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ADXRS614 Datasheet(Arkusz danych) 9 Page - Analog Devices

Numer części ADXRS614
Szczegółowy opis  ±50°/s Yaw Rate Gyro
Pobierz  12 Pages
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Producent  AD [Analog Devices]
Strona internetowa  http://www.analog.com
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ADXRS614
Rev. 0 | Page 9 of 12
THEORY OF OPERATION
The ADXRS614 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
necessary 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 through CP4 can
be omitted and this supply can be connected to CP5 (Pin 6D,
Pin 7D). Note that CP5 should not be grounded when power is
applied to the ADXRS614. Although no damage occurs, under
certain conditions the charge pump may fail to start up after the
ground is removed without first removing power from the
ADXRS614.
SETTING BANDWIDTH
External Capacitor COUT is used in combination with the on-
chip ROUT resistor to create a low-pass filter to limit the
bandwidth of the ADXRS614 rate response. The –3 dB
frequency set by ROUT and COUT is:
()
OUT
OUT
OUT
C
R
f
×
×
×
=
π
2
1
and can be well controlled because ROUT has been trimmed
during manufacturing to be 200 kΩ ±1%. Any external resistor
applied between the RATEOUT pin (1B, 2A) and SUMJ pin
(1C, 2C) results in:
(
)
()
EXT
EXT
OUT
R
R
R
+
×
=
200
200
In general, an additional hardware or software filter is added to
attenuate high frequency noise arising from demodulation
spikes at the gyro’s 14 kHz resonant frequency (the noise spikes
at 14 kHz can be clearly seen in the power spectral density
curve shown in Figure 21). Typically, this additional filter’s
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 ADXRS614 set to 40 Hz bandwidth (as shown in
Figure 21). High frequency demodulation artifacts are
attenuated by approximately 18 dB.
0.1
0.01
0.000001
0.00001
0.0001
0.001
10
100k
1k
100
(Hz)
10k
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 ADXRS614 has a
temperature proportional 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 temperature
coefficient. Therefore, buffering the output is recommended.
The voltage at the TEMP pin (3F, 3G) is nominally 2.5 V at
25°C and VRATIO = 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.
VRATIO
RTEMP
RFIXED
VTEMP
Figure 23. ADXRS614 Temperature Sensor Structure
CALIBRATED PERFORMANCE
Using a 3-point calibration technique, it is possible to calibrate
the null and sensitivity drift of the ADXRS614 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.
ADXRS614 AND SUPPLY RATIOMETRICITY
The ADXRS614 RATEOUT and TEMP signals are ratiometric
to the VRATIO voltage, that is, the null voltage, rate sensitivity, and
temperature outputs are proportional to VRATIO. Thus, the
ADXRS614 is most easily used with a supply-ratiometric ADC
that results in self-cancellation of errors due to minor supply
variations. There is some small error due to nonratiometric




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