A readout circuit for use in an image sensor includes a system ramp generator coupled to generate a system ramp signal. A plurality of analog-to-digital converters is coupled to a plurality of column bitlines from a pixel array to receive corresponding analog column image signals. An isolation ramp buffer is coupled between the system ramp generator and the analog-to-digital converters. The isolation ramp buffer includes a single input to receive the system ramp signal, and a plurality of isolated outputs. Each of the isolated outputs is coupled to provide an isolated column ramp signal to a corresponding analog-to-digital converter. Each of the of analog-to-digital converters is coupled to generate a corresponding digital column image signal in response to the corresponding analog column image signal and corresponding isolated column ramp signal.