ADXRS623
Rev. A | Page 9 of 12
THEORY OF OPERATION
The ADXRS623 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
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 ADXRS623. 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
ADXRS623.
SETTING BANDWIDTH
External Capacitor C
OUT
is used in combination with the
on-chip R
OUT
resistor to create a low-pass filter to limit the
bandwidth of the ADXRS623 rate response. The –3 dB
frequency set by R
OUT
and C
OUT
is
( )
OUTOUT
OUT
CR
f
×××
=
π2
1
and can be well controlled because R
OUT
is trimmed during
manufacturing to be 180 kΩ ± 1%. Any external resistor applied
between the RATEOUT pin (1B, 2A) and the SUMJ pin (1C,
2C) results in
( )
( )
EXT
EXT
OUT
R
R
R
+
×
=
kΩ180
kΩ180
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, the corner
frequency of this additional filter is set to greater than 5× the
required bandwidth to preserve good phase response.
Figure 22 shows the effect of adding a 250 Hz filter to the
output of an ADXRS623 set to 40 Hz bandwidth (as shown
in Figure 21). High frequency demodulation artifacts are
attenuated by approximately 18 dB.
0.1
0.01
0.000001
0.00001
0.0001
0.001
10 100k1k100
FRE
QUENCY (Hz)
(°/s/
Hz rms)
10k
08890-022
Figure 22. Noise Spectral Density with Additional 250 Hz Filter
TEMPERATURE OUTPUT AND CALIBRATION
It is common practice to temperature-calibrate gyroscopes to
improve their overall accuracy. The ADXRS623 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
RATIO
= 5 V. T h e temperature coefficient is ~9 mV/°C
at 25°C. Although the TEMP output is highly repeatable, it has
only modest absolute accuracy.
V
RATIO
R
TEMP
R
FIXED
V
TEMP
08890-023
Figure 23. ADXRS623 Temperature Sensor Structure
CALIBRATED PERFORMANCE
Using a three-point calibration technique, it is possible to
calibrate the null and sensitivity drift of the ADXRS623 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.
