Analog-to-digital conversion in image sensors

• US 8,305,474 B2
• Filed: 11/19/2009
• Issued: 11/06/2012
• Est. Priority Date: 03/21/2008
• Status: Active Grant

• Current Assignee: STMicroelectronics Limited (STMicroelectronics NV), STMicroelectronics SA (STMicroelectronics NV), STMicroelectronics SAS (STMicroelectronics NV)
• Original Assignee: STMicroelectronics Limited (STMicroelectronics NV), STMicroelectronics SA (STMicroelectronics NV), STMicroelectronics SAS (STMicroelectronics NV)
• Inventors: Purcell, Matthew, Storm, Graeme, Tolmie, Derek, Simony, Laurent, Qu, Min, El Hachimi, Mhamed
• Primary Examiner(s): Moe, Aung S
• Assistant Examiner(s): Chen, Chia-Wei A
• Application Number:
  US12/622,373
• Publication Number:
  US 20100157035A1
• Time in Patent Office: 1,083 Days
• Field of Search: 348294-324, 341126-172
• US Class Current: 348/301
• CPC Class Codes: H04N 25/617 for reducing electromagnetic interference, e.g. clocking noise
• CPC Class Codes: H04N 25/772 comprising A/D, V/T, V/F, I/T or I/F converters

Abstract

An image sensor has a per-column ADC arrangement including first and second capacitors allowing a comparator circuit to perform correlated double sampling. The capacitors are continuously connected to, respectively, the analog pixel signal and a ramp signal without use of a hold operation. The comparator circuit comprises a differential input being connected to the junction of the two capacitors and being biased by a reference signal. The reference signal is preferably sampled and held from a reference voltage. The use of a differential input as first stage of the comparator addresses problems arising from ground voltage bounce when a large pixel array images a scene with low contrast. Connectivity of the differential input stage allows the ramp signal to see a constant capacitive load thus reduce image artifacts referred to as smear.

Claims

1. A circuit, comprising:

an input circuit comprising a first input adapted to receive an analog signal from a pixel of a pixel array, the signal varying during a pixel readout period and having a first level during a first reset period and a second level during a second read period, a second input adapted to receive a time variant reference signal, and two coupling capacitors; and

a comparator circuit adapted to compare the time variant reference signal and the analog signal;

wherein the analog signal from the pixel and the time variant reference signal are constantly read onto one of the two coupling capacitors respectively of the input circuit during both the first reset period and the second read period; and

wherein the comparator circuit comprises a differential input adapted to combine the analog signal and the time variant reference signal, said differential input being biased by a fixed reference signal.

2. The circuit according to claim 1, in which the differential input is provided by a differential amplifier.

3. The circuit according to claim 2, in which the differential amplifier is a long tail pair.

4. The circuit according to claim 3 wherein the long tail pair comprises two main transistors, wherein a first of said main transistors is configured to receive the combination of the analog signal and the time variant reference signal and the fixed reference signal and wherein the output of the long tail pair is arranged to be taken from a second of said main transistors.

5. The circuit according to claim 2, in which an output of the differential amplifier is connected to an inverter via a further capacitor.

6. The circuit according to claim 1, further comprising a sample and hold circuit adapted to provide the fixed reference signal by sampling and holding a reference voltage, said sampling being performed during autozeroing of the pixel and comparator circuit.

7. The circuit according to claim 1 wherein the combination of the analog signal and the time variant reference signal is received at a first input of said differential input and the fixed reference signal is received at a second input of said differential input.

8. The circuit according to claim 1 wherein the combination of the analog signal and the time variant reference signal along with the fixed reference signal are received at a single input of said differential input.

9. The circuit according to claim 8 wherein said single input is a non-inverting input.

10. The circuit according to claim 1 wherein said differential input is comprised in a differential front end circuit comprising first and second transistor devices, and wherein the control electrode of the first transistor device provides said first input receiving the combination of the analog signal and the time variant reference signal, and the control electrode of the second transistor device provides said second input receiving a fixed reference signal, said devices being connected in series between a current source and ground.

11. The circuit according to claim 10 wherein said fixed reference signal is obtained by connecting said second input to a ground plane carrying no current, different from a main ground plane of the circuit.

12. The circuit according to claim 10 wherein the first transistor device comprises a NMOS transistor and the second transistor device comprises a PMOS transistor, the output of the differential front end circuit being taken from the first transistor'"'"'s drain electrode.

13. The circuit according to claim 1 wherein the comparator circuit operates to add the time variant reference signal and the analog signal and compare the sum with a threshold value.

14. The circuit according to claim 1 wherein the comparator circuit comprises a differential amplifier circuit and an output circuit, the output circuit comprising an output capacitor.

15. The circuit according to claim 14, wherein the differential amplifier circuit has a first selectively connectable feedback loop and the output circuit further comprises an output inverter and a second selectively connectable feedback loop such that the selective connection of the first and second feedback loops allows for the sampling of offset voltages resultant from thermal noise from the pixel, differential amplifier circuit and output circuit such that the analog signal can be appropriately corrected.

16. The circuit according to claim 15, wherein said selective connection of the first and second feedback loops results in an input offset of said output circuit being sampled onto a first plate of the output capacitor and thermal noise from the differential output circuit being sampled onto a second plate of said output capacitor.

17. An circuit according to claim 15, wherein said selective connection of the second feedback loop results in thermal noise from the output circuit being sampled onto the output capacitor.

18. A device comprising:

an image sensor array including pixel element;

an input circuit comprising a first input adapted to receive an analog signal from the pixel element, the signal varying during a pixel readout period and having a first level during a first reset period and a second level during a second read period, a second input adapted to receive a time variant reference signal, and two coupling capacitors; and

a comparator circuit adapted to compare the time variant reference signal and the analog signal;

wherein the analog signal from the pixel and the time variant reference signal are constantly read onto one of the two coupling capacitors respectively of the input circuit during both the first reset period and the second read period;

and wherein the comparator circuit comprises a differential input adapted to combine the analog signal and the time variant reference signal, said differential input being biased by a fixed reference signal.

19. The device as claimed in claim 18, wherein the device is a mobile phone having a camera including the image sensor array.

20. The device as claimed in claim 18, wherein the device is a personal digital assistant having a camera including the image sensor array.

21. The device as claimed in claim 18, wherein the device is a digital camera.

22. The device as claimed in claim 18, wherein the device is an optical pointing device such as an optical mouse.

23. The device as claimed in claim 18, wherein the device is an endoscope apparatus including the image sensor array.

24. The device as claimed in claim 18, wherein the device is an ambient light sensor.

25. A method of converting an analog signal from an image sensor pixel, during a pixel readout period to a digital signal using an analog to digital converter circuit comprising an input circuit having first and second coupling capacitors, a time varying reference circuit enabled to provide a time varying reference signal, the first coupling capacitor being continuously connected to the time varying reference circuit and the second coupling capacitor being continuously connected to the pixel during the pixel readout period, the method comprising:

(i) resetting the pixel;

(ii) removing the pixel from reset, wherein the analog signal contains the pixel reset noise on the second capacitor;

(iii) integrating light received on the pixel, wherein the analog signal is reduced according to the amount of light received;

(iv) providing the time varying reference signal;

(v) comparing the analog signal with the time varying reference signal; and

(vi) outputting an output signal when the time variant reference signal corresponds to the analog signal;

wherein;

said comparing is effectuated by applying the combination of the analog signal and the time variant reference signal to a differential front end circuit, said differential front end circuit being biased by a fixed reference signal.

26. The method according to claim 25, wherein applying the fixed reference signal comprises sampling and holding a reference voltage.

27. The method according to claim 26, wherein said sampling is performed during autozeroing of the pixel array.

28. The method according to claim 25, in which the differential front end circuit comprises a differential amplifier.

29. The method according to claim 25 wherein the combination of the analog signal and the time variant reference signal and the fixed reference signal is fed to a single input of said differential input.

30. The method according to claim 25 wherein said differential front end circuit comprises first and second transistor devices and wherein the combination of the analog signal and the time variant reference signal is fed to the control electrode of the first transistor device, and the fixed reference signal is fed to the control electrode of the second transistor device, said devices being connected in series between a current source and ground.

31. The method according to claim 30 wherein said fixed reference signal is obtained by connecting said second input to a ground plane carrying no current, different from the circuit'"'"'s main ground plane.

32. The method according to claim 30 wherein the first transistor device comprises a NMOS transistor and the second transistor device comprises a PMOS transistor, the output of the differential front end circuit being taken from the first transistor'"'"'s drain electrode.

33. The method according to claim 25 wherein the comparing step comprises adding the time variant reference signal and the analog signal and comparing the sum with a threshold value.

34. The method according to claim 25, wherein, if step (v) causes one or more associated offset voltages, the method further comprises sampling said one or more associated offset voltages such that the analog signal is appropriately corrected.

35. A circuit, comprising:

a pixel element adapted to output a voltage signal;

a first capacitor connected to receive only the voltage signal;

a second capacitor connected to receive only a ramping reference voltage signal; and

a differential comparator adapted to receive a summed output from the first and second capacitors,

said differential comparator biased by a fixed reference voltage.

36. The circuit of claim 35 further comprising a sample and hold circuit adapted to generate the fixed reference voltage.

37. The circuit of claim 35 further comprising:

a third capacitor connected to receive a signal output from the differential comparator;

an inverter circuit having an input connected to receive an output from the third capacitor; and

a feedback switching circuit connected between the input of the inverter circuit and an output of the inverter circuit.

38. The circuit of claim 35 further comprising a memory adapted to store a data signal value output from the inverter circuit.