Analogue to digital converter, and method of analogue to digital conversion

1. An analogue to digital converter comprising:

a dual differential digital to analogue converter (DAC) having first and second digital inputs for first and second digital input signals, respectively, and having first and second analogue differential output for first and second differential output signals respectively, where the first and second differential output signals are associated with the first and second digital inputs respectively;

storage for storing a DAC digital input value;

logic for deriving the first and second digital input signals from the DAC digital input value, such that the difference between the first and second differential output signals from the DAC represents the DAC digital input value;

a comparator for comparing the first and second differential output signals from the DAC with an analogue input signal and providing a comparator output; and

a controller, responsive to the comparator output, for modifying the DAC digital input value such that the DAC digital input value substantially represents the analogue input signal.

2. An analogue to digital converter as claimed in claim 1 , wherein the analogue input signal is a differential voltage.

3. An analogue to digital converter as claimed in claim 2 , wherein the comparator compares the difference between the first and second differential output signals from the DAC with twice the differential input voltage.

4. An analogue to digital converter as claimed in claim 1 , wherein the controller modifies an intermediate digital value so that the intermediate digital value substantially represents the analogue input signal and the intermediate digital value is provided as an output from the analogue to digital converter.

5. An analogue to digital converter as claimed in claim 4 , wherein the intermediate digital value is provided as the output from the analogue to digital converter wherein the intermediate digital value substantially represents the analogue input signal.

6. An analogue to digital converter as claimed in claim 4 , wherein the first input signal to the DAC comprises the intermediate digital value with the least significant bit discarded.

7. An analogue to digital converter as claimed in claim 4 , wherein the second input signal to the DAC comprises the two's complement of the intermediate digital value, with the least significant bit of the two's complement discarded.

8. An analogue to digital converter as claimed in claim 1 , comprising an input sample and hold circuit for sampling the analogue input signal and providing the sampled analogue input signal to the comparator.

9. An analogue to digital converter as claimed in claim 1 , wherein the sample and hold circuit is a differential sample and hold circuit, and the sample and hold circuit samples the analogue input signal and provides differential sampled input voltage to the comparator.

10. An analogue to digital converter as claimed in claim 4 , wherein the intermediate digital value is a n-bit value.

11. An analogue to digital converter as claimed in claim 10 , wherein the first and second digital input signals to the DAC comprise (n-1)-bit value.

12. An analogue to digital converter as claimed in claim 10 , wherein the DAC comprises a (n-2) bit resistor string.

13. An analogue to digital converter as claimed in claim 1 , wherein the controller modifies the DAC digital input value using successive approximation such that the DAC digital input value substantially represents the analogue input signal.

14. An analogue to digital converter as claimed in claim 4 , wherein the controller modifies the intermediate digital value using successive approximation such that the intermediate digital value substantially represents the analogue input signal.

15. An analogue to digital converter as claimed in claim 1 , comprising first and second output sample and hold circuits for sampling the first and second differential output signals from the DAC respectively when the analogue to digital converter is in a digital to analogue converter mode.

16. An analogue to digital converter as claimed in claim 1 , wherein the logic for deriving the first and second digital input signals for the DAC includes first and second digital inputs, and the logic passes the digital inputs to the DAC when the analogue to digital converter is in a digital to analogue converter mode.

17. A method of analogue to digital conversion, comprising:

providing a DAC digital input value to a dual differential digital to analogue converter (DAC), such that a difference between first and second differential output signals from the DAC substantially represents the DAC digital input value;

comparing the output signals from the DAC with an analogue input signal; and

modifying the DAC digital input value in response to the comparison such that the DAC digital input value substantially represents the analogue input signal.

18. A method as claimed in claim 17 , wherein providing the analogue input signal to the comparator comprises sampling and holding the analogue input signal, and providing a sampled input voltage the comparator.

19. A method as claimed in claim 18 , wherein sampled input voltage provided to the comparator is a differential voltage.

20. A method as claimed in claim 19 , comprising comparing the first and second differential output signals from the DAC with twice the differential sampled input voltage.

21. A method as claimed in claim 17 , wherein providing a DAC digital input value to the DAC comprises:

providing the DAC digital input value, with the least significant bit discarded, to a first input of the DAC; and

providing the two's complement of the DAC digital input value, with the least significant bit of the two's complement discarded, to a second input of the DAC.

22. A method as claimed in claim 17 , wherein modifying the DAC digital input value comprises using successive approximation to modify the DAC digital input value such that the DAC digital input substantially represents the analogue input signal.

23. A method as claimed in claim 17 , wherein modifying the DAC digital input value comprises using successive approximation to modify the DAC digital input value such that the DAC digital input value substantially represents the analogue input signal.

24. A system for carrying out the method as claimed in claim 17 .

25. An n-bit analogue to digital converter, comprising:

a dual differential string digital to analogue converter (DAC) having first and second digital inputs for first and second (n-1)-bit digital input signals respectively, and having first and second analogue differential outputs for first and second digital output signals respectively, where the first and second differential output signals are associated with the first and second digital input signals respectively, and where the DAC contains an (n-2)-bit resistor string;

storage for storing an n-bit DAC digital input value;

logic for deriving the first and second digital input signals from the DAC digital input value, such that the difference between the first and second digital output signals from the DAC represents the DAC digital input value;

an input sample and hold circuit for sampling a single-ended input voltage to obtain a differential sampled input voltage;

a comparator for comparing the first and second differential output signals from the DAC with the differential sampled input voltage and providing a comparator output;

a controller, responsive to the comparator output, for modifying the DAC digital input value and an intermediate digital value using successive approximation such that the DAC digital input value and the intermediate digital value substantially represents the input voltage;

first and second output sample and hold circuits for sampling and holding the first and second differential output signals respectively from the DAC; and

a n-bit output for the intermediate digital value when the intermediate digital value substantially represents the input voltage.