As a new generation radio telescope, the Allen Telescope Array (ATA) is a prototype for the Square Kilometer Array (SKA). Here we describe recently developed design constraints for the ATA digital signal processing chain as a case study for SKA processing. As radio frequency interference (RFI) becomes increasingly problematic for radio astronomy, radio telescopes must support a wide range of RFI mitigation strategies, including online adaptive RFI nulling. We observe that the requirements for digital accuracy and control speed are driven not by astronomical imaging but by RFI. This can be understood from the fact that high dynamic range and digital precision are necessary to remove strong RFI signals from the weak astronomical background and because RFI signals may change rapidly compared with celestial sources. We review and critique lines of reasoning that lead us to some of the design specifications for ATA digital processing, including these specifications: Beam former coefficients must be specified with at least 1° precision and at least once per millisecond to enable flexible RFI excision.