Method and apparatus for generating a common-mode reference signal

Abstract

A method and apparatus to generate a common-mode reference signal. A common-mode current is received at a common-mode current sensing circuit. The common-mode current is sampled at a node between the common-mode current sensing circuit and a shunt resistor. The resulting voltage across the shunt resistor from the applied common-mode current is used as a common-mode reference signal.

What is claimed is: 
     
         1 . An apparatus comprising:
 a common-mode current sensing circuit configured to receive a common-mode current;   a shunt resistor coupled between the common-mode current sensing circuit and a first low supply node and wherein the common-mode current sensing circuit comprises an instant averager circuit and receives the common-mode current from a pair of signal lines, the pair of signal lines comprising a first signal line and a second signal line.   
     
     
         2 . The apparatus of  claim 1 , wherein the instant averager circuit comprises:
 a first isolation capacitance or resistance coupled to the first signal line;   a first of the other of a matched resistance or capacitance coupled to the first isolation capacitance or resistance;   a second isolation capacitance or resistance coupled to the second signal line; and   a second of the other of a matched resistance or capacitance coupled to the second isolation capacitance or resistance.   
     
     
         3 . The apparatus of  claim 2 , wherein a first end of the shunt resistor is coupled to the first matched resistor and the second matched resistor and a second end of the shunt resistor is coupled to the first low supply node. 
     
     
         4 . The apparatus of  claim 1 , wherein the instant averager circuit comprises:
 a first matched resistance coupled to the first signal line;   a first isolation capacitance coupled to the first matched resistance;   a second matched resistance coupled to the second signal line; and   a second isolation capacitance coupled to the second matched resistance.   
     
     
         5 . The apparatus of  claim 4 , wherein a first end of the shunt resistor is coupled to the first isolation capacitance and the second isolation capacitance and a second end of the shunt resistor is coupled to the first low supply node. 
     
     
         6 . The apparatus of  claim 2 , wherein the shunt resistor has a resistance in approximately a range of 10 ohms to 1000 ohms. 
     
     
         7 . The apparatus of  claim 2 , further comprising:
 a first series winding coupled to the first signal line;   a second series winding coupled to the second signal line; and   a transformer coupled to the first series winding and the second series winding.   
     
     
         8 . The apparatus of  claim 7 , wherein the transformer has a center tap on an input side. 
     
     
         9 . The apparatus of  claim 8 , further comprising:
 a center tap potential feed circuit coupled between the center tap and a second low supply node.   
     
     
         10 . The apparatus of  claim 7 , wherein the transformer has a first primary winding and a second primary winding on an input side. 
     
     
         11 . The apparatus of  claim 8 , further comprising:
 a center tap potential feed circuit coupled between the first primary winding, the second primary winding and a second low supply node.   
     
     
         12 . The apparatus of  claim 7 , further comprising:
 a first receiver coupled to an output side of the transformer; and   a second receiver coupled to a node between the instant averager circuit and the shunt resistor.   
     
     
         13 . The apparatus of  claim 12 , further comprising:
 a signal filter coupled between the node and the second receiver.   
     
     
         14 . A method, comprising:
 receiving a common-mode current at a common-mode sensing circuit; and   sampling the common-mode current at a node coupled between the common-mode current sensing circuit and a shunt resistor to generate a common-mode reference signal wherein receiving the common-mode current comprises receiving the common-mode current from a pair of signal lines, and wherein the common-mode current sensing circuit comprises an instant averager circuit, the instant averager circuit coupled to the pair of signal lines.   
     
     
         15 . The method of  claim 14 , wherein the shunt resistor has a resistance in approximately a range of 10 ohms to 1000 ohms. 
     
     
         16 . The apparatus of  claim 1 , wherein the common-mode current sensing circuit comprises an instant averager circuit. 
     
     
         17 . The apparatus of  claim 16 , wherein the common-mode current is received from a pair of signal lines and wherein the instant averager circuit is coupled to the pair of signal lines.