A 6.9- $\mu$ m Pixel-Pitch Back-Illuminated Global Shutter CMOS Image Sensor With Pixel-Parallel 14-Bit Subthreshold ADC

In this paper, we report on a back-illuminated, global shutter, CMOS image sensor (CIS) with a pixel-parallel, single-slope analog-to-digital converter (ADC). We adopted a digital bucket relay transfer with multistage flip-flop connection, a pixel unit Cu-Cu connection, and positive-feedback circuitry, to realize a 6.9-μm pixel-pitch, 1.46-Mpixel 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 simultaneous ADC. 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 figure of merit of 0.24 nJ·e-rms/step calculated by dividing the entire sensor power by the effective ADC resolution at a subthreshold current of 111 nA during 660 frames/s operation.