The frequency-domain analysis method by Mirzaei et al. (2012) offers a systematic way to analyze the RF input impedance of $M$ -phase bandpass high-Q filters. The article also provides the mathematical expressions of the RF input impedance for $M$ -phase filters in both single-ended and differential realizations. However, the expression for the differential RF input impedance is off by a factor of two compared to simulations. In this article, we derive the corrected expression.