A signal chain for an image sensor is disclosed. The signal chain includes photo sensing elements, pixel readout circuits, and an amplifier. Each pixel readout circuit receives a charge-induced signal and a reset signal from one of the photo sensing elements. The readout circuit computes a difference signal between the charge-induced signal and said reset signal. The difference signal is measured with respect to a reference signal. The amplifier is coupled to the pixel readout circuits, and configured to supply the reference signal during computation of the difference signal. Further, the amplifier amplifies the difference signal when the computation is done.