The input impedance of recently-introduced digital impedance circuits has been discovered to be dependent on the impedance of the external signal source. To address this problem, the theory for the dependence of digital impedance on external source resistance is presented. These digital impedance circuits provide an important digitally-controlled digitally-tunable alternative approach to difficult design problems, such as design of negative capacitances for stable wideband non-Foster antennas and metamaterials. Unfortunately, undesired source-dependent variation of the digital impedance can arise in scenarios where off-the-shelf high-speed analog-to-digital and digital-to-analog converters commonly have 50 ohm impedance. Further complicating matters, the sensitivity of digital impedance on source resistance appears to also depend on other design parameters of the digital circuit. Therefore, theory and simulation results are presented to show the dependence of digital impedance on the external source resistance. Lastly, measured results for a prototype of a digital non-Foster negative capacitance confirm the theoretical results.