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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. 

It is proposed that gravitational meta-atom unit cells with gravitomagnetic moments could exhibit gravitomagnetic permeability, analogous to the magnetic permeability of materials comprised of atoms with magnetic moments. Recently, a gravitoelectromagnetic (GEM) framework was proposed to explore the possibility of a Veselago-inspired approach to gravitational metamaterials. The prospect of gravitational metamaterials motivates the consideration of candidate gravitational unit cells or gravitational meta-atoms. Although mass serves as a monopole source of a gravitoelectric field similar to positive charge, negative mass would be needed to create a gravitational analog of an electric dipole. However, moving mass is analogous to electric current, and can lead to a gravitomagnetic dipole moment analogous to magnetic dipole moments of magnetic materials and atoms. In this paper, GEM field approximations to general relativity are used to find the gravitomagnetic dipole moment of different rotating systems, ranging in scale from meters to astronomical size.