Datasheet ADXRS646 (Analog Devices) - 10

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Data Sheet ADXRS646 THEORY OF OPERATION
The ADXRS646 operates on the principle of a resonator
SETTING BANDWIDTH
gyroscope. Figure 19 shows a simplified version of one of four The combination of an external capacitor (COUT) and the polysilicon sensing structures. Each sensing structure contains on-chip resistor (ROUT) creates a low-pass filter that limits the a dither frame that is electrostatically driven to resonance. This bandwidth of the ADXRS646 rate response. The −3 dB produces the necessary velocity element to produce a Coriolis frequency set by ROUT and COUT is force when experiencing angular rate. The ADXRS646 is designed to sense a Z-axis (yaw) angular rate. fOUT = 1/(2 × π × ROUT × COUT) When the sensing structure is exposed to angular rate, the and can be well controlled because ROUT is trimmed during resulting Coriolis force couples into an outer sense frame, manufacturing to 180 kΩ ± 1%. Any external resistor applied which contains movable fingers that are placed between fixed between the RATEOUT pin (1B, 2A) and SUMJ pin (1C, 2C) pickoff fingers. This forms a capacitive pickoff structure that results in senses Coriolis motion. The resulting signal is fed to a series of ROUT = (180 kΩ × REXT)/(180 kΩ + REXT) gain and demodulation stages that produce the electrical rate An additional external filter is often added (in either hardware signal output. The quad sensor design rejects linear and angular or software) to attenuate high frequency noise arising from acceleration, including external g-forces, shock, and vibration. demodulation spikes at the 18 kHz resonant frequency of the The rejection is achieved by mechanically coupling the four gyroscope. An RC output filter consisting of a 3.3 kΩ series sensing structures such that external g-forces appear as resistor and 22 nF shunt capacitor (2.2 kHz pole) is common-mode signals that can be removed by the fully recommended. differential architecture implemented in the ADXRS646.
TEMPERATURE OUTPUT AND CALIBRATION
It is common practice to temperature-calibrate gyroscopes to improve their overall accuracy. The ADXRS646 has a temperature- dependent voltage output that provides input to such a calibration method. The temperature sensor structure is shown in Figure 20. The temperature output is characteristically nonlinear, and any load resistance connected to the TEMP output results in
X
decreasing the TEMP output and its temperature coefficient.
Y
Therefore, buffering the output is recommended.
Z
The voltage at TEMP (3F, 3G) is nominally 2.9 V at 25°C, and VRATIO = 6 V. The temperature coefficient is 10 mV/°C (typical) at 25°C; the output response over the full temperature range is shown in Figure 17. Although the TEMP output is highly repeatable, it has only modest absolute accuracy.
VRATIO VTEMP
16 0 15 1- 0 1-
RFIXED RTEMP
0977 977 0 Figure 20. Temperature Sensor Structure Figure 19. Simplified Gyroscope Sensing Structure—One Corner The electrostatic resonator requires 21 V for operation. Because only 6 V are typically available in most applications, a charge pump is included on chip. If an external 21 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 ADXRS646. 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 ADXRS646. Rev. C | Page 9 of 12 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 NULL ADJUSTMENT SELF-TEST FUNCTION CONTINUOUS SELF-TEST MODIFYING THE MEASUREMENT RANGE IMMUNITY TO VIBRATION OUTLINE DIMENSIONS ORDERING GUIDE