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ADXRS612_15 Datasheet(Arkusz danych) 9 Page - Analog Devices
AD [Analog Devices]
Rev. 0 | Page 9 of 12
THEORY OF OPERATION
The ADXRS612 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 on-chip. If an external 18 V to 20 V
supply is available, the two capacitors on CP1 to CP4 can be
omitted, and this supply can be connected to CP5 (Pin 6 D,
Pin 7D). CP5 should not be grounded when power is applied to
the ADXRS612. No damage occurs, but under certain conditions
the charge pump may fail to start up after the ground is removed
without first removing power from the ADXRS612.
External Capacitor C
is used in combination with the on-
resistor to create a low-pass filter to limit the bandwidth
of the ADXRS612 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 filter (in either hardware or software)
is added to attenuate high frequency noise arising from demodu-
lation 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. Normally, this additional
filter corner frequency is set to greater than five times the
required bandwidth to preserve good phase response.
Figure 22 shows the effect of adding a 250 Hz filter to the
output of an ADXRS612 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 ADXRS612 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 TEMP (3F, 3G) is nominally 2.5 V at 25°C, and
= 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. ADXRS612 Temperature Sensor Structure
Using a 3-point calibration technique, it is possible to calibrate
the ADXRS612 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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