Indirect-feedback Sigma-Delta CMOS image sensors with SAR-based startup
A novel type of CMOS image sensor (CIS) leveraging two types of analog-to-digital converters (ADCs) is presented in this work. Contrary to conventional ΣΔ image sensors, which implement ΣΔ ADCs at the column or pixel array level, this work utilizes the pixel structure as part of the ΣΔ modulator to enhance the noise shaping capabilities of the readout scheme. The overall speed performance of this indirectfeedback (IF) ΣΔ image sensor is enhanced with the addition of a Successive Approximation Register (SAR) startup. The SAR startup has been successfully demonstrated on two prototype image sensors developed in a generic 0.35μm digital CMOS process. For the column-level, 320x180 pixel implementation, the SAR startup is included with minimal additional overhead, 1.8% of original ΣΔ column readout circuitry.