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A speed-enhanced 10-b asynchronous SAR ADC with multistep addition-only digital error correction (ADEC) is presented with a straightforward DAC switching algorithm. The capacitor DAC is virtually divided into three sub-DACs for ADEC with negligible hardware overhead. The redundant decision cycles between stages reconfigure the capacitor connection of the DAC. These redundancies guarantee 10-b linearity under 4-b-accurate DAC settling in the MSB decision and the optimally designed ADC enhances the conversion speed by 37%. A prototype ADC was implemented in a CMOS 0.13-μm technology. The chip consumes 550 μW and achieves a 50.6-dB SNDR at 40 MS/s under a 1.2-V supply. The figure-of-merit (FOM) is 50 fJ/conversion-step.