This document analyzes and compares different 1-bit digital summing circuit topologies in terms of their robustness against process, voltage, and temperature variations at the 22nm technology node. It finds that the transmission gate-based topology is the most robust, with the tightest spread in propagation delay, power dissipation, and energy-delay product. It then proposes a transmission gate-based digital summing circuit implemented using carbon nanotube field-effect transistors, which offers even greater robustness against PVT variations compared to an implementation using traditional MOSFETs.