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US 4,010,424 AGranted
Phase-sensitive detector circuit with compensation for offset error
AssigneeBrookdeal Electronics Limited
InventorEric Andrew Faulkner
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AssigneeBrookdeal Electronics Limited
InventorEric Andrew Faulkner
Estimating the term…
A phase-sensitive detector circuit comprises a pair of field-effect transistors switched in antiphase and connected at a switching point. The switching point is supplied with a signal to be detected through an a.c. coupling. One of the transistors is connected to the input of an operational amplifier having a feedback path designed to maintain its input as a virtual earth. A compensation circuit connected from the switching point to the input side of the a.c. coupling provides a compensation signal to cancel the effect of any offset voltage in the operational amplifier. The other switching transistor may be grounded or else may be connected to a second operational amplifier whose input is also maintained as a virtual earth by a feedback path. In the latter case the output from the circuit is taken differentially from the operational amplifier. Alternatively, or in addition, a second pair of switching transistors supplied by a signal in antiphase to the signal supplied to the first-mentioned pair of transistors can be connected in a similar manner to supply the one or both operational amplifiers. BACKGROUND OF THE INVENTION
A phase-sensitive detector circuit characterised in that it comprises in combination a pair of switching devices joined at a switching point and arranged for control by reference switching signals in antiphase, an a.c. coupling between a signal input terminal and the said switching point, an output operational amplifier including a feedback path from the output thereof to an input thereof whereby to hold said input virtually at ground potential in the absence of any offset voltage in said operational amplifier, a connection from one of the switching devices to an input of the output operational amplifier, a connection from the other of the switching devices to a terminal which is at ground potential, and a compensation circuit the input to which is supplied from the said switching point and the output from which is applied to the input side of said a.c. coupling for the purpose of providing a compensating signal to tend to eliminate the effect of any offset voltage occurring in the output operational amplifier.
A phase-sensitive detector circuit characterized in that is comprises in combination a first pair of switching devices joined at a first switching point, a further pair of switching devices joined at a further switching point, said first pair of switching devices and said further pair of switching devices both being arranged for control by reference switching signals in antiphase, a first AC coupling between a signal input terminal and said first switching point, a further AC coupling taken between said signal input terminal and said further switching point, means for enabling signals in antiphase to be applied from said signal input terminal to said first AC coupling and said further AC coupling, an output operational amplifier including a feedback path from the output thereof to an input thereof whereby to hold said input virtually at ground potential in the absence of any offset voltage in said operational amplifier, a connection from one of the switching devices connected to the first switching point to an input of the output operational amplifier, a connection from one of the switching devices connected to the further switching point to the said input of the output operational amplifier, connections from the other of the switching devices of each pair to a terminal which is at ground potential, and a compensation circuit the input of which is supplied from said first switching point and the output from which is applied to the input side of the first AC coupling for the purpose of providing a compensation signal to tend to eliminate the effect of any offset voltage occuring in the output operational amplifier.
Dependencies are inferred from the claim text.