Correlated Double Sampling

Reset noise can be the dominant noise source in an imaging system but can be removed entirely using the technique of Correlated Double Sampling. This requires sampling the pixel voltage twice, immediately after reset and then again at the end of the exposure. The difference between the two samples gives the true pixel signal. This can be done in software if two complete frames are read out and subsequently subtracted in memory. It can also be done on-chip using additional circuitry at the periphery of the detector.

Correlated Double Sampling in a 3T pixel

Transistors that are switched on are marked in red.

Effective exposure lasts from t1 until t2.