A complementary dynamic single-stage residue amplifier for a pipelined SAR ADC is presented. It re-uses charge typically wasted during the reset phase, and hence improves efficiency by a factor 2× in this block that often dominates the fundamental noise/power trade-off of the ADC. The residue amplifier achieves 90 μVrms input noise for an energy consumption of 1.5 pJ. It is used in a 2-times interleaved 6b coarse/8b fine pipelined SAR ADC. The 40nm CMOS prototype achieves 11 ENOB at 20 MS/s while consuming 165 μW, leading to an energy per conversion step of 4 fJ. It maintains more than 10.8 ENOB at low input frequencies for a clock frequency up to 180 MS/s.