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ADXRS649 Datasheet(PDF) 9 Page - Analog Devices

Part No. ADXRS649
Description  Fast Starting, ±20,000°/sec Vibration Rejecting Rate Gyro
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com

ADXRS649 Datasheet(HTML) 9 Page - Analog Devices

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The ADXRS649 operates on the principle of a resonator gyro.
Figure 18 shows a simplified version of one of four polysilicon
sensing structures. Each sensing structure contains a dither
frame that is electrostatically driven to resonance. This pro-
duces the necessary velocity element to produce a Coriolis force
when experiencing angular rate. The ADXRS649 is designed to
sense a z-axis (yaw) angular rate.
When the sensing structure is exposed to angular rate, the result-
ing Coriolis force couples into an outer sense frame, which
contains movable fingers that are placed between fixed pickoff
fingers. This forms 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 quad sensor design rejects linear and angular
acceleration, including external g-forces and vibration. This is
achieved by mechanically coupling the four sensing structures
such that external g-forces appear as common-mode signals
that can be removed by the fully differential architecture
implemented in the ADXRS649.
The electrostatic resonator requires 13 V to 15 V for operation.
Because only 5 V are typically available in most applications,
a charge pump is included on chip. If an external 13 V to 15 V
supply is available, the two capacitors on CP1 to CP4 can be
omitted, and this supply can be connected to CP5 (Pin D6,
Pin D7). CP5 should not be grounded when power is applied
to the ADXRS649. No damage occurs, but under certain condi-
tions, the charge pump may fail to start up after the ground is
removed without first removing power from the ADXRS649.
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 ADXRS649 rate response. The −3 dB frequency set by
ROUT and COUT is
fOUT = 1/(2 × π × ROUT × COUT)
fOUT can be well controlled because ROUT has been trimmed
during manufacturing to be 180 kΩ ± 1%. Any external resistor
applied between the RATEOUT pin (B1, A2) and the SUMJ pin
(C1, C2) results in
ROUT = (180 kΩ × REXT)/(180 kΩ + REXT)
In general, an additional filter (in either hardware or software)
is added to attenuate high frequency noise arising from demodu-
lation spikes at the 18 kHz resonant frequency of the gyro. An
RC output filter consisting of a 3.3 kΩ series resistor and 22 nF
shunt capacitor (2.2 kHz pole) is recommended.
Figure 18. Simplified Gyro Sensing Structure—One Corner

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