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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, a conventional CDS circuit inevitably introduces new noise 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, then implements the digital CDS algorithm through a numerical method. A readout noise of 3.3 e(-) and an energy resolution of 121 eV@5.9 keV can be achieved via the digital CDS technique.

Charged Coupled Devices (CCDs) have been successfully used in several low energy X-ray astronomical satellites over the past two decades. Their high energy resolution and high spatial resolution make them a perfect tool for low energy astronomy, such as observing the formation of galaxy clusters and the environment around black holes. The Low Energy X-ray Telescope (LE) group is developing a Swept Charge Device (SCD) for the Hard Xray Modulation Telescope (HXMT) satellite. A SCD is a special low energy X-ray CCD, which can be read out a thousand times faster than traditional CCDs, simultaneously keeping excellent energy resolution. A test method for measuring the charge transfer efficiency (CTE) of a prototype SCD has been set up. Studies of the charge transfer inefficiency (CTI) with a proton-irradiated SCD have been performed at a range of operating temperatures. The SCD is irradiated by 3x10(8)cm(-2) 10 MeV protons.