An imaging system including column-parallel ADCs that operate in response to a single slope global ramp signal and a matched global ramp line signal that has a voltage representative of a dark pixel value. The signal paths of the global ramp signal and the matched global ramp line signal are matched to minimize noise effects. Prior to performing a pixel read operation, the global ramp signal is increased through a first voltage range (below the dark pixel value) to ensure that the column-parallel ADCs are operating in a linear range. The first voltage range can be adjusted to cancel offset error associated with the column parallel ADCs. The column-parallel ADCs provide output signals having a full voltage swing between V.sub.DD and ground.