Datasheet ADXRS642 (Analog Devices) - 9

link to page 9 link to page 9 link to page 9
ADXRS642 Data Sheet THEORY OF OPERATION
The ADXRS642 operates on the principle of a resonator gyro.
SETTING BANDWIDTH
Figure 13 shows a simplified version of one of four polysilicon The external capacitor, C sensing structures. Each sensing structure contains a dither OUT, is used in combination with the on-chip resistor, R frame that is electrostatical y driven to resonance. This produces OUT, to create a low-pass filter to limit the bandwidth of the ADXRS642 rate response. The −3 dB the necessary velocity element to produce a Coriolis force when frequency set by R experiencing angular rate. The ADXRS642 is designed to sense OUT and COUT is f = / 1 (2 × π × R × C ) a z-axis (yaw) angular rate. OUT OUT OUT When the sensing structure is exposed to angular rate, the and can be well control ed because ROUT has been trimmed resulting Coriolis force couples into an outer sense frame, during manufacturing to be 180 kΩ ± 1%. Any external resistor which contains movable fingers that are placed between fixed applied between the RATEOUT pin (1B, 2A) and SUMJ pin pickoff fingers. This forms a capacitive pickoff structure that (1C, 2C) results in senses Coriolis motion. The resulting signal is fed to a series of R = (180 kΩ × R )/(180 kΩ + R ) gain and demodulation stages that produce the electrical rate OUT EXT EXT signal output. The quad sensor design rejects linear and angular In general, an additional filter (in either hardware or software) acceleration, including external g-forces and vibration. This is is added to attenuate high frequency noise arising from achieved by mechanical y coupling the four sensing structures demodulation spikes at the 18 kHz resonant frequency of the such that external g-forces appear as common-mode signals gyro. An R/C output filter consisting of a 3.3k series resistor and that can be removed by the fully differential architecture 22 nF shunt capacitor (2.2 kHz pole) is recommended. Figure 13 implemented in the ADXRS642. shows the effect of adding this filter to the output of an ADXRS642 set to a 2000 Hz bandwidth.
TEMPERATURE OUTPUT AND CALIBRATION
It is common practice to temperature-calibrate gyros to improve their overall accuracy. The ADXRS642 has a temperature propor- tional voltage output that provides input to such a calibration method. The temperature sensor structure is shown in Figure 14. The temperature output is characteristical y nonlinear, and any
X
load resistance connected to the TEMP output results in decreasing
Y
the TEMP output and its temperature coefficient. Therefore,
Z
buffering the output is recommended. The voltage at TEMP (3F, 3G) is nominally 2.5 V at 25°C, and VRATIO = 5 V. The temperature coefficient is ~9 mV/°C at 25°C. Although the TEMP output is highly repeatable, it has only modest absolute accuracy.
VRATIO VTEMP
003 016
RFIXED RTEMP
09770- 09770- Figure 13. Simplified Gyro Sensing Structure–One Corner Figure 14. Temperature Sensor Structure The electrostatic resonator requires 18 V to 20 V for operation.
SUPPLY RATIOMETRICITY
Because only 5 V are typically available in most applications, a The ADXRS642 RATEOUT, ST1, ST2, and TEMP signals are charge pump is included on chip. If an external 18 V to 20 V ratiometric to the VRATIO voltage; for example, the null voltage, supply is available, the two capacitors on CP1 to CP4 can be rate sensitivity and temperature outputs are proportional to omitted, and this supply can be connected to CP5 (Pin 6D, VRATIO. Therefore, it is most easily used with a supply-ratiometric Pin 7D). CP5 should not be grounded when power is applied to ADC, which results in self-cancel ation of errors due to minor the ADXRS642. No damage occurs, but under certain conditions, supply variations. There is some small, usual y negligible, error the charge pump may fail to start up after the ground is removed due to nonratiometric behavior. Note that to guarantee ful rate without first removing power from the ADXRS642. range, VRATIO should not be greater than AVCC. Rev. A | Page 8 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 Bandwidth Temperature Output and Calibration Supply Ratiometricity Modifying the Measurement Range Null Adjustment Self-Test Function Continuous Self-Test Mechanical Performance Outline Dimensions Ordering Guide