A comparator circuit with a built-in programmable threshold is proposed. The threshold is embedded by providing different initial source voltages to each of the input transistors. This source-voltage shifting is done by means of a tunable capacitance, which can synthesise a very wide range of initial voltages, even voltages beyond the supply. This results in a power-efficient comparator structure with a very wide programmable tuning range and low decision delay. The inherently small integrating capacitance of the structure allows for high speed operation even when using small device sizes, minimising input capacitance and maximising efficiency. A practical transistor-level implementation is simulated in a standard 16 nm CMOS technology on a 0.8 V supply, with a tuning range of at least 568 mV.