DIGITIZER

Published:2009/7/8 2:12:00 Author:May | From:SeekIC

If the tube is level with respect to gravity, the bubble resides in the tube's center and the electrode resistances to common are identical. As the tube shifts avqr from level, the resistances increase and decrease proportionally. Transducers of this type must be excited with an ac waveform to avoid damage to the partially conductive liquid inside the tube. The level transducer is configured with a pair of 2-KΩ resistors to form a bridge. The required ac bridge excitation is developed at C1A, configured as a multivibrator. C1 biases Q1, which switches the LT1009's 2.5-V potential through the 100-μF capacitor to provide the ac bridge drive. The bridge differential output ac signal is converted to a current by A1, operating as a Howland current pump. This current, whose polarity reverses as bridge drive polarity switches, is rectified by the diode bridge. Thus, the 0.03-μF capacitor receives unipolar charge. A2, running at a differential gain of 2, senses the voltage across the capacitor and presents its single-ended output to C1B. When the voltage across the 0.03-μF capacitor becomes high enough, C1B's output becomes high, turning on the paralleled sections of the LTC1043 switch. This discharges the capacitor. The 47-pF capacitor provides enough ac feedback around CIB to allow a complete zero reset for the capacitor. When the ac feedback ceases, C1B's output decreases and the LTC1043 switch goes off. The 0.03-μF unit again receives constant current charging and the entire cycle repeats. The frequency of this oscillation is determined by the magnitude of the constant current delivered to the bridge-capacitor configuration. This current's magnitude is determined by the transducer bridge's offset, which is level related.



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