A current mode subscriber line interface circuit (SLIC) for telephone system in which AC and De impedances for tip and ring connections of the SLIC are separately set. A DC filter separates the DC and AC components of tip and ring currents, and a current amplifier amplifies the AC components separately...http://www.google.com/patents/US5515434?utm_source=gb-gplus-sharePatent US5515434 - Impedance setting circuit for a SLIC
