When designing a mixed-signal system, many designers have a tendency to examine and optimize each component separately. Given the array of different components in a system, designers must have a complete understanding of not only the individual components but also their impact on the overall system performance. When a design has an analog-to- digital converter (ADC), it is critical to understand how this device interacts with the voltage reference and voltage reference buffer. This article, the first of a three-part series, looks at the fundamental operation of an ADC independently, exactly as many designers do, and then at the performance characteristics that have an impact on the accuracy and repeatability of the system.