This paper presents a foreground calibration technique with LSB capacitors for an 18-bit SAR ADC. Calibration using LSB capacitors suffers from poor accuracy owing to the comparator static offset and the random mismatch in LSB capacitors. The proposed floating capacitors operation addresses the miscalibration due to the saturation of sub-ADC resulting from the static offset. In addition, calibration using multiple groups of LSB capacitors is taken for each capacitor measurement to average out the effect of random mismatch in LSB capacitors. Behavioral simulations of an 18-bit SAR ADC demonstrate the availability and robustness of the proposed calibration techniques.