System and method for AC coupling

1. An AC coupling circuit comprising:

a level shifter circuit having a p input voltage and an n input voltage and producing a p output voltage and an n output voltage;

a common mode voltage adjustment feedback circuit configured to cause a common mode voltage output to tend towards a specified reference voltage, the common mode voltage output being an average of the p output and n output voltages of the level shifter circuit;

wherein the level shifter circuit comprises an RC (resistor capacitor) network;

wherein the RC network comprises a first RC network that produces the p output voltage and a second RC network that produces the n output voltage;

the first RC network comprises a first resistor divider in parallel with a first capacitor divider;

the second RC network comprises a second resistor divider in parallel with a second capacitor divider.

2. The circuit of claim 1 wherein components of the level shifter circuit comprise adjustable components such that a frequency response of the level shifter circuit is adjusted by adjusting the adjustable components.

3. The circuit of claim 1 wherein the first and second RC networks each comprise adjustable resistors and adjustable capacitors such that a frequency response of the RC network is adjusted by adjusting the adjustable resistors and adjustable capacitors.

4. The circuit of claim 3 wherein the resistors and capacitors are adjusted such that the frequency response of the level shifter circuit cancels out at least part of another frequency response.

5. The circuit of claim 1 wherein the common mode voltage adjusting feedback circuit sources current or sinks current through a respective resistor in each of the first and second RC networks.

6. The circuit of claim 5 wherein:

the respective resistor in each of the first and second RC networks is a respective first resistor in each of the first and second resistor dividers.

7. The circuit of claim 1 wherein the common mode voltage adjusting feedback circuit comprises:

a comparator that compares the common mode output voltage to the reference voltage and produces an output based on the comparison;

a filter that filters the output of the comparator to produce a filtered output;

a digital to analog converter that receives the filtered output and produces a control signal.

8. The circuit of claim 1 wherein the common mode voltage adjusting feedback circuit comprises an analog feedback loop.

9. The circuit of claim 7 wherein:

the common mode voltage adjusting feedback circuit comprises at least one current source;

the control signal is for the at least one current source.

10. The circuit of claim 9 wherein the at least one current source comprises a first pull-up current source and a first pull-down current source each connected to the first RC network, and a second pull-up current source and a second pull-down current source each connected to the second RC network.

11. The circuit of claim 10 wherein at any instant, either:

the first pull-up current source sources current through a resistor in the first RC network, and the second pull-up current source sources current through a resistor in the second RC network; or

the first pull-down current source sinks current through a resistor in the first RC network, and the second pull-down current source sinks current through a resistor in the second RC network.

12. The circuit of claim 10 wherein:

the first pull-up current source and the first pull-down current source both form part of a first sourcing and sinking current source; and

the second pull-up current source and the second pull-down current source each form part of a second sourcing and sinking current source.

13. An integrated circuit comprising the AC coupling circuit of claim 1 .

14. The integrated circuit of claim 13 wherein the reference voltage is set to equal a common mode voltage suitable for input to a remainder of the integrated circuit.

15. A method of AC coupling:

performing level shifting with a level shifter circuit having a p input voltage and an n input voltage and producing a p output voltage and an n output voltage;

using a feedback circuit to cause a common mode voltage output to tend towards a specified reference voltage, the common mode voltage output being an average of the p output and n output voltages of the level shifter circuit;

wherein the level shifter circuit comprises an RC (resistor capacitor) network;

wherein the RC network comprises a first RC network that produces the p output voltage and a second RC network that produces the n output voltage;

the first RC network comprises a first resistor divider in parallel with a first capacitor divider;

the second RC network comprises a second resistor divider in parallel with a second capacitor divider.

16. The method of claim 15 further comprising:

adjusting at least one adjustable component in the level shifter circuit to adjust a frequency response of the level shifter circuit.

17. The method of claim 15 further comprising adjusting at least one adjustable component in the level shifter circuit to adjust a frequency response of the level shifter circuit to cancel out at least part of a frequency response of an interconnect between a first circuit and a second circuit, the level shifter circuit and the feedback circuit both forming part of the second circuit.

18. The method of claim 15 further comprising:

transmitting a signal across an interconnect;

receiving the signal, wherein the signal received by the second circuit contains said p input voltage and said n input voltage.

19. A system comprising:

a first circuit;

a second circuit comprising the AC coupling circuit of claim 1 ;

an interconnect that connects the first circuit to the second circuit.

20. The system of claim 19 wherein the first circuit is a first integrated circuit, the second circuit is a second integrated circuit, and the interconnect comprises a circuit board trace.