Many existing results on the analysis of OFDM systems are based on an analog representation. The actual implementation of OFDM transmitters typically consists of a discrete DFT matrix and a digital-to-analog (DAC) converter. In this paper, we show that the analogrepresentation admitsthe implementation of a DFT matrix followed by a DAC converter only in special cases. Necessary and sufficient conditions for such cases will be given. Analyses based on the analog representation, e.g., spectral roll-off of transmitter outputs and carrier frequency offset, may not be appropriate when digital implementation is employed.