Fully Differential AC-Coupling Networks: A Comparative Study

Fully differential AC amplifiers can easily be designed by placing an AC-coupling network in front of a fully differential DC amplifier. Such a network, however, must have a high common-mode rejection ratio (CMRR) to keep the overall CMRR high. In principle, passive AC-coupling networks that include no grounded components have an infinite CMRR, but they must provide a path for input bias currents. This paper analyzes how component tolerances limit the CMRR and affect the transient response of three different AC-coupling networks. To evaluate these networks in real conditions, the common-mode input impedance and bias currents of the ensuing differential amplifier are considered. Experimental results confirm the theoretical predictions.