This chapter first describes the fundamentals and principles of the analog-to-digital converters. Several conventional ADC structures are analyzed to show their advantages and disadvantages for different requirements and applications. Then, the trade-offs between the conventional ADC and the radiation-hardened ADC are discussed. Four-factor design trade-offs for radiation-tolerant ADC between power, speed, accuracy, and radiation tolerance are concluded. These trade-offs provide fundamental guidelines for the following ADC architecture selection and design.