Fast current mode sigma-delta analog-to-digital converter

1. A current mode sigma-delta modulator, comprising:

an input node;

a comparator configured to compare a voltage of the input node to a reference voltage and to output a comparison result;

an integrating capacitor connected to an input of the comparator; and

a switched capacitor circuit connected at a first end to the input node, the input of the comparator, and the integrating capacitor, and connected at a second end to an output of the comparator,

wherein the switched capacitor circuit includes:

a switched capacitor having a first electrode and a second electrode, the second electrode connected to a predetermined voltage;

a first switch directly connected to the first electrode of the switched capacitor and the input node so as to selectively allow a charge to flow therebetween; and

a second switch connected between the predetermined voltage and the first electrode of the switched capacitor.

2. The current mode sigma-delta modulator according to claim 1 , wherein the first switch is controlled by a signal corresponding to the output of the comparator.

3. The current mode sigma-delta modulator according to claim 1 , wherein the second switch is controlled by a signal corresponding to an inverse of the output of the comparator.

4. The current mode sigma-delta modulator according to claim 1 , wherein the first switch is a first transistor and the second switch is a second transistor.

5. The current mode sigma-delta modulator according to claim 1 , wherein

in a case where the output of the comparator is at a high level of two possible levels, the first switch is closed and the second switch is opened, and

in a case where the output of the comparator is at a low level of the two possible levels, the first switch is opened and the second switch is closed.

6. A solid-state imaging device, comprising:

a pixel circuit configured to convert an incident light into an analog signal;

the current mode sigma-delta modulator according to claim 1 ; and

a vertical signal line configured to conduct the analog signal to the input node of the current mode sigma-delta modulator.

7. An analog-to-digital converter, comprising:

the current mode sigma-delta modulator according to claim 1 ; and

a decimation filter configured to receive the output of the comparator and output a value corresponding to a digital representation of an analog signal received at the input node.

8. A method of operating a current mode sigma-delta modulator, comprising:

inputting a voltage at an input node;

comparing the voltage of the input node to a reference voltage with a comparator, and outputting a comparison result; and

providing an output of the comparator to an integrating capacitor that is connected to an input of the comparator and the input node, by way of a switched capacitor circuit connected at a first end to the input node, the input of the comparator, and the integrating capacitor and at a second end to an output of the comparator,

wherein the switched capacitor circuit includes:

a switched capacitor having a first electrode and a second electrode, the second electrode connected to a predetermined voltage;

a first switch directly connected to the first electrode of the switched capacitor and the input node so as to selectively allow a charge to flow therebetween; and

a second switch connected between the predetermined voltage and the first electrode of the switched capacitor.

9. A current mode sigma-delta modulator, comprising:

an input node;

an transconductance amplifier configured to input a voltage of the input node and a reference voltage to output a signal;

a first integrating capacitor connected to an input of the transconductance amplifier;

a comparator configured to compare an output of the transconductance amplifier to the reference voltage and to output a comparison result;

a second integrating capacitor connected to an input of the comparator;

a first switched capacitor circuit connected at a first end to the input node, the input of the transconductance amplifier, and the first integrating capacitor, and connected at a second end to an output of the comparator; and

a second switched capacitor circuit connected at a first end to the output of the transconductance amplifier, the input of the comparator, and the second integrating capacitor, and connected at a second end to the output of the comparator,

wherein the first switched capacitor circuit includes:

a first switched capacitor having a first electrode and a second electrode, the second electrode being connected to a predetermined voltage,

a first switch directly connected to the first electrode of the first switched capacitor and the input node so as to selectively allow a charge to flow therebetween, and p 2 a second switch connected between the predetermined voltage and the first electrode of the first switched capacitor.

10. The current mode sigma-delta modulator according to claim 9 , wherein the second switched capacitor circuit comprises:

a second switched capacitor having a first electrode and a second electrode, the second electrode being connected to the predetermined voltage,

a third switch connected between the first electrode of the second switched capacitor and an output node of the transconductance amplifier, and

a fourth switch connected between the predetermined voltage and the first electrode of the second switched capacitor.

11. The current mode sigma-delta modulator according to claim 10 , wherein the first switch and the third switch are controlled by a signal corresponding to the output of the comparator.

12. The current mode sigma-delta modulator according to claim 10 , wherein the second switch and the fourth switch are controlled by a signal corresponding to an inverse of the output of the comparator.

13. The current mode sigma-delta modulator according to claim 10 , wherein the first switch is a first transistor, the second switch is a second transistor, the third switch is a third transistor, and the fourth switch is a fourth transistor.

14. The current mode sigma-delta modulator according to claim 10 , wherein

in a case where the output of the comparator is at a high level of two possible levels, the first and third switches are closed and the second and fourth switches are opened, and

in a case where the output of the comparator is at a low level of the two possible levels, the first and third switches are opened and the second and fourth switches are closed.

15. The current mode sigma-delta modulator according to claim 10 , further comprising a replica current source connected to the second integrating capacitor.

16. The current mode sigma-delta modulator according to claim 15 , wherein a current value of the replica current source is equal to a current value of the input node times a ratio of a capacitance of the second switched capacitor to a capacitance of the first switched capacitor.

17. The current mode sigma-delta modulator according to claim 9 , further comprising a replica current source connected to the second switched capacitor circuit.

18. A solid-state imaging device, comprising:

a pixel circuit configured to convert an incident light into an analog signal;

the current mode sigma-delta modulator according to claim 9 ; and

a vertical signal line configured to conduct the analog signal to the input node of the current mode sigma-delta modulator.

19. A solid-state imaging device, comprising:

a pixel circuit configured to convert an incident light into an analog signal;

the current mode sigma-delta modulator according to claim 9 ; and

a vertical signal line configured to conduct the analog signal to the input node of the current mode sigma-delta modulator.