2. The apparatus of claim 1 , wherein the excitation circuit is configured to generate a current excitation at the first terminal combination; and wherein the detector is configured receive a voltage developed at the first terminal combination in response to the current excitation.
3. The apparatus of claim 2 , wherein the excitation circuit is configured to inject a pulsed current excitation into the first terminal combination, the first terminal combination comprising a reference terminal; and wherein the detector circuit is configured to measure a voltage developed across the first terminal combination.