ADXRS646 Analog Devices, Inc., ADXRS646 Datasheet - Page 7

• Manufacturer Part Number: ADXRS646
• Description: High Stability, Low Noise Vibration Rejecting Yaw Rate Gyro
• Manufacturer: Analog Devices, Inc.

THEORY OF OPERATION
The ADXRS646 operates on the principle of a resonator gyro.
Figure 10 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
produces the necessary velocity element to produce a Coriolis
force when experiencing angular rate. The ADXRS646 is
designed to sense a Z-axis (yaw) angular rate.
When the sensing structure is exposed to angular rate, the
resulting 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 ADXRS646.
The electrostatic resonator requires 21V for operation. Because
only 6V are typically available in most applications,
a charge pump is included on chip. If an external 21V supply is
available, the two capacitors on CP1 to CP4 can be omitted, and
this supply can be connected to CP5 (Pin 6D,
Pin 7D). CP5 should not be grounded when power is applied to
the ADXRS646. 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 ADXRS646.
Preliminary Technical Data
Z
X
Figure 10. Simplified Gyro Sensing Structure – One Corner
Y
Rev. PrA | Page 7 of 12
SETTING BANDWIDTH
External Capacitor C
chip R
of the ADXRS646 rate response. The −3 dB frequency set by
R
and can be well controlled because R
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 18 kHz resonant frequency of the gyro. An
R/C output filter consisting of a 3.3k series resistor and 22nF
shunt capacitor (2.2kHz pole) is recommended. Figure 11.
Power Spectral Density Output With and Without Additional
Output Filtering shows the effect of adding a this filter to the
output of an ADXRS646 set to 2000Hz bandwidth
TEMPERATURE OUTPUT AND CALIBRATION
It is common practice to temperature-calibrate gyros to improve
their overall accuracy. The ADXRS646 has a temperature propor-
tional voltage output that provides input to such a calibration
method. The temperature sensor structure is shown in Figure .
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.
OUT
Figure 11. Power Spectral Density Output With and Without Additional
and C
R
f
OUT
OUT
OUT
resistor to create a low-pass filter to limit the bandwidth


OUT
/ 1

180

is
2

kΩ
π

OUT

R
R
OUT
EXT
is used in combination with the on-
Output Filtering
 

/
180
C
OUT
kΩ

OUT

R
has been trimmed
EXT

ADXRS646