ADXRS624BBGZ Analog Devices Inc, ADXRS624BBGZ Datasheet - Page 9

• Manufacturer Part Number: ADXRS624BBGZ
• Description: Rollover Detection Gyro +/- 75
• Manufacturer: Analog Devices Inc

Specifications of ADXRS624BBGZ

• Sensor Case Style: BGA
• No. Of Pins: 32
• Supply Voltage Range: 4.75V To 5.25V
• Operating Temperature Range: -40°C To +105°C

THEORY OF OPERATION
The ADXRS624 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 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 CP5 (Pin 6D,
Pin 7D). Note that CP5 should not be grounded when power is
applied to the ADXRS624. 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
ADXRS624.
SETTING BANDWIDTH
External Capacitor C
chip R
bandwidth of the ADXRS624 rate response. The –3 dB
frequency set by R
and can be well controlled because R
manufacture to be 200 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 band-
width to preserve good phase response.
Figure 22 shows the effect of adding a 250 Hz filter to the
output of an ADXRS624 set to 40 Hz bandwidth (as shown in
Figure 21). High frequency demodulation artifacts are
attenuated by approximately 18 dB.
R
f
OUT
OUT
OUT
resistor to create a low-pass filter to limit the
=
=
(
(
(
2
200
200
×
π
OUT
kΩ
kΩ
×
OUT
R
and C
×
+
OUT
1
R
R
is used in combination with the on-
EXT
EXT
×
OUT
)
)
C
OUT
is
)
OUT
is trimmed during
Rev. A | Page 9 of 12
TEMPERATURE OUTPUT AND CALIBRATION
It is common practice to temperature-calibrate gyros to improve
their overall accuracy. The ADXRS624 has a temperature
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 three-point calibration technique, it is possible to
calibrate the null and sensitivity drift of the ADXRS624 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.
ADXRS624 AND SUPPLY RATIOMETRICITY
The ADXRS624 RATEOUT and TEMP signals are ratiometric
to the V
temperature outputs are proportional to V
ADXRS624 is most easily used with a supply-ratiometric ADC
that results in self-cancellation of errors due to minor supply
variations. There is some small error due to nonratiometric
0.000001
0.00001
0.0001
0.001
Figure 22. Noise Spectral Density with Additional 250 Hz Filter
0.01
0.1
RATIO
10
Figure 23. ADXRS624 Temperature Sensor Structure
RATIO
voltage; that is, the null voltage, rate sensitivity, and
= 5 V. The temperature coefficient is ~9 mV/°C
V
RATIO
100
R
FIXED
R
TEMP
(Hz)
1k
V
TEMP
RATIO
10k
. Thus, the
ADXRS624
100k