Successive-approximation register (SAR) ADCs are susceptible to non-linearities due to capacitor mismatch. This paper proposes a transition points-based calibration of the SAR ADC using an RC-filtered square wave by looking at the histogram of output digital codes to improve the linearity without any additional analog circuitry. The proposed method is validated with an 8-bit SAR ADC fabricated in UMC 180-nm CMOS. The signal-to-noise ratio (SNR) of the 8-bit ADC is improved from 42.87 dB to 46.91 dB, and the spurious-free dynamic range (SFDR) is improved from 51.95 dB to 60.61 dB with the proposed calibration method.