Current sensing echo cancellation device

A line card for coupling to a subscriber line, comprising:

• a balancing network;
• and
• a subscriber line interface circuit adapted to drive a downstream signal on the subscriber line and receive an upstream signal on the subscriber line, the subscriber line interface circuit comprising:
• a first output terminal coupled to the subscriber line;
• an output driver coupled to the output terminal and adapted to drive the downstream signal on the subscriber line;
• first sensing circuitry coupled to the output driver adapted to sense a current on the subscriber line to generate a first voltage;
• a second output terminal coupled to the balancing network to provide at least a portion of the downstream signal to the balancing network;
• second sensing circuitry adapted to sense a current in the portion of the downstream signal passing through the balancing network to generate a second voltage;
• and
• an upstream driver adapted to subtract the second voltage from the first voltage to generate the upstream signal.

A method for canceling echoes on a subscriber line, comprising:

• driving a downstream signal on the subscriber line;
• sensing a current on the subscriber line to generate a first voltage;
• providing a portion of the downstream signal to a balancing network;
• sensing a current in the portion of the downstream signal passing through the balancing network to generate a second voltage;
• and
• subtracting the second voltage from the first voltage to generate an upstream signal.

A passive echo cancellation circuit for reducing echoes on a subscriber line having first and second differential lines, comprising:

• a first output driver coupled to the first differential line and being adapted to drive a first component of a downstream signal on the first differential line;
• a second output driver coupled to the second differential line and being adapted to drive a second component of the downstream signal on the second differential line;
• first sensing circuitry coupled to the first output driver and being adapted to sense a current on the first differential line to generate a first voltage;
• second sensing circuitry coupled to the second output driver and being adapted to sense a current on the second differential line to generate a second voltage;
• a balancing network coupled between the first and second differential lines and being adapted to receive at least a portion of the downstream signal;
• third sensing circuitry adapted to sense a current in the portion of the first component of the downstream signal passing through the balancing network to generate a third voltage;
• and
• fourth sensing circuitry adapted to sense a current in the portion of the second component of the downstream signal passing through the balancing network to generate a fourth voltage;
• a first upstream driver adapted to subtract the third voltage from the first voltage to generate a first differential component of an upstream signal;
• and
• a second upstream driver adapted to subtract the fourth voltage from the second voltage to generate a second differential component of the upstream signal.

A passive echo cancellation circuit for reducing echoes on a subscriber line having first and second differential lines, comprising:

• a first output driver coupled to the first differential line and being adapted to drive a first component of a downstream signal on the first differential line;
• a second output driver coupled to the second differential line and being adapted to drive a second component of the downstream signal on the second differential line;
• a first sensing resistor coupled to the first output driver;
• a second sensing resistor coupled to the second output driver;
• a balancing network coupled between the first and second differential lines;
• a third sensing resistor coupled between the first output driver and the balancing network;
• a fourth sensing resistor coupled between the second output driver and the balancing network;
• a first upstream driver coupled to the first sensing resistor and the third sensing resistor and being adapted to generate a first differential component of an upstream signal;
• and
• a second upstream driver coupled to the second sensing resistor and the fourth sensing resistor and being adapted to generate a second differential component of the upstream signal.