High-CMRR instrumentation amp works with low supply voltages

摘要

Modern battery-cell voltages of 3 to 3,6V require circuits that offer efficient low-voltage operation. This Design Idea proposes an ac-coupled instrumenta-tion-amplifier design that features high CMRR (common-mode-rejection ratio), wide dc input-voltage tolerance, and a first-order highpass characteristic. Most of these features stem from a high-gain first-stage design. The circuit uses popular-value and -tolerance components. Figure la shows the simplified amplifier circuit. The general principle is that the capacitor, C, and the R_3 resistors buffer and ac-couple the input signal. The second stage comprises two differential amplifiers, A_D. Each of them amplifies half the differential input signal.