An imager including sub-imager pixel arrays having a plurality of four-side buttable imagers distributed on a substrate and an on-chip digitizing readout circuit. Pixel groupings formed from among the plurality of four-side buttable imagers. The readout electronics including a buffer amplifier for each of the pixel groupings and connected to respective outputs of each four-side buttable imager of the pixel grouping. A plurality of shared analog front ends, each shared analog front end connected to respective multiple buffer amplifiers from among the plurality of pixel groupings. An analog-to-digital converter located at a common centroid location relative to the plurality of shared analog front ends, the analog-to-digital converter having a fully addressable input selection to individually select an output from each of the plurality of shared analog front ends. An output of the analog-to-digital converter connected to a trace on a back surface of the wafer substrate by a through-substrate-via.