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ADXRS622WBBGZA Datasheet(Arkusz danych) 9 Page - Analog Devices
AD [Analog Devices]
Rev. C | Page 9 of 12
THEORY OF OPERATION
The ADXRS622 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 neces-
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
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 onchip. If an external 18 V to 20 V
supply is available, the two capacitors on CP1 from CP4 can
be omitted, and this supply can be connected to the CP5 pin
(6D, 7D). Note that CP5 should not be grounded when power is
applied to the ADXRS622. Although no damage occurs, under
certain conditions the charge pump may fail to start up after the
ground is removed if power is not first removed from the
External Capacitor C
is used in combination with the on-
resistor to create a low-pass filter to limit the bandwidth
of the ADXRS622 rate response. The −3 dB frequency set by
and can be well controlled because R
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
In general, an additional hardware or software filter is added to
attenuate high frequency noise arising from demodulation spikes
at the 14 kHz resonant frequency of the gyro. The noise spikes
at 14 kHz can be clearly seen in the power spectral density
curve, shown in Figure 21. Typically, this additional filter corner
frequency is set to greater than 5× the required bandwidth to
preserve good phase response.
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.
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 ADXRS622 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 its temperature coefficient. Therefore,
buffering the output is recommended.
The voltage at the TEMP pin (3F, 3G) is nominally 2.5 V at 25°C,
= 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.
Figure 23. ADXRS622 Temperature Sensor Structure
Using a three-point calibration technique, it is possible to
calibrate the ADXRS622 null and sensitivity drift 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 measurement
accuracy at each calibration point.
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