In-pixel ADC for a vision architecture on CMOS-3D technology

This paper addresses the design of an 8-bit single-slope in-pixel ADC for a 3D chip architecture intended for airborne surveillance and reconnaissance applications. The 3D chip architecture comprises a sensor layer with a resolution of 320 × 240 pixels bump-bonded to a three-tier chip on the 150 nm FDSOI CMOS-3D technology from MIT-Lincoln Laboratories. The top tier is a mixed-signal layer with 160 × 120 processing elements. The ADC is distributed between the top two tiers. The top tier contains both global and local circuitry. The ramp generation is implemented with global circuitry through an 8-bit unary current-steering DAC. The end of conversion at every pixel or processing element is triggered by a local comparator. The digital words are stored in a frame-buffer in an intermediate tier. The area of the local circuitry in the ADC is consumed by the comparator, capable of reaching less than 3 mV of resolution in less than 150 ns with less than 220 μm², and by the memory cells, each one storing 6 8-bit words along with two additional bits in less than 50 μm × 50 μm. Every ADC conversion is performed in less than 120 μs.