RTS noise reduction using selective averaging in multi-aperture system

This paper reports on a back-illuminated, global-shutter, CMOS image sensor with a pixel-parallel, single-slope analogto-digital converter (ADC). We adopted a digital bucket relay transfer with a multistage flip-flop connection, a pixel unit Cu–Cu connection, and positive-feedback circuitry to realize a 6.9 μm × 6.9 μm, 1.46 MP pixel-parallel ADC. By operating the comparator with a bias current in the subthreshold region of 7.74-111 nA, we succeeded in reducing the peak current during the simultaneous analog-to-digital conversions. In combination with an ADC standby operation, we succeeded in further reducing the pixel-parallel ADC power consumption. With these techniques, we realized a normalized index of figure-of-merit of 0.24 nJ·e-rms/step, which was calculated by dividing the entire sensor power by the effective ADC resolution at a subthreshold current of 111 nA during 660 fps operation.