Resistor-input transconductor including common-mode compensation

1. A system comprising:

a first voltage input;

a second voltage input;

a first voltage-to-current converter comprising:

a first circuit configured to provide a virtual ground,

an input resistor R p disposed between the first voltage input and the virtual ground, and

a circuit configured to generate an output current I outp responsive to current flowing through the resistor R p ;

a second voltage-to-current converter comprising:

a second circuit configured to provide the virtual ground,

an input resistor R n disposed between the second voltage input and the virtual ground, and

a circuit configured to generate an output current I outn responsive to current flowing through the resistor R n ;

a third voltage-to-current converter comprising:

a third circuit configured to provide the virtual ground,

an input resistor R cmp disposed between the first voltage input and the virtual ground,

an input resistor R cmn disposed between the second voltage input and the virtual ground, and

a circuit configured to combine currents flowing through the input resistors R cmp and R cmn , and to generate an output current I Rcm proportional to the combined currents; and

a circuit configured to provide the output current I Rcm to the first voltage-to-current converter and the second voltage-to-current converter so as to compensate for a common-mode voltage between the first voltage input and the second voltage input.

2. The system of claim 1 , wherein the circuit configured to provide the output current I Rcm to the first voltage-to-current converter is configured to provide the output current to the output of the first voltage-to-current converter.

3. The system of claim 1 , wherein the circuit configured to provide the output current I Rcm to the first voltage-to-current converter is configured to provide the output current to the first voltage-to-current converter at a point between the input resistor R p and the virtual ground.

4. The system of claim 1 , wherein the input resistors R cmp and R cmn have approximately the same resistance.

5. The system of claim 1 , wherein the input resistors R cmp and R cmn have approximately twice the resistance of the input resistors R n and R p .

6. The system of claim 1 , further including a current-to-voltage converter configured to receive the output currents I outn and I outp and generate a voltage proportional to a difference between these currents.

7. The system of claim 1 , further including an analog-to-digital converter configured to generate a digital value representative of a difference between V inp and V inn , responsive to the output currents I outn and I outp .

8. The system of claim 1 , wherein the circuit configured to generate an output current I outp responsive to current flowing through the resistor R p is configured to generate output current I outp proportional to current flowing through the resistor R p .

9. The system of claim 1 , wherein the circuit configured to generate an output current I outp responsive to current flowing through the resistor R p is configured to generate output current I outp proportional to current flowing through the resistor R p combined with the output current I Rcm .

10. The system of claim 1 , wherein the circuit configured to provide the output current I Rcm to the first voltage-to-current converter is configured to do so at the output of the first voltage-to-current converter.

11. The system of claim 1 , further comprising an interface configured to receive a signal from a power line and provide the signal across the first voltage input and the second voltage input.

12. The system of claim 1 , further comprising an interface configured to receive a signal from a telephone line or cable line and provide the signal across the first voltage input and the second voltage input.

13. The system of claim 1 , wherein the virtual ground is established using a gm-boosting loop.

14. A system comprising:

means for receiving a first input voltage and generating a first output current using the first input voltage;

means for receiving a second input voltage and generating a second output current using the second input voltage;

means for generating a common-mode current representative of a common-mode voltage between the first input voltage and the second input voltage; and

means for providing the common-mode current to the means for generating a first output current and to the means for generating a second output current, such that the first output current and the second output current are compensated for the common-mode voltage.

15. The system of claim 14 , further including an interface configured to receive a signal from a power line and to provide the received signal to the means for receiving a first input voltage.

16. The system of claim 14 , wherein the means for receiving a first input voltage and generating a first output current includes a circuit configured to establish a virtual ground.

17. A method comprising the steps of:

receiving a first voltage at a first input;

applying the first voltage to a first input resistor to generate a first input current;

receiving a second voltage at a second input;

applying the second voltage to a second input resistor to generate a second input current;

generating a common-mode current representative of a common-mode voltage of the first voltage and the second voltage;

using the first input current and the common-mode current to generate a first output current; and

using the second input current and the common-mode current to generate a second output current, a difference between the first output current and the second output current being proportional to a difference between the first voltage and the second voltage.

18. The method of claim 17 , further comprising communicating the first voltage from a power line to the first input.

19. The method of claim 17 , wherein the first voltage and second voltage are part of a digitally encoded signal, and further including receiving the digitally encoded signal via a telephone line, a cable system, an antenna, or a coax cable.

20. The method of claim 17 , further comprising converting the first output current and the second output current to digital data and communicating the digital data to a computing system.

21. The method of claim 17 , wherein using the first input current and the common-mode current to generate the first output current comprises combining the first input current and the common-mode current at a virtual ground node.

22. The method of claim 17 , wherein using the first input current and the common-mode current to generate the first output current comprises combining the first input current and the common-mode current at the output of a current-copying circuit.