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ADXRS645 Datasheet(Arkusz danych) 9 Page - Analog Devices

Numer części ADXRS645
Szczegółowy opis  High Temperature, Vibration Rejecting plus,minus2000temperature/sec Gyroscope
Pobierz  10 Pages
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Producent  AD [Analog Devices]
Strona internetowa  http://www.analog.com
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Data Sheet
ADXRS645
THEORY OF OPERATION
The ADXRS645 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 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 15 V for operation. Because
only 5 V is typically available in most applications, a charge
pump is included on chip. If an external 17 V to 22 V supply is
available, the two capacitors on CP1 to CP4 can be omitted, and
this supply can be connected to CP5 (Pin A2) through a 1 kΩ
series resistor. Do not ground CP5 when power is applied to the
ADXRS645. 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 ADXRS645.
SETTING BANDWIDTH
The external capacitor, COUT, is used in combination with the
on-chip resistor, ROUT, to create a low-pass filter to limit the
bandwidth of the ADXRS645 rate response. The −3 dB
frequency set by ROUT and COUT is
fOUT = 1/(2 × π × ROUT × COUT)
This frequency can be well controlled because ROUT has been
trimmed during manufacturing to be 180 kΩ ± 1%. Any external
resistor applied between the RATEOUT pin (D2) and SUMJ pin
(E2) results in ROUT = (180 kΩ × REXT)/(180 kΩ × REXT).
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
gyroscope. An RC output filter consisting of a 3.3 kΩ series
resistor and 22 nF shunt capacitor (2.2 kHz pole) is recommended.
TEMPERATURE OUTPUT AND CALIBRATION
It is common practice to temperature calibrate gyroscopes to
improve their overall accuracy. The ADXRS645 has a temperature
proportional voltage output that provides input to such a calibration
method. The temperature sensor structure is shown in Figure 21.
The voltage at TEMP (Pin C3) is nominally 2.4 V at 25°C, and
VRATIO = 5 V. The temperature coefficient is ~9 mV/°C at 25°C.
Although the TEMP output is highly repeatable, it has only
modest absolute accuracy.
Figure 21. Temperature Sensor Structure
SUPPLY RATIOMETRICITY
The RATEOUT, ST1, ST2, and TEMP signals of the ADXRS645
are ratiometric to the VRATIO voltage, that is, the null voltage, rate
sensitivity, and temperature outputs are proportional to VRATIO.
Therefore, it is most easily used with a supply ratiometric analog-
to-digital converter (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, VRATIO must not be greater than AVCC.
RANGE EXTENSION
The ADXRS645 scale factor can be reduced to extend the
measurement range to as much as ±5000°/sec by adding a single
120 kΩ resistor between the RATEOUT and SUMJ pins. If an
external resistor is added between the RATEOUT and SUMJ
pins, proportionally increase COUT to maintain correct
bandwidth (that is, if adding a 180 kΩ resistor, double COUT).
SELF-TEST FUNCTION
The ADXRS645 includes a self-test feature that actuates each of
the sensing structures and associated electronics in the same
manner, as if subjected to angular rate. It is activated by standard
logic high levels applied to ST1 (Pin A3), ST2 (Pin B3), or both.
ST1 causes the voltage at RATEOUT to change about −1.3 V,
and ST2 causes an opposite change of +1.3 V. The self-test
response follows the viscosity temperature dependence of the
package atmosphere, approximately 0.25%/°C.
Activating both ST1 and ST2 simultaneously is not damaging.
ST1 and ST2 are fairly closely matched (±1%), but actuating
both simultaneously may result in a small apparent null bias
shift proportional to the degree of self-test mismatch.
ST1 and ST2 are activated by applying a voltage equal to VRATIO
to the ST1 pin and the ST2 pin. The voltage applied to ST1 and
ST2 must never be greater than AVCC.
CONTINUOUS SELF-TEST
The on-chip integration of the ADXRS645 gives it higher reliability
than is obtainable with any other high volume manufacturing
method. In addition, it is manufactured under a mature BiMOS
process that has field proven reliability. As an additional failure
detection measure, power-on self-test can be performed.
However, some applications may warrant continuous self-test
while sensing rate.
VRATIO
VTEMP
RFIXED
RTEMP
Rev. A | Page 9 of 10




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