A Low-Power Column-Parallel ΣΔ ADC with Shared OTAs and Single-Bit-BWI Decimation Filter for CMOS Image Sensor

This article presents a low-power column-parallel sigma Delta analog-to-digital converter (ADC) with shared operational transconductance amplifiers (OTAs) for CMOS image sensors (CISs). Through the proposed time division multiplexing of OTAs, the power consumption of the modulator achieves a reduction of about 40%. The proposed structure alleviates the layout requirement of OTAs, enabling implementation of a high-resolution low-power image sensor in advanced CMOS technology nodes. As the coupled noise introduced by the proposed column-shared scheme is suppressed to a lower level than the inherent crosstalk of adjacent pixels, the proposed structure only contributes a negligible inter-column coupled noise from the ADCs. Moreover, a compact digital decimation filter with a single-bit-bit-wise-inversion (BWI) topology is also proposed, which can reduce chip area significantly. The prototype sensor is fabricated in a 40-nm standard CMOS technology with 256 x 256 pixel array and 256 proposed column-parallel sigma Delta ADCs. Each sigma Delta ADC occupies a core area of 4.5 mu m x 310 mu m, while consuming a power of 58.8 mu W. The measured differential nonlinearity (DNL) and integral nonlinearity (INL) are +0.49/-0.65 LSB and +5.1/-4.8 LSB, respectively. Measurement results show a dynamic range of 79.9 dB and an effective-number-of-bit (ENOB) of 11.4 bit. This work achieves a figure of merit (FOM) of 97.2 fJ/step.