RF DAC's: output impedance and distortion

Abstract

The output impedance of a current-steering DAC is setting a lower limit for the second-order distortion [1]. At low frequencies it is not much of a factor. The output resistance can be quite high. At higher frequencies the capacitances gravely reduce the output impedance. As the distortion is mainly second order, some propose to use differential outputs for high frequency signals [2]. Unfortunately, this analysis is not complete. Likewise effects cause severe third-order distortion. Before envisaging RF DAC’s this problem must be identified. In this paper we study the problem in depth and give design guidelines.