A circuit includes sensing unit, first and second group of switches, capacitor, and readout circuit. The sensing unit is configured to receive light and generate sensing voltage at sensing node in response to the light. The first group of switches is coupled to the sensing node, and configured to generate first transfer voltage to first node and generate first auxiliary voltage to second node. The second group of switches is coupled to the sensing node, and configured to generate second transfer voltage to third node and generate second auxiliary voltage to fourth node. The capacitor is coupled to the first group of switches, and configured to store charges generated from the sensing unit. The readout circuit is configured to read at least one of the first transfer voltage and the first auxiliary voltage, and read at least one of the second transfer voltage and the second auxiliary voltage.