Active pixel sensor with current mode readout

• US 6,194,696 B1
• Filed: 03/10/1999
• Issued: 02/27/2001
• Est. Priority Date: 03/10/1998
• Status: Expired due to Term

• Current Assignee: Aptina Imaging Corporation (ON Semiconductor Corporation)
• Original Assignee: Photobit Corporation (Micron Technology, Inc.)
• Inventors: Krymski, Alexander, Fossum, Eric R., Mansoorian, Barmak
• Primary Examiner(s): MACK, RICKY LEVERN
• Application Number:
  US09/265,936
• Time in Patent Office: 720 Days
• Field of Search: 250/208.1, 348/294, 348/300, 348/301, 348/302, 348/308, 348/241, 348/250, 348/303, 348/304, 348/307, 348/309, 348/310, 348/315
• US Class Current: 250/208.1
• CPC Class Codes: H04N 25/616 involving a correlated sampling function, e.g. correlated double sampling [CDS] or triple sampling
• CPC Class Codes: H04N 25/75 Circuitry for providing, modifying or processing image signals from the pixel array
• CPC Class Codes: H04N 3/155 Control of the image-sensor operation, e.g. image processing within the image-sensor

Abstract

A current mode device for an active pixel sensor includes a voltage to current converter which includes an operation that occurs inside a source follower. The device operates to draw no current while the current is incoming. The device uses two stages of double sampling—one before the current-mode op amp and one after.

Claims

1. An image sensor device, comprising:

a plurality of image sensor elements, each element detecting a pixel of an overall image, said image sensor elements arranged in groups such that a group of said pixels can be output at one time to produce first pixel outputs;

a first circuit element, which changes said first pixel outputs to a current that is indicative of the first pixel output and adjusts said current to remove at least one offset of associated circuitry used in obtaining the first pixel output, to produce a first corrected pixel output; and

a common bus, selectively connected to a plurality of said pixels; and

a second circuit element which prevents current from flowing during at least a portion of a time in which said pixel outputs are being obtained.

2. A device as in claim 1, further comprising an operational amplifier, with a high impedance current mode input maintained at a substantially constant voltage, connected to said common bus.

3. A device as in claim 2, further comprising a current source, feeding a bias current to one input of said operational amplifier, said bias current selected to correspond to an average current flow.

4. A device as in claim 1, wherein said second circuit element is a select transistor, which turns on to select one of a plurality of groups to be connected to said bus at any one time.

5. A device as in claim 4, further comprising a second correlated double sampling element, which removes at least one additional offset other than said at least one offset.

6. A device as in claim 5, wherein said at least one offset includes at least signal minus reset, and said at least one additional offset includes at least an offset of said transistor.

7. A device as in claim 1, wherein said first circuit element comprises a source follower with a gate connected to receive an input voltage, a first output leg having a resistor connected to ground and a second output leg producing an output current indicative of the input voltage.

8. A device as in claim 1, wherein said pixels are arranged in an array, and said groups are columns of the array.

9. An image sensor system, comprising:

an image sensor element which produces an image sensor output indicative of incoming light;

a converter, connected to receive said image sensor output, said converter including a transistor having an input connected to receive said image sensor output, an output leg having a resistor connected to produce a current flow proportional to said voltage, and another output voltage producing a current proportional to said voltage, thereby producing an output current proportional to said voltage.

10. A system as in claim 9, wherein said converter is a source follower, further comprising a group select switch, coupled to the output of the source follower, and turning on to enable current flow therethrough.

11. A system as in claim 9, further comprising a plurality of said converters, each connected to a separate image sensor element, and each having a group select switch, all of said group select switches of all of said converters being connected together at one end to form a bus.

12. A system as in claim 11, further comprising a current mode operational amplifier, connected to said bus.

13. A system as in claim 12, further comprising a current source, connected to said operational amplifier, and biasing the operational amplifier to a specified operation point which enhances dynamic range of the signals.

14. An active pixel sensor device, comprising:

a plurality of pixel elements, each pixel element including a photoreceptor and at least one buffer, and producing an output indicating a value representing an amount of charge accumulated by the photoreceptor; and

a sensing circuit, connected to receive said signal, and operating such that no current flows during at least a portion of a time when charge is being read out from said photoreceptor.

15. A device as in claim 14, further comprising a correlated double sampling circuit, using a capacitor element to determine a difference between signal level and reset level of said photoreceptor.

16. A device as in claim 15, wherein said converter device comprises a source follower, receiving said value at its input, and a second electronic switch, in series with an output leg of said source follower, selectively actuated such that no current flows through said source follower until said second electronic switch is actuated.

17. A device as in claim 16, further comprising a controller, controlling a select signal to said second electronic switch, and controlling a time of acquiring said signal, said select signal provided after a time of acquiring said signal.

18. A device as in claim 14, further comprising a sample and hold switch, selectively actuated to allow a signal from an associated photoreceptor to pass.

19. A device as in claim 18, further comprising a source follower, which does not conduct current at its gate until actuated and a selective actuator, connected to an output leg of said source follower, said selective actuator controlled to conduct during a different time than a time of conduction of said sample and hold circuit.

20. A device as in claim 14, wherein said sensing circuit includes a buffer circuit, receiving said signal at its input, and outputting said signal to its output.

21. A device as in claim 14, wherein said sensing circuit includes a source follower which converts a voltage input to a current output.

22. A device as in claim 21, wherein said source follower includes a transistor, receiving said voltage at its input gate, and including a resistor, said transistor having an output leg connected to ground via said resistor.

23. A device as in claim 14, further comprising a first correlated double sampling circuit which removes offsets of the transistors thereby providing an output proportional to signal minus reset.

24. A device as in claim 23, further comprising a current mode amplifier device, receiving an output from said first correlated double sampling circuit.

25. A device as in claim 24, further comprising a second correlated double sampling circuit, coupled to an output of said current mode amplifier device, producing an output which compensates for offsets of said current mode amplifier device.

26. A device as in claim 25, wherein said current mode amplifier device is an operational amplifier.

27. A method of operating an image sensor, comprising:

attaching a plurality of image sensor outputs to a common bus;

driving said outputs into a charge storage device, which stores a signal indicative of the charge from said image sensor outputs, during a time when no current is flowing; and

after storing the charge, allowing current to flow, to thereby obtain an output indicative of said image sensor outputs.

28. A method as in claim 27, further comprising carrying out a first correlated double sampling on the input pixel information, to remove buffer offsets therefrom, and to provide a pure output indicative of signal minus reset.

29. A method as in claim 28, further comprising, when current flows, converting the first correlated double sampling signal to a current.

30. A method as in claim 28, further comprising amplifying the current indicative of the first correlated double sampling signal.

31. A method as in claim 29, further comprising amplifying the first correlated double sampling signal.

32. A method as in claim 30, further comprising carrying out a second correlated double sampling, to remove offsets caused by said amplifying.

33. A method as in claim 31, further comprising carrying out a second correlated double sampling in a current mode, to remove offsets caused by said amplifying.

34. A method as in claim 27, further comprising

converting, when current flows, the output to a current; and

amplifying said current to produce an amplified current output.

35. A method as in claim 34, further comprising converting said amplified current back to a voltage to output a voltage output signal.

36. A method as in claim 27, further comprising biasing the current mode device to an amount of an average dynamic range of the signals.

37. A method as in claim 28, wherein said first correlated double sampling by using two capacitors connected to a node receiving said signal, and wherein a first of said capacitors is connected to ground, and a second of said capacitors is connected in series with said node.

38. A double sampling pixel sensor device, comprising:

a plurality of pixel elements producing an output indicating a value representing an amount of charge accumulated by the photoreceptor at a signal node;

a first double sampling element, including two capacitors connected to said signal node, one connected to ground, and a difference between signal and reset levels as a first double sampled result;

a clamp circuit, connected to said second node, and selectively producing a reference voltage on said second node;

a buffer circuit, passing said first double sampled result;

a second double sampling element, connected to an output of said clamp circuit, and operating to remove at least said reference voltage from said first double sampled result, and to remove at least one offset from said buffer circuit;

a sensing circuit, connected to receive said signal, and operating such that no current flows during a least a portion of a time when charge is being read out from said photoreceptor.