Analog to digital converters (ADCs) are the front end of most of the modern measuring instruments. They affect crucially the interpretation of signals acquired from the real world into the digital domain. ADCs influence dominantly the accuracy of the instruments and limit their bandwidth. The exact ADC error description using standardized testing procedures are needed to evaluate instruments performance. This will be an important task for metrologists in the next future. The paper is aimed at providing a metrological overview of ADC testing.