Chopper-multiplexer system for measurement of remote low-level signals
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United States Patent and Trademark Office
Abstract
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A balanced bridge chopper circuit including four metal oxide semiconductor field effect transistors (MOSFETs) is driven by a remote oscillator connected through an isolator to modulate low-voltage low-frequency input signals at their remote source into a square wave A.C. output signal that can be conducted to a common point where it is amplified and demodulated for better linear measurement of said low-voltage signals. The oscillator is connected through the isolator to the bridge circuit in such a manner that opposing pairs of the bridge transistors are simultaneously conducting. The isolator may comprise a novel photovoltaic isolation device or it may consist of an isolation transformer or of other types of electrical isolation devices. The transistors and input and output terminals for the bridge circuit are shielded and are mounted on a heat sink consisting of a thermally conducting metal plate and/or a metal oxide single crystal which is in common with the shield and/or one terminal of the low-voltage source such that all leads and terminals are maintained at a constant temperature and preferably at the same temperature as the low-voltage source. Air currents are prevented from causing thermally induced voltages within the bridge circuit by the circuit shield and in some extreme cases by the use of a hermetically sealed shield for the critical circuit elements and connections. For accurately balancing out interelectrode capacitance-coupled gate drive to low-level channel signals, a pair of variable capacitors are connected between one output terminal of the bridge circuit and the gate of each of its two adjacent transistors. The balanced bridge chopper may be produced as a monolithic planar-silicon integrated circuit in which gate-to-channel capacitance balancing is accomplished during manufacturing in place of the variable capacitors. A novel balanced input filter and trimmer capacitors between the bridge output terminals and local ground greatly reduce 60 Hz. common mode noise. In a hybrid circuit embodiment of the bridge circuit, the individual MOSFET chips forming the bridge circuit are bonded and interconnected within a hermetically sealed and magnetically transparent metal oxide single crystalline container. Multiplexing circuitry is also disclosed for driving several chopper circuits from a single oscillator and for connecting the outputs from the chopper circuits to a single amplifier and demodulator at a remote location.
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Keywords
370/532, 340/870.13