Programmable impedance

1. A programmable impedance providing an impedance value across two nodes and being responsive to an input code word, the impedance comprising:

a first branch couplable between the two nodes for use in providing impedance values across a first range of the input code word; and

a second branch couplable between the two nodes for use in providing impedance values across a second range of the input code word;

wherein the input code word comprises a plurality of bits and the first branch is responsive to all bits of the plurality of bits.

2. A programmable impedance according to claim 1 , wherein the second branch is responsive to a subset of the plurality of bits.

3. A programmable impedance according to claim 1 , wherein the first and second ranges overlap.

4. A programmable impedance according to claim 1 , where the first and second ranges do not overlap.

5. A programmable impedance according to claim 1 , wherein the first branch has a minimum impedance which it can present across the two nodes and where the second branch is configured to allow an impedance value below the minimum impedance to be presented across the two nodes.

6. A programmable impedance according to claim 1 , wherein the second branch is configured to provide an impedance value in response to a input code word value in the range from 0 to M, where M≧1 and where the first branch is configured to provide an impedance value for an input code word value in a range comprising at least M+1 to a maximum value of the input word.

7. A programmable impedance according to claim 1 , wherein the second branch is configured to provide an impedance value in response to a input code word value in the range from 0 to M, where M>1, and where the first branch is configured to provide an impedance value for an input code word value in a range comprising at least M+1 to the maximum value of the input word.

8. A programmable impedance according to claim 1 , wherein the second branch comprises a plurality of switches arranged in parallel, and where the plurality of the switches are selectably switchable to effect a desired impedance reflective of the input digital word.

9. A programmable impedance according to claim 8 , wherein the plurality of switches are scaled relative to one and other.

10. A programmable impedance according to claim 1 , wherein the second branch comprises at least one controllable impedance and where the impedance is controllable to effect a desired impedance reflective of the input digital word.

11. An impedance according to claim 1 , wherein the impedance of the first branch is configured to be decoupled between the two nodes when the second branch is coupled between the two nodes.

12. An impedance according to claim 1 , wherein the second branch is configured to be decoupled when the first branch is coupled.

13. An impedance according to claim 1 , wherein the first branch comprises a multi stage controlled impedance.

14. An impedance according to claim 1 , wherein the first branch comprises a switchable resistor array.

15. A digital to analog converter comprising the impedance of claim 1 .

16. A method of providing an impedance in response to an input value, the method comprising:

determining that the value is within a determined range of operation of a first programmable impedance; and

using the first programmable impedance to provide the impedance value;

wherein the input value comprises a plurality of bits and the first programmable impedance is responsive to all bits of the input.

17. The method of claim 16 , further comprising:

determining that the value is not within a determined range of operation of a first programmable impedance; and

using a second programmable impedance to provide an extended range of operation, where the extended range of operation provides lower impedance values than those of the determined range of operation.

18. A method according to claim 17 , further comprising coupling the first and second programmable impedances in parallel for the extended range of operation.

19. A circuit element providing a impedance value across two nodes in response to an input code word, the circuit element comprising:

a primary branch, coupled between the two nodes, comprising a programmable impedance configured to be responsive to the input code word to provide a corresponding impedance between the two nodes;

an auxiliary branch coupled between the two nodes and comprising at least one impedance switchably couplable between the two nodes, wherein the auxiliary branch is configured to be employed to augment the performance of the primary branch allowing for a lower impedance value to be provided between the two nodes than the programmable impedance is capable of providing;

wherein the input code word comprises a plurality of bits and the primary branch is responsive to all bits of the plurality of bits.