Digital camera and image sensor readout method with black level calibration

• US 8,749,670 B1
• Filed: 02/27/2013
• Issued: 06/10/2014
• Est. Priority Date: 02/27/2013
• Status: Active Grant

• Current Assignee: Himax Imaging Limited (Himax Technologies Incorporated)
• Original Assignee: Himax Imaging Limited (Himax Technologies Incorporated)
• Inventors: Liu, Chin-Min, Chen, Ching-Fong
• Primary Examiner(s): VU, NGOC YEN T
• Application Number:
  US13/778,899
• Time in Patent Office: 468 Days
• Field of Search: 348/241, 348/243
• US Class Current: 348/243
• CPC Class Codes: H04N 25/63 applied to dark current

Abstract

A digital camera and an image sensor readout method with black level calibration are disclosed. In a readout circuit of the digital camera, a black reference for black level calibration and an analog-to-digital conversion reference voltage for analog-to-digital conversion are generated from reference voltages provided by one voltage ladder. Device-to-device variations of sensor characteristics are considered when selecting the reference voltage for generating the black reference, and, a gain between the selected reference voltage and the black reference varies with a gain of a programmable gain amplifier coupled after a compensator. The compensator subtracts the black reference from a sensed signal that the readout circuit receives from an image sensor of the digital camera.

Claims

1. A digital camera, comprising

an image sensor comprising an array of pixel sensors;

a readout circuit, coupled to the image sensor to receive a sensed signal and comprising;

a compensator, subtracting a black reference from the sensed signal to generate a first analog signal;

a programmable gain amplifier, amplifying the first analog signal to generate a second analog signal;

an analog-to-digital converter, controlled by an analog-to-digital conversion reference voltage to convert the second analog signal to a first digital signal;

a digital black level calibration unit, generating black level calibrated digital data in different color spaces based on the first digital signal; and

a feed-forward control design comprising;

a voltage ladder, generating reference voltages of different levels;

a first gain stage, amplifying a specific one of the reference voltages to generate the analog-to-digital conversion reference voltage;

a multiplexer, receiving the reference voltages generated by the voltage ladder and outputting one of the received reference voltages according to sensor characteristics of the image sensor; and

a second gain stage, amplifying the reference voltage output from the multiplexer to generate the black reference, wherein a gain of the second gain stage varies with a gain of the programmable gain amplifier.

2. The digital camera as claimed in claim 1, wherein the first and second gain stages are switched-capacitor gain stages synchronized with each other.

3. The digital camera as claimed in claim 1, wherein the digital black level calibration unit comprises:

a digital gain stage, amplifying the first digital signal by a digital gain to generate a second digital signal; and

a digital black level calibrator, transforming the second digital signal to the black level calibrated digital data,

wherein the digital gain provided within the digital gain stage is compensated for by the digital black level calibrator.

4. The digital camera as claimed in claim 1, having a single-ended circuit configuration.

5. The digital camera as claimed in claim 1, having a fully differential circuit configuration.

6. The digital camera as claimed in claim 1, wherein the pixel sensors within a dark region of the array are shielded from light.

7. The digital camera as claimed in claim 6, further comprising:

an offset calibrator, optimizing a selection signal of the multiplexer based on the first digital signal when the sensed signal is from the pixel sensors within the dark region.

8. The digital camera as claimed in claim 7, wherein the multiplexer is controlled by the optimized selection signal when the sensed signal is from the pixel sensors beyond the dark region.

9. An image sensor readout method with black level calibration for a digital camera, comprising:

receiving a sensed signal from the image sensor which comprises an array of pixel sensors;

subtracting a black reference from the sensed signal to generate a first analog signal;

amplifying the first analog signal to generate a second analog signal;

converting the second analog signal to a first digital signal in accordance with an analog-to-digital conversion reference voltage; and

generating black level calibrated digital data in different color spaces based on the first digital signal,

wherein;

the analog-to-digital conversion reference voltage is generated by amplifying a specific one of a plurality of reference voltages generated by a voltage ladder; and

the black reference is generated by amplifying one of the plurality of reference voltages selected according to sensor characteristics of the image sensor, wherein a gain between the selected reference voltage and the black reference varies with a gain between the first and second analog signals.

10. The image sensor readout method as claimed in claim 9, further providing switched-capacitor gain stages, synchronized with each other, to implement the generation of the analog-to-digital conversion reference voltage and the black reference.

11. The image sensor readout method as claimed in claim 9, wherein a scheme for generating the black level calibrated digital data comprises:

amplifying the first digital signal by a digital gain to generate a second digital signal; and

transforming the second digital signal to the black level calibrated digital data,

wherein the digital gain between the first and the second digital signals is compensated for when transforming the second digital signal to the black level calibrated digital data.

12. The image sensor readout method as claimed in claim 9, wherein the reference voltage selected for generating the black reference is determined based on the first digital signal when the sensed signal is from the pixel sensors within a dark region of the array, and, the pixel sensors within the dark region are shielded from light.