A foreground calibration for successive approximation register analog-to-digital converter (SAR ADC) is introduced in this paper. This calibration system is based on the fact that capacitor mismatch can result in the discontinuity of output code density. In the calibration mode, a triangular signal with a small swing generated on chip is input and the Calibration Controller utilizes a data comparator and a SAR logic to trim the MSB capacitors based on the result of code density test. A MATLAB behavioral simulation with 30 times Monte Carlo is done when the calibration method is applied in a 10-bit SAR ADC. Simulation results show that the mean values of the signal-to-noise-plus-distortion ratio (SNDR) and the spurious-free dynamic range (SFDR) are improved to 63.24 dB and 70.61 dB from 53.61 dB and 59.23 dB, respectively.