Datasheet ADXRS620 (Analog Devices) - 10

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ADXRS620 THEORY OF OPERATION
The ADXRS620 operates on the principle of a resonator gyro. Figure 22 shows the effect of adding a 250 Hz filter to the output Two polysilicon sensing structures each contain a dither frame of an ADXRS620 set to 40 Hz bandwidth (as shown in Figure 21). that is electrostatically driven to resonance, producing the High frequency demodulation artifacts are attenuated by necessary velocity element to produce a Coriolis force during approximately 18 dB. angular rate. At two of the outer extremes of each frame,
0.1
orthogonal to the dither motion, are movable fingers that are placed between fixed pickoff fingers to form a capacitive pickoff
rms)
structure that senses Coriolis motion. The resulting signal is fed
0.01 /√Hz
to a series of gain and demodulation stages that produces the
/sec
electrical rate signal output. The dual-sensor design rejects
Y(° T 0.001
external g-forces and vibration. Fabricating the sensor with the
SI EN
signal conditioning electronics preserves signal integrity in
D L A 0.0001
noisy environments.
R T
The electrostatic resonator requires 18 V to 20 V for operation.
0.00001
Because only 5 V are typically available in most applications,
ISE SPEC O
a charge pump is included on chip. If an external 18 V to 20 V
N
supply is available, the two capacitors on CP1 through CP4 can
0.000001 10 100 1k 10k 100k
021 be omitted and this supply can be connected to CP5 (Pin 6D,
FREQUENCY (Hz)
08887- Pin 7D). Note that CP5 should not be grounded when power is Figure 22. Noise Spectral Density with Additional 250 Hz Filter applied to the ADXRS620. Although no damage occurs, under certain conditions the charge pump may fail to start up after the
TEMPERATURE OUTPUT AND CALIBRATION
ground is removed without first removing power from the It is common practice to temperature-calibrate gyros to improve ADXRS620. their overal accuracy. The ADXRS620 has a temperature propor- tional voltage output that provides input to such a calibration
SETTING BANDWIDTH
method. The temperature sensor structure is shown in Figure 23. External Capacitor C The temperature output is characteristical y nonlinear, and any OUT is used in combination with the on- chip R load resistance connected to the TEMP output results in decreasing OUT resistor to create a low-pass filter to limit the bandwidth of the ADXRS620 rate response. The −3 dB the TEMP output and temperature coefficient. Therefore, buf- frequency set by R fering the output is recommended. OUT and COUT is f = 1 The voltage at the TEMP pin (3F, 3G) is nominally 2.5 V at 25°C, O UT (2 × π × R ×C ) OUT OUT and VRATIO = 5 V. The temperature coefficient is ~9 mV/°C at 25°C. Although the TEMP output is highly repeatable, it has This frequency can be well controlled because ROUT has been only modest absolute accuracy. trimmed during manufacturing to be 180 kΩ ± 1%. Any external resistor applied between the RATEOUT pin (1B, 2A)
VRATIO VTEMP
and SUMJ pin (1C, 2C) results in 022 (
RFIXED RTEMP
180 kΩ× R ) 08887- = Figure 23. Temperature Sensor Structure O R ( EXT UT 180 kΩ + R ) EXT
CALIBRATED PERFORMANCE
In general, an additional hardware or software filter is added Using a three-point calibration technique, it is possible to to attenuate high frequency noise arising from demodulation calibrate the nul and sensitivity drift of the ADXRS620 to spikes at the gyro’s 14 kHz resonant frequency. (The noise spikes an overal accuracy of nearly 200°/hour. An overall accuracy at 14 kHz can be clearly seen in the power spectral density curve of 40°/hour or better is possible using more points. shown in Figure 21). Typically, this additional filter’s corner frequency is set to greater than 5× the required bandwidth to Limiting the bandwidth of the device reduces the flat-band preserve good phase response. noise during the calibration process, improving the measure- ment accuracy at each calibration point. Rev. B | 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 ADXRS620 and Supply Ratiometricity Null Adjustment Self-Test Function Continuous Self-Test Outline Dimensions Ordering Guide Automotive Products