An input folding high speed cyclic ADC for column-parallel readout in CMOS image sensors

An input folding cyclic ADC for column-parallel readout in CMOS image sensor is proposed. A double sampling circuit in the CMOS image sensor is reused to perform input folding operation in cyclic ADC. In addition, a push-pull configuration based slew rate enhancement technique is used to reduce the settling time of multiplying digital to analog converter. The ADC results in a conversion rate of 1.38 MS/s while consuming 560 $\mu$W of power. A prototype CMOS image sensor, with 12-bit column-parallel cyclic ADC, is designed and fabricated in AMS 350 nm CMOS OPTO process at 3.3 V power supply. For a $96\times 64$ pixel array, the row readout time of 720 ns is achieved, which is two to five times smaller compared to the state-of-the-art.