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 this dependence of digital impedance on external source impedance is presented. These circuits offer an important digitally-controlled alternative to achieve challenging impedances, such as design of stable negative capacitances for wideband non-Foster antennas and metamaterials. Further complicating matters, the dependence on source impedance appears to manifest itself over limited ranges of design parameters. Simulation and measured results for a prototype non-Foster negative capacitance are provided in support of the theoretical results.