Compared with the traditional electronic analog-to-digital converter, the photonic analog-to-digital converter (PADC) has the advantage of high sampling rate, large bandwidth, low timing jitter and low comparator ambiguity. PADC provides an effective solution for high-speed, large-bandwidth, and high-resolution processing in ultra-wideband radar, ultra-high speed oscilloscope, high-capacity optical communication and so on. In this paper, different architectures of PADC are classified and compared. Then, the recent progresses of our group in channel-interleaved PADC are introduced. Besides, the status of integrated PADC are analyzed briefly and the main challenges of future integrated PADC are discussed.