A single-channel 12-bit SAR ADC achieving 250–340 MS/s and consuming 4.8–8.0 mW from 0.75 to 0.9 V is presented. At 300 MS/s, the ADC exhibits 61.6-dB peak SNDR and reaches 60.5-dB SNDR and 78.7-dB SFDR with 0.8- $\text{V}_{\text {pp, diff}}$ input amplitude at Nyquist. It consumes 7.0 mW from a single 0.85-V supply, where 3.7 mW is contributed by the reference buffer. The key element is a comparator with an inverter-based preamplifier to achieve low-noise performance with below 1- $\text{V}_{\text {pp, diff}}$ input amplitude. The common-mode (CM) sensitivity of the inverter is counteracted by an SAR-based CM regulation (CMREG). The regulation adjusts the sampled CM to the optimal CM for the maximum inverter gain using a second capacitive DAC. It adjusts the CM on a sample-by-sample basis and, thus, can correct time-varying CM. The implemented CMREG maintains SNDR above 60 dB for a differential input amplitude mismatch of up to 2 dB or a phase mismatch of up to 15°.