Datasheet ADXRS622 (Analog Devices) - 10

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ADXRS622 THEORY OF OPERATION
The ADXRS622 operates on the principle of a resonator gyro.
0.1
Two polysilicon sensing structures each contain a dither frame that is electrostatical y driven to resonance, producing the neces-
0.01
sary 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
0.001 rms)
fixed pickoff fingers to form a capacitive pickoff structure that
Hz
senses Coriolis motion. The resulting signal is fed to a series of
0.0001 (°/sec/
gain and demodulation stages that produce the electrical rate signal output. The dual-sensor design rejects external g-forces and
0.00001
vibration. Fabricating the sensor with the signal conditioning electronics preserves signal integrity in noisy environments.
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The electrostatic resonator requires 18 V to 20 V for operation.
10 100 1k 10k 100k
021 Because only 5 V are typically available in most applications,
(Hz)
07754- a charge pump is included onchip. If an external 18 V to 20 V Figure 22. Noise Spectral Density with Additional 250 Hz Filter supply is available, the two capacitors on CP1 from CP4 can
TEMPERATURE OUTPUT AND CALIBRATION
be omitted, and this supply can be connected to the CP5 pin It is common practice to temperature-calibrate gyros to improve (6D, 7D). Note that CP5 should not be grounded when power is their overall accuracy. The ADXRS622 has a temperature propor- applied to the ADXRS622. Although no damage occurs, under tional voltage output that provides input to such a calibration certain conditions the charge pump may fail to start up after the method. The temperature sensor structure is shown in Figure 23. ground is removed if power is not first removed from the The temperature output is characteristical y nonlinear, and any ADXRS622. load resistance connected to the TEMP output results in decreasing
SETTING BANDWIDTH
the TEMP output and its temperature coefficient. Therefore, External Capacitor C buffering the output is recommended. OUT is used in combination with the on- chip ROUT resistor to create a low-pass filter to limit the bandwidth The voltage at the TEMP pin (3F, 3G) is nominally 2.5 V at 25°C, of the ADXRS622 rate response. The −3 dB frequency set by and VRATIO = 5 V. The temperature coefficient is ~9 mV/°C at ROUT and COUT is 25°C. Although the TEMP output is highly repeatable, it has f = / 1 (2 × π × R × C ) only modest absolute accuracy. OUT OUT OUT
V
and can be well control ed because R
RATIO VTEMP
OUT has been trimmed during manufacturing to be 180 kΩ ± 1%. Any external resistor 022
R
applied between the RATEOUT pin (1B, 2A) and SUMJ pin
FIXED RTEMP
07754- (1C, 2C) results in Figure 23. ADXRS622 Temperature Sensor Structure R = (180 kΩ × R )/(180 kΩ + R ) OUT EXT EXT
CALIBRATED PERFORMANCE
In general, an additional hardware or software filter is added to Using a three-point calibration technique, it is possible to attenuate high frequency noise arising from demodulation spikes calibrate the ADXRS622 null and sensitivity drift to an overall at the 14 kHz resonant frequency of the gyro. The noise spikes accuracy of nearly 200°/hour. An overal accuracy of 40°/hour at 14 kHz can be clearly seen in the power spectral density or better is possible using more points. curve, shown in Figure 21. Typically, this additional filter corner Limiting the bandwidth of the device reduces the flat-band noise frequency is set to greater than 5× the required bandwidth to during the calibration process, improving the measurement preserve good phase response. accuracy at each calibration point. Figure 22 shows the effect of adding a 250 Hz filter to the output of an ADXRS622 set to 40 Hz bandwidth (as shown in Figure 21). High frequency demodulation artifacts are attenuated by approximately 18 dB. Rev. C | 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 Bandwidth Temperature Output and Calibration Calibrated Performance ADXRS622 and Supply Ratiometricity Null Adjustment Self-Test Function Continuous Self-Test Outline Dimensions Ordering Guide Automotive Products