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EVAL-ADXRS624Z Datasheet(PDF) 9 Page - Analog Devices |
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EVAL-ADXRS624Z Datasheet(HTML) 9 Page - Analog Devices |
9 / 12 page ADXRS624 Rev. A | Page 9 of 12 THEORY OF OPERATION The ADXRS624 operates on the principle of a resonator gyro. Two polysilicon sensing structures each contain a dither frame that is electrostatically driven to resonance, producing the necessary velocity element to produce a Coriolis force during angular rate. At two of the outer extremes of each frame, orthogonal to the dither motion, are movable fingers that are placed between fixed pickoff fingers to form 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 dual-sensor design rejects external g forces and vibration. Fabricating the sensor with the signal conditioning electronics preserves signal integrity in noisy environments. The electrostatic resonator requires 18 V to 20 V for operation. Because only 5 V are typically available in most applications, a charge pump is included on-chip. If an external 18 V to 20 V supply is available, the two capacitors on CP1 through CP4 can be omitted, and this supply can be connected to CP5 (Pin 6D, Pin 7D). Note that CP5 should not be grounded when power is applied to the ADXRS624. Although no damage occurs, under certain conditions the charge pump may fail to start up after the ground is removed without first removing power from the ADXRS624. SETTING 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 ADXRS624 rate response. The –3 dB frequency set by ROUT and COUT is ( ) OUT OUT OUT C R f × × × = π 2 1 and can be well controlled because ROUT is trimmed during manufacture to be 200 kΩ ± 1%. Any external resistor applied between the RATEOUT pin (1B, 2A) and SUMJ pin (1C, 2C) results in ( ) ( ) EXT EXT OUT R R R + × = k Ω 200 k Ω 200 In general, an additional hardware or software filter is added to attenuate high frequency noise arising from demodulation spikes at the gyro’s 14 kHz resonant frequency (the noise spikes at 14 kHz can be clearly seen in the power spectral density curve shown in Figure 21). Typically, this additional filter’s corner frequency is set to greater than 5× the required band- width to preserve good phase response. Figure 22 shows the effect of adding a 250 Hz filter to the output of an ADXRS624 set to 40 Hz bandwidth (as shown in Figure 21). High frequency demodulation artifacts are attenuated by approximately 18 dB. 0.1 0.01 0.000001 0.00001 0.0001 0.001 10 100k 1k 100 (Hz) 10k Figure 22. Noise Spectral Density with Additional 250 Hz Filter TEMPERATURE OUTPUT AND CALIBRATION It is common practice to temperature-calibrate gyros to improve their overall accuracy. The ADXRS624 has a temperature proportional voltage output that provides input to such a calibration method. The temperature sensor structure is shown in Figure 23. The temperature output is characteristically nonlinear, and any load resistance connected to the TEMP output results in decreasing the TEMP output and temperature coefficient. Therefore, buffering the output is recommended. The voltage at the TEMP pin (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 RTEMP RFIXED VTEMP Figure 23. ADXRS624 Temperature Sensor Structure CALIBRATED PERFORMANCE Using a three-point calibration technique, it is possible to calibrate the null and sensitivity drift of the ADXRS624 to an overall accuracy of nearly 200°/hour. An overall accuracy of 40°/hour or better is possible using more points. Limiting the bandwidth of the device reduces the flat-band noise during the calibration process, improving the measure- ment accuracy at each calibration point. ADXRS624 AND SUPPLY RATIOMETRICITY The ADXRS624 RATEOUT and TEMP signals are ratiometric to the VRATIO voltage; that is, the null voltage, rate sensitivity, and temperature outputs are proportional to VRATIO. Thus, the ADXRS624 is most easily used with a supply-ratiometric ADC that results in self-cancellation of errors due to minor supply variations. There is some small error due to nonratiometric |
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