Image readout circuit with capacitor that is shared between sample and hold and buffer

• US 8,610,046 B2
• Filed: 06/16/2011
• Issued: 12/17/2013
• Est. Priority Date: 06/16/2011
• Status: Active Grant

• Current Assignee: Forza Silicon Corporation (Ametek Incorporated)
• Original Assignee: Forza Silicon Corporation (Ametek Incorporated)
• Inventors: Van Blerkom, Daniel A.
• Primary Examiner(s): Luu, Thanh
• Assistant Examiner(s): Naphas, Renee
• Application Number:
  US13/162,220
• Publication Number:
  US 20120318963A1
• Time in Patent Office: 915 Days
• Field of Search: 250/214.P
• US Class Current: 250/214.P
• CPC Class Codes: H04N 25/75 Circuitry for providing, modifying or processing image signals from the pixel array
• CPC Class Codes: H04N 25/78 Readout circuits for addressed sensors, e.g. output amplifiers or A/D converters

Abstract

A switched capacitor sample and hold buffer that can be used, for example, in a CMOS image sensor. The switched capacitor sample and hold circuit has large output swings, low noise and reduced capacitor area compared to the traditional switched capacitor approach.

Claims

1. An image readout device, comprising:

a connection to at least one pixel output receiving a pixel output from an image sensor, said connection including a sample and hold capacitor which is used for sampling and holding the pixel output, said sample and hold capacitor including first and second terminals;

a buffer amplifier, connected to buffer and amplify said pixel output; and

a circuit reconfiguration element which reconfigures the connection such that the same sample and hold capacitor which is used for sampling and holding signals from the image sensor is reused as a part of said buffer amplifier by switching both the first and second terminals of the sample and hold capacitor into a feedback loop of the buffer amplifier.

2. An image readout device as in claim 1, wherein said sample and hold capacitor is used to sample and hold the pixel output, and is also switched into a feedback loop of the buffer amplifier.

3. A device as in claim 2, further comprising a turn on circuit, that connects the sample and hold capacitor into said feedback loop in different stages of turn on, with each stage having a defined voltage to current transfer function, to avoid losing charge from the sample and hold capacitor when said sample and hold capacitor is switched into said feedback loop.

4. The device as in claim 3, wherein said sample and hold capacitor is connected to a node of said buffer amplifier, and said turn on circuit is turned on in said stages that avoid the amplifier node being biased to a level below ground.

5. The device as in claim 3, wherein said turn on circuit allows only a specified amount of charge to be released from the capacitor per unit time.

6. The device as in claim 3, wherein said turn on circuit operates to match a specified slew time.

7. The device as in claim 6, further comprising an auxiliary circuit that simulates a slew time to find a simulated slew time, and using said simulated slew time to determine a speed of turn on a said turn on circuit.

8. The device as in claim 7, wherein said sample and hold and buffer amplifier are formed on a chip, and said auxiliary circuit is also formed on the chip.

9. A device as in claim 8, further comprising a plurality of additional readout connections, buffer amplifiers, and circuit reconfiguration elements, also formed on said chip.

10. The device as in claim 7, wherein said auxiliary circuit simulates a slew time which replicates the buffer amplifier slew time.

11. An image readout device, comprising:

a sample and hold capacitor which is used for sampling and holding a pixel output from a pixel of an image sensor, said sample and hold capacitor including first and second terminals;

a buffer amplifier, connected to buffer and amplify said pixel output; and

a switch which switches both said first and second terminals of the sample and hold capacitor between a first configuration where it samples and holds said pixel output and a second configuration where it is used in a feedback loop of the buffer amplifier.

12. An image readout device, comprising:

sample and hold capacitor is used for sampling and holding a pixel output from a pixel of an image sensor;

buffer amplifier, connected to buffer and amplify said pixel output; and

a switch which switches the sample and hold capacitor between a first configuration where it samples and holds said pixel output and a second configuration where it is used as a part of the buffer amplifier, wherein said sample and hold capacitor is used to sample and hold the pixel output, and is also switched into a feedback loop of the buffer amplifier.

13. A device as in claim 12, wherein said switch includes a turn on circuit, that gradually connects the sample and hold capacitor into said feedback loop in different stages of turn on, with each stage having a defined voltage to current transfer function to avoid losing charge from the sample and hold capacitor when said sample and hold capacitor is switched into said feedback loop.

14. The device as in claim 13, wherein said sample and hold capacitor is connected to a node of said buffer amplifier, and said turn on circuit is turned on in stages to avoid the amplifier node being biased to a level below ground.

15. The device as in claim 14, wherein said turn on circuit allows only a specified amount of charge to be released from the capacitor per unit time.

16. The device as in claim 14, wherein said turn on circuit operates to match a specified slew time.

17. The device as in claim 16, further comprising an auxiliary circuit that simulates a slew time a simulated slew time, and using said simulated slew time to determine a speed of turn on a said turn on circuit.

18. The device as in claim 17, wherein said sample and hold and buffer amplifier are formed on a chip, and said auxiliary circuit is also formed on the chip.

19. A device as in claim 18, further comprising a plurality of additional readout connections, buffer amplifiers, and circuit reconfiguration elements, also formed on said chip.

20. The device as in claim 17, wherein said auxiliary circuit simulates a slew time which replicates the buffer amplifier slew time.

21. An image readout method, comprising:

sampling and holding a pixel output from a pixel of an image sensor on to a sample and hold capacitor that has first and second terminals;

switching both said first and second terminals of the sample and hold capacitor, with charge thereon from said sampling and holding, into a feedback loop of a buffer amplifier to use the charge in said sample and hold capacitor in said buffer amplifier, to produce an output indicative of said pixel output.

22. A method as in claim 21, further comprising connecting the sample and hold capacitor into said circuit configuration of said buffer amplifier using a turn on circuit that avoids losing charge from the sample and hold capacitor when said sample and hold capacitor is connected into said circuit configuration.

23. The method as in claim 22, wherein said connecting allows only a specified amount of charge to be released from the capacitor per unit time.

24. The method as in claim 22, wherein said connecting operates to match a specified slew time.

25. The method as in claim 21, wherein said sample and hold capacitor is connected to a node of said buffer amplifier, and said connecting turns on in stages to avoid the amplifier node being biased to a level below ground.

26. An image readout method, comprising:

sampling and holding a pixel output from a pixel of an image sensor on to a sample and hold capacitor;

switching the sample and hold capacitor, with charge thereon from said sampling and holding, into a circuit configuration of a buffer amplifier to use the charge in said sample and hold capacitor in said buffer amplifier, to produce an output indicative of said pixel output; and

switching again said sample and hold capacitor, out of said circuit configuration of said buffer amplifier, to again receive a second pixel output indicative of different information from said image sensor, wherein said switching comprises switching into a feedback loop of the buffer amplifier.

27. An image readout method, comprising:

sampling and holding a pixel output from a pixel of an image sensor on to a sample and hold capacitor;

switching the sample and hold capacitor, with charge thereon from said sampling and holding, into a circuit configuration of a buffer amplifier to use the charge in said sample and hold capacitor in said buffer amplifier, to produce an output indicative of said pixel output; and

switching again said sample and hold capacitor, out of said circuit configuration of said buffer amplifier, to again receive a second pixel output indicative of different information from said image sensor, connecting the sample and hold capacitor into said circuit configuration of said buffer amplifier using a turn on circuit that avoids losing charge from the sample and hold capacitor when sample and hold capacitor is connected into said circuit configuration, wherein said slow turn on circuit operates at a specified slew time; and

using an auxiliary circuit form simulating a slew time that will avoid a node of said buffer amplifier being biased to a level below ground in to find a simulated slew time, and using said simulated slew time to determine a speed of turn on of said connecting.

28. The method as in claim 27, wherein said sample and hold and buffer amplifier are formed on a chip, and said auxiliary circuit is also formed on the chip.