of the ADXRS649 rate response. The −3 dB frequency set by ing Coriolis force couples into an outer sense frame, which R contains movable fingers that are placed between fixed pickoff OUT and COUT is fingers. This forms a capacitive pickoff structure that senses fOUT = 1/(2 × π × ROUT × COUT) Coriolis motion. The resulting signal is fed to a series of gain fOUT can be well controlled because ROUT has been trimmed and demodulation stages that produce the electrical rate signal during manufacturing to be 180 kΩ ± 1%. Any external resistor output. The quad sensor design rejects linear and angular applied between the RATEOUT pin (B1, A2) and the SUMJ pin acceleration, including external g-forces and vibration. This is (C1, C2) results in achieved by mechanical y coupling the four sensing structures R such that external g-forces appear as common-mode signals OUT = (180 kΩ × REXT)/(180 kΩ + REXT) that can be removed by the fully differential architecture In general, an additional filter (in either hardware or software) implemented in the ADXRS649. is added to attenuate high frequency noise arising from demodu- lation spikes at the 18 kHz resonant frequency of the gyro. An The electrostatic resonator requires 13 V to 15 V for operation. RC output filter consisting of a 3.3 kΩ series resistor and 22 nF Because only 5 V are typically available in most applications, shunt capacitor (2.2 kHz pole) is recommended. 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,
X Y Z
018 09573- Figure 18. Simplified Gyro Sensing Structure—One Corner Rev. B | Page 9 of 12 Document Outline Features Applications General Description Functional Block Diagram Revision History Specifications Absolute Maximum Ratings Rate Sensitive Axis ESD Caution Pin Configuration and Function Descriptions Typical Performance Characteristics Theory of Operation Setting the Bandwidth Temperature Output and Calibration Modifying the Measurement Range Null Bias Adjustment Self-Test Function Continuous Self-Test Outline Dimensions Ordering Guide