A 12-Bit Column-Parallel Two-Step Single-Slope ADC With a Foreground Calibration for CMOS Image Sensors

This paper proposes a novel 12-bit column-parallel two-step single-slope (SS) analog-to-digital converter (ADC) for high-speed CMOS image sensors. Cooperating with the output offset storage (OOS) technique, a new correlated double sampling (CDS) is adopted to reduce the non-uniformity in column-level ADCs. In the proposed structure, the decision point of the comparator is independent of the input signal. The variation of the comparator offset caused by the input level is eliminated. Through a foreground calibration, the non-idealities from the ramp generator and the column ADC are both corrected. Design and simulation in a 130nmCMOSprocess, the proposed ADC achieves the differential nonlinearity (DNL) of C0.76/0.8 LSB and the integral nonlinearity (INL) of +1.06/-0.84 LSB at a sampling frequency of 100 KS/s with the calibration. The effective number of bits (ENOB) is also improved from 4.66 bits to 11.25 bits. The single ADC occupies an active area of 7.5 x 775 mu m(2) and the power consumption is 72 mu W.