An apparatus comprises a readout circuit configured to be disconnected from a pixel output, and to connect a pixel reset signal received by the readout circuit to a pixel output signal received by the readout circuit. The apparatus also comprises at least one programmable gain amplifier coupled with the readout circuit. The apparatus further comprises an analog-to-digital converter coupled with the programmable gain amplifier. The readout circuit is configured to be calibrated based on a comparison of a measured output of the readout circuit to a predetermined value, the predetermined value being equal to (2n/2)−1, where n is the number of bits of the analog-to-digital converter.