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ADXRS620 Scheda tecnica(PDF) 9 Page - Analog Devices |
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ADXRS620 Scheda tecnica(HTML) 9 Page - Analog Devices |
9 / 12 page ADXRS620 Rev. 0 | Page 9 of 12 () THEORY OF OPERATION Figure 22 shows the effect of adding a 250 Hz filter to the output of an ADXRS620 set to 40 Hz bandwidth (as shown in Figure 21). High frequency demodulation artifacts are attenuated by approximately 18 dB. The ADXRS620 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 produces 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. 0.1 0.01 0.000001 0.00001 0.0001 0.001 10 100k 1k 100 FREQUENCY (Hz) 10k 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 ADXRS620. 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 ADXRS620. 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 ADXRS620 has a temperature propor- tional 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, buf- fering the output is recommended. 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 ADXRS620 rate response. The −3 dB frequency set by ROUT and COUT is OUT OUT UT O C R f × × × = π 2 1 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. This frequency can be well controlled because ROUT has been trimmed during manufacturing to be 180 kΩ ± 1%. Any external resistor applied between the RATEOUT pin (1B, 2A) and SUMJ pin (1C, 2C) results in VRATIO VTEMP RFIXED RTEMP ( ) () EXT UT O R R + = kΩ 180 EXT R × kΩ 180 Figure 23. Temperature Sensor Structure CALIBRATED PERFORMANCE 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 bandwidth to preserve good phase response. Using a three-point calibration technique, it is possible to calibrate the null and sensitivity drift of the ADXRS620 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. |
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