EVAL-ADXRS620Z Analog Devices Inc, EVAL-ADXRS620Z Datasheet - Page 9

• Manufacturer Part Number: EVAL-ADXRS620Z
• Description: EB ?300?/sec Yaw Rate Gyroscope
• Manufacturer: Analog Devices Inc

Specifications of EVAL-ADXRS620Z

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

THEORY OF OPERATION
The ADXRS620 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 produces 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 ADXRS620. 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
ADXRS620.
SETTING BANDWIDTH
External Capacitor C
chip R
bandwidth of the ADXRS620 rate response. The −3 dB
frequency set by R
This frequency can be well controlled because R
trimmed 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 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.
R
f
O
OUT
O
UT
UT
resistor to create a low-pass filter to limit the
=
=
(
(
(
2
180
180
×
π
kΩ
kΩ
OUT
×
OUT
R
×
+
and C
OUT
1
R
R
is used in combination with the on-
EXT
EXT
×
OUT
)
)
C
OUT
is
)
OUT
has been
Rev. B | Page 9 of 12
Figure 22 shows the effect of adding a 250 Hz filter to the output
of an ADXRS620 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 gyros to improve
their overall accuracy. The ADXRS620 has a temperature propor-
tional 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, buf-
fering the output is recommended.
The voltage at the TEMP pin (3F, 3G) is nominally 2.5 V at 25°C,
and V
25°C. Although the TEMP output is highly repeatable, it has
only modest absolute accuracy.
CALIBRATED PERFORMANCE
Using a three-point calibration technique, it is possible to
calibrate the null and sensitivity drift of the ADXRS620 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
0.001
RATIO
Figure 22. Noise Spectral Density with Additional 250 Hz Filter
0.01
0.1
10
= 5 V. The temperature coefficient is ~9 mV/°C at
Figure 23. Temperature Sensor Structure
V
RATIO
100
R
FIXED
FREQUENCY (Hz)
R
TEMP
1k
V
TEMP
10k
ADXRS620
100k