  Readout noise is a critical parameter for characterizing the performance of
charge-coupled devices (CCDs), which can be greatly reduced by the correlated
double sampling (CDS) circuit. However, conventional CDS circuit inevitably
introduces new noises since it consists of several active analog components
such as operational amplifiers. This paper proposes a digital CDS circuit
technique, which transforms the pre-amplified CCD signal into a train of
digital presentations by a high-speed data acquisition card directly without
the noisy CDS circuit first, then implement the digital CDS algorithm through
numerical method. The readout noise of 3.3 e$^{-}$ and the energy resolution of
121<EMAIL_ADDRESS>can be achieved via the digital CDS technique.
