Zero-crossing-prediction-based Single-slope ADC with a Constant Charge Bias Amplifier for Low Power Image Sensors

This paper presents a single-slope analog-to-digital converter (SS-ADC) with a proposed zero-crossing (ZC)-prediction-based power-gating technique for low-power complementary metal-oxide-semiconductor (CMOS) image sensor applications. The proposed ZC prediction utilizes the DC gain difference between the preamplifier and main comparator in the SS-ADC without an additional ramp signal. For a consistent prediction with a low power operation, the constant charge bias amplifier (CQBA) is implemented as a preamplifier, which exhibits input level independent output common mode and gain response. The proposed SS-ADC is demonstrated for a 110-nm CMOS image sensor process, assuming a 640 times 480 image resolution, a 10-bit ADC resolution, and 120 frames/s. The simulation results show that the CQBA output common mode and gain response only vary by 1% and 6.4%, respectively. Furthermore, the overall power consumption of the sensor is reduced by about 80% compared with the static comparator by the prediction-based power-gating, achieving high energy efficiency (51. 5 pJ/pixel of the Figure of merit).