John Fluke Mfg. Co., Inc.

Craig E Johnston

Ramesh C Goyal

An operational amplifier (33) is continuously switched back-and-forth between an auto-zero mode of operation and an accurate amplification mode of operation. During the auto-zero mode of operation, the junction between an offset voltage compensation (e.g., auto-zero) capacitor (35) and the noninverting input of the operational amplifier (33) is referenced to the input signal voltage via a bootstrap amplifier (31), rather than being tied to ground. Further, during the auto-zero mode of operation, the other end of the auto-zero capacitor (35) is referenced to the output of the operational amplifier (33), which is also connected to the inverting input of the operational amplifier. As a result, the auto-zero capacitor (35) is charged to a voltage level equal to the input offset voltage of the operational amplifier, but in polarity opposition thereto. During the accurate amplification mode of operation, the input signal is applied to the noninverting input of the operational amplifier (33) via the auto-zero capacitor (35). As a result, the charge on the auto-zero capacitor cancels the input offset voltage of the amplifier whereby the output of the operational amplifier (33) is accurately related to the voltage of the input signal by the gain of the operational amplifier. Because the auto-zero capacitor (35) is referenced to the input signal voltage during the auto-zero mode of operation, rather than being tied to ground, voltage swings at the output of the operational amplifier (33) and across the various switch elements (37, 39, 41) are kept low. As a result, the auto-zero amplifier circuit is operable over a wide dynamic range.

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Woodinville, WA, United States of America

Craig E Johnston