Analog-to-digital conversion in image sensors using a differential comparison

• US 8,134,623 B2
• Filed: 03/11/2009
• Issued: 03/13/2012
• Est. Priority Date: 03/21/2008
• Status: Active Grant

• Current Assignee: STMicroelectronics Limited (STMicroelectronics NV)
• Original Assignee: STMicroelectronics Limited (STMicroelectronics NV)
• Inventors: Purcell, Matthew, Storm, Graeme, Qu, Min
• Primary Examiner(s): Ye, Lin
• Assistant Examiner(s): CHEN, CHIA WEI A
• Application Number:
  US12/402,397
• Publication Number:
  US 20090237536A1
• Time in Patent Office: 1,098 Days
• Field of Search: 348/245, 348/294, 348/300, 348/301
• US Class Current: 348/301
• CPC Class Codes: H04N 25/616 involving a correlated sampling function, e.g. correlated double sampling [CDS] or triple sampling
• CPC Class Codes: H04N 25/617 for reducing electromagnetic interference, e.g. clocking noise
• CPC Class Codes: H04N 25/75 Circuitry for providing, modifying or processing image signals from the pixel array
• 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 for correlated double sampling, and a comparator circuit. 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 amplifier having one input connected to the junction of the two capacitors and another input connected to a reference signal. The reference signal is preferably sampled and held from a reference voltage. The use of a differential amplifier as first stage of the comparator addresses problems arising from ground voltage bounce when a large pixel array images a scene with low contrast.

Claims

1. A circuit, comprising:

a correlated double sample circuit comprising a first input for receiving 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 for receiving a time variant reference signal, and two sample capacitors; and

a comparator circuit that compares 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 sample capacitors respectively of the correlated double sample circuit during both the first reset period and the second read period; and

wherein the comparator circuit comprises a differential amplifier having one input receiving the combination of the analog pixel signal and the time variant reference signal, and a second input receiving a fixed reference signal.

2. The circuit according to claim 1, in which the differential amplifier is a long tail pair.

3. The circuit according to claim 1, in which an output of the differential amplifier is connected to an inverter via a further capacitor.

4. The circuit according to claim 1, in which the fixed reference signal is provided by sampling and holding a reference voltage, said sampling being performed during autozeroing of the pixel array.

5. A device comprising:

an image sensor array including pixel element;

a correlated double sample circuit comprising a first input for receiving 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 for receiving a time variant reference signal, and two sample capacitors; and

a comparator circuit that compares 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 sample capacitors respectively of the correlated double sample circuit during both the first reset period and the second read period; and

wherein the comparator circuit comprises a differential amplifier having one input receiving the combination of the analog pixel signal and the time variant reference signal, and a second input receiving a fixed reference signal.

6. The device as claimed in claim 5, wherein the device is a mobile phone having a camera including the image sensor array.

7. The device as claimed in claim 5, wherein the device is a personal digital assistant having a camera including the image sensor array.

8. The device as claimed in claim 5, wherein the device is a digital camera.

9. The device as claimed in claim 5, wherein the device is an optical pointing device such as an optical mouse.

10. The device as claimed in claim 5, wherein the device is an endoscope apparatus including the image sensor array.

11. The device as claimed in claim 5, wherein the device is an ambient light sensor.

12. 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 a correlated double sample circuit having first and second sample capacitors, a time varying reference circuit enabled to provide a time varying reference signal, the first sample capacitor being continuously connected to the time varying reference circuit and the second sample capacitor being continuously connected to the pixel during the pixel readout period, the method comprising:

(i) resetting the pixel, thereby sampling an analog signal equivalent to a pixel reset voltage onto the second capacitor;

(ii) removing the pixel from reset, wherein the analog signal is corrected for 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 pixel signal and the time variant reference signal to one input of a differential amplifier, and applying a fixed reference signal to the other input of the differential amplifier.

13. The method according to claim 12, wherein applying the fixed reference signal comprises sampling and holding a reference voltage.

14. The method according to claim 13, wherein said sampling is performed during autozeroing of the pixel array.

15. A circuit, comprising:

a pixel element configured to output a voltage signal;

a first capacitor connected to receive the voltage signal;

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

a differential comparator having a first input connected to receive a summed output from the first and second capacitors and having a second input connected to receive for receiving a fixed reference voltage.

16. The circuit of claim 15 further comprising a sample and hold circuit for generating the fixed reference voltage.

17. The circuit of claim 15 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.

18. The circuit of claim 15 further comprising a memory for storing a data signal value output from the inverter circuit.