Datasheet ADXRS623 (Analog Devices) - 10

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ADXRS623 THEORY OF OPERATION
The ADXRS623 operates on the principle of a resonator Figure 22 shows the effect of adding a 250 Hz filter to the gyroscope. Two polysilicon sensing structures each contain a output of an ADXRS623 set to 40 Hz bandwidth (as shown dither frame that is electrostatical y driven to resonance, in Figure 21). High frequency demodulation artifacts are producing the necessary velocity element to produce a Coriolis attenuated by approximately 18 dB. force while rotating. At two of the outer extremes of each frame,
0.1
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
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to a series of gain and demodulation stages that produce the electrical rate signal output. The dual-sensor design rejects
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external g forces and vibration. Fabricating the sensor with
rms) Hz
signal conditioning electronics preserves signal integrity in
0.0001
noisy environments.
(°/s/
The electrostatic resonator requires 18 V to 20 V for operation.
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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
0.000001 10 100 1k 10k 100k
022 be omitted, and this supply can be connected to the CP5 pin
FREQUENCY (Hz)
08890- (6D, 7D). Note that CP5 should not be grounded when power is Figure 22. Noise Spectral Density with Additional 250 Hz Filter applied to the ADXRS623. Although no damage occurs, under certain conditions, the charge pump may fail to start up after
TEMPERATURE OUTPUT AND CALIBRATION
the ground is removed without first removing power from the It is common practice to temperature-calibrate gyroscopes to ADXRS623. improve their overal accuracy. The ADXRS623 has a tempera- ture proportional voltage output that provides input to such a
SETTING BANDWIDTH
calibration method. The temperature sensor structure is shown External Capacitor C in Figure 23. The temperature output is characteristically OUT is used in combination with the on-chip R nonlinear, and any load resistance connected to the TEMP OUT resistor to create a low-pass filter to limit the bandwidth of the ADXRS623 rate response. The –3 dB output results in decreasing the TEMP output and temperature frequency set by R coefficient. Therefore, buffering the output is recommended. OUT and COUT is f = 1 The voltage at the TEMP pin (3F, 3G) is nominally 2.5 V at OUT ( 2 × π × R × C ) OUT OUT 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 and can be well controlled because ROUT is trimmed during only modest absolute accuracy. manufacturing to be 180 kΩ ± 1%. Any external resistor applied between the RATEOUT pin (1B, 2A) and the SUMJ pin (1C,
VTEMP
2C) results in
VRATIO
(180 kΩ × R ) 023 R = ( EXT
R R FIXED TEMP
OUT 08890- 180 kΩ + R ) EXT Figure 23. ADXRS623 Temperature Sensor Structure In general, an additional hardware or software filter is added to
CALIBRATED PERFORMANCE
attenuate high frequency noise arising from demodulation Using a three-point calibration technique, it is possible to spikes at the gyroscope’s 14 kHz resonant frequency (the noise calibrate the nul and sensitivity drift of the ADXRS623 to an spikes at 14 kHz can be clearly seen in the power spectral overal accuracy of nearly 200°/hour. An overal accuracy of density curve shown in Figure 21). Typically, the corner 40°/hour or better is possible using more points. frequency of this additional filter is set to greater than 5× the required bandwidth to preserve good phase response. Limiting the bandwidth of the device reduces the flat-band noise during the calibration process, improving the measure- ment accuracy at each calibration point. Rev. A | Page 9 of 12 Document Outline Features Applications General Description Functional Block Diagram 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 Calibrated Performance ADXRS623 and Supply Ratiometricity Null Adjustment Self-Test Function Continuous Self-Test Outline Dimensions Ordering Guide Automotive Products