Contemporary CCD especially the scientific grade CCD dynamic range is up to 100,000 electrons, and even capacity over 100,000 electrons. In order to take full advantage of these characteristics, it is necessary that the dynamic range of Analog-to-Digital Converter (ADC) must exceed the dynamic range of the CCD. The number of bits provided bythe ADC must exceed 16 bits, while the high reliability and inexpensive 18～20 bit A/D converter is few. Firstly, we analyze CCD noise, and then the principle to extend the dynamic of the CCD in signal processing chain is presented using two low resolution ADC with different sensitivity. Then, we present a concrete example of improving the resolution of the ADC by two parallel low resolution ADCs. Finally, the laboratory imaging test results show that two low-resolution ADCs are used by combining raw quantization with fine quantization to achieve a high resolution, and improve the dynamic range of CCD camera.