Datasheet ADXRS645 (Analog Devices) - 10

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Data Sheet ADXRS645 THEORY OF OPERATION
The ADXRS645 operates on the principle of a resonator gyroscope.
SUPPLY RATIOMETRICITY
Two polysilicon sensing structures each contain a dither frame that The RATEOUT, ST1, ST2, and TEMP signals of the ADXRS645 is electrostatically driven to resonance, producing the necessary are ratiometric to the VRATIO voltage, that is, the null voltage, rate velocity element to produce a Coriolis force during angular rate. sensitivity, and temperature outputs are proportional to V At two of the outer extremes of each frame, orthogonal to the RATIO. Therefore, it is most easily used with a supply ratiometric analog- dither motion, are movable fingers that are placed between to-digital converter (ADC), which results in self cancellation of fixed pickoff fingers to form a capacitive pickoff structure that errors due to minor supply variations. There is some small, usually senses Coriolis motion. The resulting signal is fed to a series of negligible, error due to nonratiometric behavior. Note that, to gain and demodulation stages that produce the electrical rate guarantee full rate range, VRATIO must not be greater than AVCC. signal output. The dual sensor design rejects external g-forces and vibration. Fabricating the sensor with the signal conditioning
RANGE EXTENSION
electronics preserves signal integrity in noisy environments. The ADXRS645 scale factor can be reduced to extend the The electrostatic resonator requires 15 V for operation. Because measurement range to as much as ±5000°/sec by adding a single only 5 V is typically available in most applications, a charge 120 kΩ resistor between the RATEOUT and SUMJ pins. If an pump is included on chip. If an external 17 V to 22 V supply is external resistor is added between the RATEOUT and SUMJ available, the two capacitors on CP1 to CP4 can be omitted, and pins, proportionally increase COUT to maintain correct this supply can be connected to CP5 (Pin A2) through a 1 kΩ bandwidth (that is, if adding a 180 kΩ resistor, double COUT). series resistor. Do not ground CP5 when power is applied to the
SELF-TEST FUNCTION
ADXRS645. No damage occurs, but under certain conditions, The ADXRS645 includes a self-test feature that actuates each of the charge pump may fail to start up after the ground is the sensing structures and associated electronics in the same removed without first removing power from the ADXRS645. manner, as if subjected to angular rate. It is activated by standard
SETTING BANDWIDTH
logic high levels applied to ST1 (Pin A3), ST2 (Pin B3), or both. The external capacitor, C ST1 causes the voltage at RATEOUT to change about −1.3 V, OUT, is used in combination with the on-chip resistor, R and ST2 causes an opposite change of +1.3 V. The self-test OUT, to create a low-pass filter to limit the bandwidth of the ADXRS645 rate response. The −3 dB response fol ows the viscosity temperature dependence of the frequency set by R package atmosphere, approximately 0.25%/°C. OUT and COUT is f Activating both ST1 and ST2 simultaneously is not damaging. OUT = 1/(2 × π × ROUT × COUT) ST1 and ST2 are fairly closely matched (±1%), but actuating This frequency can be well control ed because ROUT has been both simultaneously may result in a small apparent null bias trimmed during manufacturing to be 180 kΩ ± 1%. Any external shift proportional to the degree of self-test mismatch. resistor applied between the RATEOUT pin (D2) and SUMJ pin (E2) results in R ST1 and ST2 are activated by applying a voltage equal to V OUT = (180 kΩ × REXT)/(180 kΩ × REXT). RATIO to the ST1 pin and the ST2 pin. The voltage applied to ST1 and In general, an additional filter (in either hardware or software) ST2 must never be greater than AV is added to attenuate high frequency noise arising from CC. demodulation spikes at the 18 kHz resonant frequency of the
CONTINUOUS SELF-TEST
gyroscope. An RC output filter consisting of a 3.3 kΩ series The on-chip integration of the ADXRS645 gives it higher reliability resistor and 22 nF shunt capacitor (2.2 kHz pole) is recommended. than is obtainable with any other high volume manufacturing
TEMPERATURE OUTPUT AND CALIBRATION
method. In addition, it is manufactured under a mature BiMOS process that has field proven reliability. As an additional failure It is common practice to temperature calibrate gyroscopes to detection measure, power-on self-test can be performed. improve their overall accuracy. The ADXRS645 has a temperature However, some applications may warrant continuous self-test proportional voltage output that provides input to such a calibration while sensing rate. method. The temperature sensor structure is shown in Figure 21. The voltage at TEMP (Pin C3) is nominal y 2.4 V at 25°C, and V RATIO = 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.
VRATIO VTEMP
010
RFIXED RTEMP
1958- 1 Figure 21. Temperature Sensor Structure Rev. A | Page 9 of 10 Document Outline Features Applications General Description Functional Block Diagram Table of Contents Revision History Specifications Absolute Maximum Ratings Rate Sensitive Axis ESD Caution Pin Configuration and Function Descriptions Typical Performance Characteristics Theory of Operation Setting Bandwidth Temperature Output and Calibration Supply Ratiometricity Range Extension Self-Test Function Continuous Self-Test Outline Dimensions Ordering Guide