Image sensor with correlated double sampling

A radiation sensor such as a photodiode is coupled via a sampling switch to memory elements such as three sample and hold stages indicated by capacitors C1, C2, C3. A signal integrated in the photodiode is sampled and can be stored in the sample and hold stages. Charge levels for the photodiode are stored in two of the memory elements at the beginning and end of a first integration period. Correlated double sampling can then be used to remove the effects of reset noise and dc offset from the sensor signal i.e. the value of the signal at the beginning of the integration period is subtracted from the value at the end of the integration period. To improve the operating rate of the double correlation process, while the subtraction of the reset level from the end signal for the first integration period is being performed, a reset level at the beginning of a second, subsequent integration period can be stored in the third memory element.