ADXRS613 Analog Devices, Inc., ADXRS613 Datasheet - Page 9

• Manufacturer Part Number: ADXRS613
• Description: ?150?/sec Yaw Rate Gyroscope
• Manufacturer: Analog Devices, Inc.

THEORY OF OPERATION
The ADXRS613 operates on the principle of a resonator
gyroscope. 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 while rotating. 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 the CP5 pin
(6D, 7D). Note that CP5 should not be grounded when power is
applied to the ADXRS613. 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
ADXRS613.
SETTING BANDWIDTH
External Capacitor C
on-chip R
bandwidth of the ADXRS613 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 hardware or software filter is added to
attenuate high frequency noise arising from demodulation
spikes at the gyroscope’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.
R
f
OUT
OUT
OUT
=
=
(
(
(
resistor to create a low-pass filter to limit the
2
180
180
×
π
kΩ
kΩ
OUT
×
R
OUT
×
+
OUT
and C
1
R
R
is used in combination with the
EXT
EXT
×
C
OUT
)
)
OUT
is
)
OUT
has been trimmed
Rev. 0 | Page 9 of 12
Figure 22 shows the effect of adding a 250 Hz filter to the
output of an ADXRS613 set to 40 Hz bandwidth (as shown
in Figure 21). High frequency demodulation artifacts are
attenuated by approximately 18 dB.
TEMPERATURE OUTPUT AND CALIBRATION
It is common practice to temperature-calibrate gyroscopes to
improve their overall accuracy. The ADXRS613 has a tempera-
ture 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 V
at 25°C. Although the TEMP output is highly repeatable, it has
only modest absolute accuracy.
CALIBRATED PERFORMANCE
Using a 3-point calibration technique, it is possible to calibrate
the null and sensitivity drift of the ADXRS613 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 measure-
ment accuracy at each calibration point.
0.000001
0.00001
0.0001
Figure 22. Noise Spectral Density with Additional 250 Hz Filter
0.001
0.01
0.1
Figure 23. ADXRS613 Temperature Sensor Structure
10
RATIO
= 5 V. The temperature coefficient is ~9 mV/°C
V
RATIO
R
100
FIXED
R
TEMP
(Hz)
1k
V
TEMP
10k
ADXRS613
100k