In the last few years, fully differential amplifiers (FDAs) have grown in popularity; and, while similar in theory to inverting operational amplifiers, they have important differences that need to be understood when input impedance matching is considered. This article shows how to analyze the input impedance of an FDA. Circuit analysis is performed to aid understanding of the key design points, and a methodology is presented to illustrate how to approach the design variables and calculate component values. A spreadsheet and TINA-TI SPICE models are available as design aids.