A Low Power Digitizer Array With Adaptive Split Current Source for 3-D-Stacked 100 MP High Dynamic Range Imager

The increasing demand for high dynamic range (HDR), power efficiency, and high resolution has driven the adoption of single-exposure dual conversion gain (DCG) techniques. This article proposes an adaptive split current source to optimize power consumption in single-exposure DCG readouts. By splitting the comparator bias current, the power consumption of the digitizer array can be adaptively optimized based on the conversion gain (CG) of pixel in single-exposure DCG operation. Additional power-saving features including decision-feedback and auto-zeroing (AZ) power-down techniques are also implemented to further improve power efficiency. The proposed digitizer chip was fabricated in a 28 nm CMOS process, achieving a power consumption reduction of 44.5% in comparator. The integral nonlinearity (INL) was measured as +2.67/–2.34 LSB in high CG (HCG) and +2.95/–1.92 LSB in low CG (LCG). The input-referred random noise (RN) values of 2.17 LSB (HCG) and 2.41 LSB (LCG) were measured at an analog gain of 16, corresponding to $93~\mu \! {V}_{\text {rms}}$ and $103~\mu \! {V}_{\text {rms}}$ , respectively. The prototype chip shows a highly competitive figure of merit (FoM) of 2.41 mV $\cdot $ pJ/pixel/frame.