Method and apparatus for a current control

A method of controlling current, comprising:

• coupling discrete impedance elements to one another to form at least one impedance ladder circuit which exhibits a fixed impedance at an input and current divider steps each differing in a current magnitude by a multiple of three with respect to the current magnitude in an adjacent less significant step;
• switchably coupling each current divider step to a selected one of three discrete current sources or sinks;
• coupling three discrete current sources or sinks to corresponding ones of the three outputs of each SPTT switch;
• and
• digitally controlling each SPTT switch to additively deliver currents from selected current divider steps of the impedance ladder circuit formed in the first coupling act to corresponding selected ones of the three discrete current sources or sinks.

A differential hybrid circuit for communicating a differential transmit signal from a differential transmit signal source and a differential receive signal across a digital subscriber line, and the hybrid circuit comprising:

• a transformer with a primary and a secondary inductor and the primary inductor coupled to the differential transmit signal source and the secondary inductor coupled to the subscriber line;
• and
• a balance circuit differentially coupling the transmit signal to and the receive signal from the primary inductor and the balancing circuit including components forming a differential hybrid path for providing a signal inversely proportional to an amount of leakage of the transmit signal into the receive signal at corresponding differential summing nodes thereof on the receive path;
• and the balance circuit hybrid path components including:
• a differential pair of current controllers each including a fixed impedance input coupled to the transmit signal source, a switch control input and successive triple pole outputs each configured to deliver currents corresponding to the transmit signal in amounts differing in magnitude by a multiple of three with respect to an adjacent less significant switch, and each switch configured to switchably couple to a selected one of the positive and negative differential nodes or to a ground to switchably provide at the differential summing nodes differential currents an aggregate of which substantially corresponds inversely with an amount of leakage of the transmit signal into the receive signal, thereby substantially removing leakage from the receive signal.

An apparatus comprising:

• a signal source input that presents a fixed impedance;
• a digital control input;
• a plurality of switch elements coupled to the signal source input and the digital control input, each of the switch elements providing respective differential outputs having respective different magnitudes corresponding to the signal source input;
• and
• a pair of differential summing nodes,
• wherein the digital control input selectively causes the differential outputs of one and only one of the plurality of switch elements to couple to the pair of differential summing nodes.