ANALOG TO DIGITAL CONVERTER CIRCUIT WITH OFFSET REDUCTION AND IMAGE SENSOR USING THE SAME

• US 20070257720A1
• Filed: 07/13/2007
• Published: 11/08/2007
• Est. Priority Date: 12/16/2005
• Status: Active Grant

• Current Assignee: Alexander Krymski
• Original Assignee: Alexander Krymski
• Inventors: Krymski, Alexander
• Granted Patent: US 7,400,279 B2
• Time in Patent Office: Days
• US Class Current: 327/306
• CPC Class Codes: H03M 1/0607 Offset or drift compensation removal of offset already present on the analogue input signal H03M1/1295

Abstract

An image sensor may be improved by using ADCs that compensate for the effect of comparator input offset on comparator decisions. Offset compensation may be implemented in an ADC by using an amplifier section between the input of the ADC and a comparator section of the ADC to amplify the signals supplied to the comparator inputs and thereby reduce the effect of comparator offset on the comparator decision. The comparator section may be an autozeroing comparator section that is capable of performing an offset reduction operation to store offset compensation values at capacitors provided at its inputs. The amplifier section may be an autozeroing amplifier section having one or more amplifier stages that are capable of performing an offset reduction operation to store offset compensation values at capacitors provided at their inputs. Offset compensation may also be implemented using an autozeroing comparator section without a preceding amplifier section. Related methods of operation cause the circuits to perform the offset reduction operations.

Claims

1. A circuit, comprising:

a comparator comprising a plurality of input nodes;

said comparator configured to perform an offset reduction operation to compensate for offset between the plurality of input nodes; and

said comparator configured to perform a decision operation using positive feedback to decide which of the plurality of input nodes is being provided with a highest input voltage.

2. The circuit of claim 1,

said comparator further comprising a plurality of output nodes;

and

said comparator configured to perform a preset operation in which a potential from a voltage source is applied to the plurality of output nodes.

3. The circuit of claim 2, said comparator further comprising:

two PMOS preset transistors, each PMOS preset transistor of the two PMOS preset transistors coupled between a corresponding output node of the plurality of output nodes and the voltage source.

4. The circuit of claim 3,

wherein, during said preset operation, a latch control signal with a LO level is applied to a gate of each of the two PMOS preset transistors.

5. The circuit of claim 1, said comparator further comprising:

a plurality of output nodes;

two NMOS reset transistors, each NMOS reset transistor coupled between a respective output node of the plurality of output nodes and a corresponding input node of the plurality of input nodes;

two PMOS load transistors, each of the two PMOS load transistors coupled between a corresponding output node of the plurality of output nodes and a voltage source; and

two PMOS autozero enable transistors, a gate of each PMOS load transistor of the two PMOS load transistors coupled to a drain of the PMOS load transistor through a respective PMOS autozero enable transistor of the two PMOS autozero enable transistors.

6. The circuit of claim 5,

wherein, during said offset reduction operation, an autozero control signal with a LO level is applied to a gate of each of the two PMOS autozero enable transistors and a reset signal is applied to a gate of each of the two NMOS reset transistors.

7. The circuit of claim 1, said comparator further comprising:

a first output node;

a second output node;

a first PMOS load transistor coupled between a voltage source and the first output node;

a second PMOS load transistor coupled between the voltage source and the second output node;

a first PMOS comparator enable transistor coupled between the first output node and a gate of the second PMOS load transistor; and

a second PMOS comparator enable transistor coupled between the second output node and a gate of the first PMOS load transistor.

8. The circuit of claim 7,

wherein, during said decision operation, a comparator control signal with a LO level is applied to a gate of the first PMOS comparator enable transistor and to a gate of the second PMOS comparator enable transistor.

9. The circuit of claim 1,

wherein the circuit is an analog to digital converter circuit.

10. A method, comprising:

performing, by a comparator having a plurality of input nodes, an offset reduction operation to compensate for offset between the plurality of input nodes; and

performing, by the comparator, a decision operation using positive feedback to decide which of the plurality of input nodes is being provided with a highest input voltage.

11. The method of claim 10, further comprising:

performing, by the comparator, a preset operation in which a potential from a voltage source is applied to a plurality of output nodes of the comparator.

12. The method of claim 11, said performing, by the comparator, the preset operation, comprising:

causing the plurality of output nodes to be charged to a particular voltage level.

13. The method of claim 11,

wherein the offset reduction operation is performed before the preset operation is performed;

and

wherein the preset operation is performed before the decision operation is performed.

14. The method of claim 11, said performing, by the comparator having the plurality of input nodes, the offset reduction operation, comprising:

enabling a current to flow between each input node of the plurality of input nodes and a corresponding output node of the plurality of output nodes.

15. The method of claim 11, said performing, by the comparator, the decision operation using positive feedback, comprising:

controlling current flow from the voltage source to each output node of the plurality of output nodes based on a value at a corresponding opposing output node of the plurality of output nodes.

16. A circuit, comprising:

comparator means for performing an offset reduction operation to compensate for offset between a plurality of input nodes of the comparator means, and for performing a decision operation using positive feedback to decide which of the plurality of input nodes is being provided with a highest input voltage.

17. The circuit of claim 16,

said comparator means for performing a preset operation in which a potential from a voltage source is applied to a plurality of output nodes of the comparator means.

18. The circuit of claim 17,

said comparator means including preset means for causing the plurality of output nodes to be charged to a particular voltage level during the preset operation.

19. The circuit of claim 17,

said comparator means including reset means for enabling a current to flow between each input node of the plurality of input nodes and a corresponding output node of the plurality of output nodes during the offset reduction operation.

20. The circuit of claim 17,

said comparator means including comparator enable means for controlling current flow from the voltage source to each output node of the plurality of output nodes based on a value at a corresponding opposing output node of the plurality of output nodes during the decision operation.