EVAL-ADXRS646Z AD [Analog Devices], EVAL-ADXRS646Z Datasheet - Page 9

• Manufacturer Part Number: EVAL-ADXRS646Z
• Description: High Stability, Low Noise Vibration Rejecting Yaw Rate Gyro
• Manufacturer: AD [Analog Devices]

Data Sheet
THEORY OF OPERATION
The
gyroscope. Figure 19 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
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, shock, and vibration.
The rejection 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 21 V for operation. Because
only 6 V are typically available in most applications, a charge
pump is included on chip. If an external 21 V 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.
Z
ADXRS646
Figure 19. Simplified Gyroscope Sensing Structure—One Corner
X
Y
is designed to sense a Z-axis (yaw) angular rate.
operates on the principle of a resonator
Rev. 0 | Page 9 of 12
SETTING BANDWIDTH
The combination of an external capacitor (C
on-chip resistor (R
bandwidth of the
frequency set by R
and can be well controlled because R
manufacturing to 180 kΩ ± 1%. Any external resistor applied
between the RATEOUT pin (1B, 2A) and SUMJ pin (1C, 2C)
results in
An additional external filter is often added (in either hardware
or software) to attenuate high frequency noise arising from
demodulation spikes at the 18 kHz resonant frequency of the
gyroscope. An RC output filter consisting of a 3.3 kΩ series
resistor and 22 nF shunt capacitor (2.2 kHz pole) is
recommended.
TEMPERATURE OUTPUT AND CALIBRATION
It is common practice to temperature-calibrate gyroscopes
to improve their overall accuracy. The
temperature-dependent voltage output that provides input
to such a calibration method. The temperature sensor structure
is shown in Figure 20. The temperature output is characteristi-
cally 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 2.9 V at 25°C, and
V
at 25°C; the output response over the full temperature range is
shown in Figure 17. Although the TEMP output is highly
repeatable, it has only modest absolute accuracy.
RATIO
f
R
OUT
OUT
= 6 V. The temperature coefficient is 10 mV/°C (typical)
= 1/(2 × π × R
= (180 kΩ × R
Figure 20. Temperature Sensor Structure
ADXRS646
V
OUT
OUT
RATIO
) creates a low-pass filter that limits the
R
and C
FIXED
OUT
EXT
× C
)/(180 kΩ + R
OUT
rate response. The −3 dB
R
OUT
TEMP
is
)
OUT
ADXRS646
V
TEMP
is trimmed during
EXT
)
OUT
) and the
ADXRS646
has a