Fundamental physical constraints, such as noise and channel capacity, set an upper bound to theoretical performance of many systems. In this work we examine the limitations of high-speed data in modern digital wireless radio systems. Although physical constraints provide an ultimate limit, we show that signal-to-noise-plus-distortion-ratio (SNDR) and nonlinearity of modern CMOS and Bipolar technologies limit current performance well before physical limitations. By examining several example circuit topologies used in data conversion in digital radios, we show that 3rd order harmonics (HD3) likely set the practical barrier to high data rates at SNDRs above 40 dB and sampling rates above 20 GS/s.