Rail to rail flash

An analog to digital converter comprising:

• a first set comprising nodes defined by circuitry such that each node is at a respective voltage, the voltages on the nodes being in an orderly progression between the voltage of a first current source and the voltage of a first input node;
• a second set comprising nodes defined by circuitry such that each node is at a respective voltage, the voltages on the nodes being in an orderly progression between the voltage of a second current source and the voltage of a second input node;
• a plurality of comparators, each comparator being arranged to compare the voltage on a node in the first set with the voltage on a respective node in the second set;
• and
• a pair of switching devices arranged between the first current source and the first set of nodes, the first switching device of the pair connected so as to supply the current from the first current source to all said nodes in the first set, and the second switching device of the pair connected so as to supply the current from the first current source to a subset of said nodes in the first set which are closest in voltage to the voltage of the first input node;
• wherein control voltages of the first and second switching devices of the pair are set such that:
• (a) when the potential difference between the voltage at the first current source and the voltage at the first input node is greater than a predetermined potential difference the first switching device is in a first state and the current from the first current source flows through the first switching device;
• and
• (b) when the potential difference between the voltage at the first current source and the voltage at the first input node is less than the predetermined potential difference the first switching device is in a second state and the current from the first current source flows through the second switching device.