Conversion of digital signals into spiking analog signals

1. A method of converting a digital signal into a spiking analog signal, comprising:

applying a different constant current to each of a plurality of switch circuits;

applying each bit of the digital signal to a corresponding one of the plurality of switch circuits;

each switch circuit applying the corresponding constant current to a common output when the corresponding bit has a predetermined value and not applying the corresponding constant current to the common output when the corresponding bit does not have the predetermined value; and

applying a common current at the common output to a spiking neuron circuit.

2. The method of claim 1 , wherein each switch circuit applies the corresponding constant current to a sink when the corresponding bit does not have the predetermined value.

3. The method of claim 1 , wherein the constant current applied to each switch circuit is double the current applied to the switch circuit corresponding to the next less significant bit in the digital word, except for the switch circuit corresponding to the least significant bit in the digital word.

4. The method of claim 1 , wherein the applying the current further comprises:

charging a capacitor with the current at the common output; and

discharging the capacitor when a voltage across the capacitor reaches a predetermined value.

5. The method of claim 4 , wherein the charging includes continuously applying a charging current to the capacitor as long as all of the bits of the digital signal are not zero.

6. The method of claim 1 , wherein the sum of the constant current sources is no more than in the micro-ampere range.

7. A circuit for converting a digital signal into a spiking analog signal, comprising:

a digital-to-analog converter including a plurality of switch circuits, each switch circuit receiving a different constant current and a corresponding bit of the digital signal, each switch circuit connecting the corresponding constant current to a common output when the corresponding bit has a predetermined value and not applying the corresponding constant current to the common output when the corresponding bit does not have the predetermined value; and

a spiking neuron circuit having an input connected to the common output.

8. The circuit of claim 7 , wherein each switch circuit applies the corresponding constant current to a sink when the corresponding bit does not have the predetermined value.

9. The circuit of claim 7 , further comprising a constant current source producing the constant currents, wherein the constant current applied to each switch circuit is double the current applied to the switch circuit corresponding to the next less significant bit in the digital word, expect for the switch circuit corresponding to the least significant bit in the digital signal.

10. The circuit of claim 9 , wherein the constant current source comprises a current mirror circuit.

11. The circuit of claim 10 , wherein the current mirror circuit includes a plurality of pairs of transistors, each pair of transistors producing a different constant current applied to a corresponding switching circuit, a ratio of width to length of transistors in each pair of transistors corresponding to each switch circuit being double that of the pair of transistors corresponding to the switch circuit receiving the next less significant bit in the digital word, except for the switch circuit receiving the least significant bit of the digital word.

12. The circuit of claim 7 , wherein the spiking neuron circuit further comprises:

a capacitor having a terminal coupled to the common output;

an amplifier having an input coupled to the terminal of the capacitor; and

a switch having current conducting terminals connected across the capacitor and a control terminal connected to the output of the amplifier.

13. The circuit of claim 12 , wherein the common output continuously provides a current to the capacitor as long as all of the bits of the digital signal are not zero.

14. The circuit of claim 7 , wherein the sum of the constant currents is no more than the micro-ampere range.