  Analog to digital conversion is a very important part of almost all beam
instrumentation systems. Ideally, in a properly designed system, the used
analog to digital converter (ADC) should not limit the system performance.
However, despite recent improvements in ADC technology, quite often this is not
possible and the choice of the ADC influences significantly or even restricts
the system performance. It is therefore very important to estimate the
requirements for the analog to digital conversion at an early stage of the
system design and evaluate whether one can find an adequate ADC fulfilling the
system specification. In case of beam instrumentation systems requiring both,
high time and amplitude resolution, it often happens that the system
specification cannot be met with the available ADCs without applying special
processing to the analog signals prior to their digitisation. In such cases the
requirements for the ADC even influence the system architecture. This paper
aims at helping the designer of a beam instrumentation system in the process of
selecting an ADC, which in many cases is iterative, requiring a trade off
between system performance, complexity and cost. Analog to digital conversion
is widely and well described in the literature, therefore this paper focusses
mostly on aspects related to beam instrumentation. The ADC fundamentals are
limited to the content presented as an introduction during the CAS one-hour
lecture corresponding to this paper.
