Compensated signal isolator

Abstract

A device for coupling a signal value out of a circuit which itself is floating at some relatively high voltage (termed common-mode voltage). A sensor for converting the parameter to be measured to a signal voltage, together with an amplifier and light-emitting diode are powered by a floating power supply of minimal power rating at the location of the common-mode voltage. An optical fiber link, which itself has great electrical insulation characteristics, joins the LED with a solid state light-to-electric transducer at a relatively remote location. Circuit means responsive to the light-to-electric transducer are included for outputting an analog value representative of the quantity to be measured at the point of the common-mode voltage. This LED, fiber optic link and light-to-electric transducer constitute a first OCI (optically coupled isolator). An additional OCI having the same characteristics as the first is located at the remote point and includes a second LED, optical fiber linked and a second light-to-electric transducer which generates a signal representative only of the extraneous variations produced through the first OCI. The resulting signal is differentially added as a feedback term to the signal produced by the first OCI to provide a compensated analog signal output.