Output impedance requirements for DACs

Communications applications are driving the development of high output frequency digital to analog converters (DACs) with tough SNR and SFDR frequency-domain specifications. Static measures of performance such as INL and DNL are becoming irrelevant. This paper derives and compares the output impedance required for a DAC to meet static and dynamic specifications in both single-ended and fully-differential implementations. It is shown that the output impedance required in a fully-differential implementation is much lower than that required in a single-ended one. This reduced output impedance requirement has the potential to eliminate traditional impedance-boosting cascode transistors, allowing DACs to scale with technology to reduced supply voltages.