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Dynamic Offset Null

IP.com Disclosure Number: IPCOM000051603D
Original Publication Date: 1981-Feb-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 2 page(s) / 23K

Publishing Venue

IBM

Related People

Martin, WJ: AUTHOR [+2]

Abstract

In certain types of precision operational amplifier circuits, the effects of the input offset error voltage of the operational amplifier must be minimized. The conventional approaches to the problem do not compensate for later drifts in the error voltage, or introduce additional sources of offset error which may become significant.

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Dynamic Offset Null

In certain types of precision operational amplifier circuits, the effects of the input offset error voltage of the operational amplifier must be minimized. The conventional approaches to the problem do not compensate for later drifts in the error voltage, or introduce additional sources of offset error which may become significant.

One method of nulling out the offset error dynamically is illustrated in the above circuit which depicts correction within a simple inverting amplifier. Normally, the circuit operates as an inverting amplifier with switches 2 and 3 open, and switch 1 closed. The voltage across capacitor 4 is so low as to be ignored in the calculations. During a nulling cycle, the positions of switches 1, 2, and 3 are reversed. In this mode, the ratio of resistors 5 and 6 are specified to produce a large gain by amplifier 7. The output of amplifier 7, in turn, comprises a voltage which is a large multiple of the input offset voltage applied to amplifier 8. Amplifier 8 is a unity gain inverter which inverts the output of amplifier 7 and applies it through switch 3 to the positive (+) input of amplifier 7. The steady-state voltage V(c) produced at the positive input of amplifier 7 will be:

V(c) = - (R(f) R(I)) E(7) over (R(f) 2R(I) - R(I)

E(8) over (R(f) 2R(I)). where E(7) and E(8) are the input offset errors of amplifiers 7 and 8, respectively. As the gain of amplifier 7 is set high (R(f) >> R(I)), the equation simplifies to: V(c) = -...