Datasheet ADXRS624 (Analog Devices) - 10

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ADXRS624 THEORY OF OPERATION
The ADXRS624 operates on the principle of a resonator gyro.
0.1
Two polysilicon sensing structures each contain a dither frame that is electrostatically driven to resonance, producing the
0.01
necessary velocity element to produce a Coriolis force during angular rate. At two of the outer extremes of each frame,
0.001
orthogonal to the dither motion, are movable fingers that are
rms)
placed between fixed pickoff fingers to form a capacitive pickoff
Hz
structure that senses Coriolis motion. The resulting signal is fed
0.0001 (°/s/
to a series of gain and demodulation stages that produce the electrical rate signal output. The dual-sensor design rejects
0.00001
external g forces and vibration. Fabricating the sensor with the signal conditioning electronics preserves signal integrity in noisy environments.
0.000001 10 100 1k 10k 100k
022
(Hz)
08999- The electrostatic resonator requires 18 V to 20 V for operation. Figure 22. Noise Spectral Density with Additional 250 Hz Filter 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
TEMPERATURE OUTPUT AND CALIBRATION
supply is available, the two capacitors on CP1 through CP4 can It is common practice to temperature-calibrate gyros to improve be omitted, and this supply can be connected to CP5 (Pin 6D, their overal accuracy. The ADXRS624 has a temperature Pin 7D). Note that CP5 should not be grounded when power is proportional voltage output that provides input to such a applied to the ADXRS624. Although no damage occurs, under calibration method. The temperature sensor structure is shown certain conditions the charge pump may fail to start up after the in Figure 23. The temperature output is characteristically ground is removed without first removing power from the nonlinear, and any load resistance connected to the TEMP ADXRS624. output results in decreasing the TEMP output and temperature
SETTING BANDWIDTH
coefficient. Therefore, buffering the output is recommended. External Capacitor C The voltage at the TEMP pin (3F, 3G) is nominally 2.5 V at OUT is used in combination with the on- chip R 25°C and V OUT resistor to create a low-pass filter to limit the RATIO = 5 V. The temperature coefficient is ~9 mV/°C bandwidth of the ADXRS624 rate response. The –3 dB at 25°C. Although the TEMP output is highly repeatable, it has frequency set by R only modest absolute accuracy. OUT and COUT is 1
VTEMP
f =
V
OUT (
RATIO
2 × π × R × C OUT OUT ) 023 and can be well controlled because R
R R FIXED TEMP
OUT is trimmed during 08999- manufacture to be 200 kΩ ± 1%. Any external resistor applied Figure 23. ADXRS624 Temperature Sensor Structure between the RATEOUT pin (1B, 2A) and SUMJ pin (1C, 2C)
CALIBRATED PERFORMANCE
results in ( Using a three-point calibration technique, it is possible to 200 kΩ × REXT ) R = calibrate the nul and sensitivity drift of the ADXRS624 to an OUT (200 kΩ + REXT ) overall accuracy of nearly 200°/hour. An overall accuracy of In general, an additional hardware or software filter is added to 40°/hour or better is possible using more points. attenuate high frequency noise arising from demodulation Limiting the bandwidth of the device reduces the flat-band spikes at the gyro’s 14 kHz resonant frequency (the noise spikes noise during the calibration process, improving the measure- at 14 kHz can be clearly seen in the power spectral density ment accuracy at each calibration point. curve shown in Figure 21). Typically, this additional filter’s corner frequency is set to greater than 5× the required band-
ADXRS624 AND SUPPLY RATIOMETRICITY
width to preserve good phase response. The ADXRS624 RATEOUT and TEMP signals are ratiometric Figure 22 shows the effect of adding a 250 Hz filter to the to the VRATIO voltage; that is, the null voltage, rate sensitivity, and output of an ADXRS624 set to 40 Hz bandwidth (as shown in temperature outputs are proportional to VRATIO. Thus, the Figure 21). High frequency demodulation artifacts are ADXRS624 is most easily used with a supply-ratiometric ADC attenuated by approximately 18 dB. that results in self-cancel ation of errors due to minor supply variations. There is some smal error due to nonratiometric Rev. A | 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 ADXRS624 and Supply Ratiometricity Null Adjustment Self-Test Function Continuous Self-Test Outline Dimensions Ordering Guide Automotive Products