This paper presents a 10-bit column-parallel single slope analog-to-digital converter (SS ADC) with a two-step time-to-digital converter (TDC) to overcome the long conversion time problem in conventional SS ADC for high-speed CMOS image sensors (CIS). The time interval proportional to the input signal is generated by a ramp generator and a comparator, which is digitized by a two-step TDC consisting of coarse and fine conversions to achieve a high-precision time-interval measurement. To mitigate the impact of propagation delay mismatch, a calibration circuit is also proposed to calibrate the delay skew within -T/2 to T/2. The proposed ADC is designed in 0.18 μm CMOS process. The power dissipation of each column circuit is 232 μW at supply voltages of 3.3 V for the analog circuits and 1.8 V for the digital blocks. The post simulation results indicate that the ADC achieves a SNDR of 60.89 dB (9.82 ENOB) and a SFDR of 79.98 dB at a conversion rate of 2 MS/s after calibration, while the SNDR and SFDR are limited to 41.52 dB and 67.64 dB, respectively before calibration. The differential nonlinearity (DNL) and integral nonlinearity (INL) without calibration are +15.80/-15.29 LSB and +1.68/-15.34 LSB while they are reduced down to +0.75/-0.25 LSB and +0.76/-0.78 LSB with the proposed calibration.
A column-parallel clock skew self-calibration circuit for time-resolved CMOS image sensors
