A column parallel image sensor such as an active pixel sensor includes a calibration circuit for the readout circuit within the active pixel sensor. The calibration circuit produces a value that is stored and used to offset any noise in the A/D converter. The calibration is carried out each time that each row is read out so in effect the calibration's individual for each pixel. Hence, any noise within the calibration evens out within the image, and is effectively random within the image and hence becomes less noticeable within the image.