The capacitor digital-to-analog converter (CDAC) which affects the system performance of speed and linearity occupies the most area in successive approximation register (SAR) analog-to-digital converter (ADC). The performance of tri-level SAR ADC is well balanced between power and speed comparing to the conventional CDAC based architecture. In order to further improve the ADC performance in light of area and energy efficiencies, a partially asymmetric tri-level CDAC design technique is proposed to save the silicon cost and power as well. Combining the asymmetric CDAC approach with the tri-level charge redistribution technique makes it possible for the SAR ADC to achieve a 9-bit resolution with 4-bit + 3-bit split capacitor arrays. A 9-bit SAR ADC with CDAC calibration has been implemented in a 65nm CMOS technology and it achieves a peak SNDR of 50.1 dB and consumes 1.26 mW from a 1.2-V supply, corresponding to a FOM of 45fJ/conv.-step. The static performance of +0.4/0.5 LSB DNL and +0.5/0.7 LSB INL is achieved. The ADC has input capacitance of 180 fF and occupies an active area of 0.10.13 mm 2.