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-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> 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<inline-formula> <tex-math notation="LaTeX">$\cdot \text{e}-$ </tex-math></inline-formula>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.

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