ADXRS642 Data Sheet
Rev. A | Page 8 of 12
THEORY OF OPERATION
The ADXRS642 operates on the principle of a resonator gyro.
Figure 13 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 ADXRS642 is designed to sense
a z-axis (yaw) angular rate.
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 and vibration. This 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 ADXRS642.
Figure 13. Simplified Gyro Sensing Structure–One Corner
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 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 ADXRS642. 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 ADXRS642.
SETTING BANDWIDTH
The external capacitor, C
OUT
, is used in combination with the
on-chip resistor, R
OUT
, to create a low-pass filter to limit the
bandwidth of the ADXRS642 rate response. The −3 dB
frequency set by R
OUT
and C
OUT
is
( )
OUTOUTOUT
CRf ×××= π2/1
and can be well controlled because R
OUT
has been 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
( ) ( )
EXTEXTOUT
RRR +×= kΩ180/kΩ180
In general, an additional filter (in either hardware or software)
is added to attenuate high frequency noise arising from
demodulation spikes at the 18 kHz resonant frequency of the
gyro. An R/C output filter consisting of a 3.3k series resistor and
22 nF shunt capacitor (2.2 kHz pole) is recommended. Figure 13
shows the effect of adding this filter to the output of an ADXRS642
set to a 2000 Hz bandwidth.
TEMPERATURE OUTPUT AND CALIBRATION
It is common practice to temperature-calibrate gyros to improve
their overall accuracy. The ADXRS642 has a temperature propor-
tional voltage output that provides input to such a calibration
method. The temperature sensor structure is shown in Figure 14.
The temperature output is characteristically 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 2.5 V at 25°C, and
V
RATIO
= 5 V. T he temperature coefficient is ~9 mV/°C at 25°C.
Although the TEMP output is highly repeatable, it has only modest
absolute accuracy.
V
RATIO
V
TEMP
R
FIXED
R
TEMP
09770-003
Figure 14. Temperature Sensor Structure
SUPPLY RATIOMETRICITY
The ADXRS642 RATEOUT, ST1, ST2, and TEMP signals are
ratiometric to the V
RATIO
voltage; for example, the null voltage,
rate sensitivity and temperature outputs are proportional to
V
RATIO
. Therefore, it is most easily used with a supply-ratiometric
ADC, which results in self-cancellation of errors due to minor
supply variations. There is some small, usually negligible, error
due to nonratiometric behavior. Note that to guarantee full rate
range, V
RATIO
should not be greater than AV
CC
.
