Abstract

In a practical instrumentation amplifier for low-level input of 10 mV or less, a high common-mode rejection ratio (CMRR) of more than 140 dB (107) is desired. This paper considers a three-operational amplifier-type instrumentation amplifier as an example and examines the CMRR. Common-mode sampling feedback is proposed as a means to improve the CMRR. A detailed design procedure is described and an experimental circuit is constructed. The CMRR of 160 dB is obtained, which is comparable to that possible with an insulation amplifier. As a promising application example of the proposed amplifier, the circuit is applied to a high-speed, low-level multiplexor system. Satisfactory results are demonstrated. The proposed instrumentation amplifier is suitable for present monolithic technologies.

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