TWI386051B - Analog dark average circuit and method for an image sensor - Google Patents

Description

Analog black level averaging circuit and method

The present invention relates to black level (or dark level) correction of image sensors, and more particularly to analog black level averaging circuits and methods for image sensors.

Semiconductor image sensors, such as charge coupled devices (CCDs) or complementary metal oxide semiconductor (CMOS) sensors, are commonly used in cameras or cameras to convert visible light images into electrical signals for subsequent storage and transmission. Or display.

Due to the imperfect nature of the electronic circuitry, the image sensor will still have leakage current (or dark signal) without receiving any light. This harmful dark signal is combined with a useful data signal, and worse, this dark signal cannot be distinguished from the data signal. This combined dark signal sacrifices the dynamic range of the image and reduces image contrast, thus reducing image quality. To avoid or correct this dark signal, a black level calibration (BLC) is usually applied. In the black-order calibration procedure, it is necessary to collect one or more dark-lighted pixels as a black level reference for subtraction from the integrated signal, thereby improving image quality.

Traditional image sensors typically use digital-based black-order correction (BLC). The first figure shows a conventional digital black level correction system. The photomasked pixel signal is amplified by a programmable gain amplifier (PGA) 11 and converted into a digital format by an analog to digital converter (ADC) 12. The digital output of analog to digital converter (ADC) 12 is averaged via a digital circuit, such as a digital signal processor (DSP) 14. This average output is then converted to an analog format by a digital to analog converter (DAC) 16 to compensate for the programmable gain amplifier (PGA) 11. In the conventional digital black-order correction system, it is necessary to obtain the statistical value of the black level before the black level is fed back for correction. Since the acquisition of the black-order statistical value in the digital domain takes a considerable amount of time, it causes a limitation of the frame rate and a decrease in the video playback rate. Furthermore, if the image sensor has a fault pixel (such as a hot pixel), the performance of the digital black level correction will become worse.

Since the conventional black-order correction method (especially the digital black-order correction) cannot quickly correct the black level of the image sensor, it is urgent to propose a novel mechanism for obtaining the black-order value quickly and efficiently for the black-order correction.

In view of the above, one of the objects of the present invention is to provide an analogous (domain) black level averaging circuit and method for quickly and efficiently acquiring black level values for black level correction. Another object of the present invention is to reduce the impact of faulty pixel points on black level correction.

According to one of the features of the present invention, each circuit of the black level averaging circuit inputs a signal from a pixel circuit of the image sensor. Each circuit contains an associated double sampling (CDS) circuit. The capacitances of the secondary circuits are coupled together by an average switch, thereby averaging the reset signals and image signals between the black-order pixels.

According to another feature of the invention, the reset signal of the black level pixel is sampled and held, and the image signal of the black level pixel is sampled and held. Next, the reset signals held between the black-order pixels are averaged, and the image signals held between the black-order pixels are averaged.

The second figure shows an analog black level averaging circuit 10 of an image sensor of an embodiment of the present invention. For the sake of simplicity, the transistors (or switches) and their control signals in the figures use the same symbols. The third A diagram shows the timing diagram of the correlation signal of the second diagram, and the third B diagram shows the steps of the analog black scale correction method of the embodiment of the present invention. The analog black level averaging circuit 10 of the present embodiment can be applied to a digital image processing device such as, but not limited to, a camera or a video camera. The image sensor can be, but is not limited to, a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) sensor for converting an image of visible light into an electrical signal.

The analog black level averaging circuit 10 includes a plurality of sub-circuits 100 (typically three sub-circuits are shown in the figure), and each time the circuit 100 inputs a signal from a pixel circuit of the image sensor. Typically, the signal is first amplified by a column amplifier (CA) before entering the black level averaging circuit 10. The output of the black-order averaging circuit 10 is typically amplified by an amplifier, such as a programmable gain amplifier (PGA), and processed by a digital circuit, such as an analog-to-digital converter (ADC) or a digital signal processor. Since the signal is processed in the analog domain in the black level averaging circuit 10, the circuit 10 is also referred to as an "analog" black level averaging circuit.

In this embodiment, each circuit 100 includes a correlated double sampling (CDS) circuit including a sample-and-hold-reset_signal (SHR) switch controlled by The SHR control signal; and a sample-and-hold-image_signal (SHS) switch controlled by the SHS control signal. The SHR switch and the SHS switch are respectively coupled to the SHR capacitor C _SHR and the SHS capacitor C _SHS . The SHR capacitance C _SHR of all (or part of) sub-circuits 100 of the analog black level averaging circuit 10 is electrically coupled together by an average switch DARK_AVG, wherein the average switch DARK_AVG is controlled by the black level average control signal DARK_AVG. That is, the SHR switch in all of the secondary circuits 100 and the connecting nodes of the SHR capacitor C _{SHR are} in communication with each other. Thereby, when the average switches DARK_AVG are closed, the SHR capacitors C _{SHR of} all (or part of) the secondary circuits 100 are electrically coupled together. In the present specification, the term "electrically coupled" means that nodes or elements are directly connected together by wires or indirectly via other electronic components. Similarly, the SHS capacitor C _SHS of all (or part of) the sub-circuit 100 of the analog black level averaging circuit 10 is electrically coupled together by the average switch DARK_AVG, wherein the average switch DARK_AVG is controlled by the black level average Control signal DARK_AVG. That is, the contacts of the SHS switch and the SHS capacitor C _SHS in all of the secondary circuits 100 are in communication with each other. Thereby, when the average switches DARK_AVG are closed, the SHR capacitors C _{SHR of} all (or part of) the secondary circuits 100 are electrically coupled together.

During the operation of the black level correction, when in the reset period (for example, t1 to t2 of the third A picture), the SHR switch is closed, so that the reset signal of the light-shielding pixel is sampled and held at the SHR capacitance C _SHR ( Step 31). While in the integration period (e.g., t2 to t3 of the third A-picture), the SHS switch is closed, so that the image signal of the light-shielding pixel is sampled and held at the SHS capacitor C _SHS (step 32). In practice, the active SHR signal and the active SHS signal will overlap between t1 and t2 in order to prevent the coupling effect. In another embodiment, the SHS open relationship between t1 and t2 is open. After the reset and integration are completed, the black level average control signal DARK_AVG will close all of the average switches DARK_AVG (step 33), for example between t4 and t5 of the third A picture. Thereby, the SHR capacitors C _{SHR of} all the sub-circuits 100 are electrically coupled together, thereby averaging the black-order pixel reset signals stored in the SHR capacitor C _SHR . In a similar situation, the SHS capacitors C _{SHS of} all the sub-circuits 100 are electrically coupled together, thereby averaging the black-order pixel image signals stored in the SHS capacitor C _SHS .

According to the embodiment of the present invention, in the analogy field, the average black level value can be obtained among several pulse period, which is much faster than the digital field. Moreover, by analogous black level averaging circuit 10 and method, the effect of fault pixels of the image sensor (eg, hot pixels) on black level correction can be minimized.

The above description is only the preferred embodiment of the present invention, and is not intended to limit the scope of the present invention; all other equivalent changes or modifications which are not departing from the spirit of the invention should be included in the following Within the scope of the patent application.

10. . . Analog black level averaging circuit

11. . . Programmable gain amplifier (PGA)

12. . . Analog to digital converter (ADC)

14. . . Digital circuit

16. . . Digital to analog converter (DAC)

31-33. . . Black level averaging step

100. . . Average circuit

SHR. . . Sample-and-hold-reset signal (SHR) switch

C _SHR . . . SHR capacitor

SHS. . . Sample-and-hold-image signal (SHS) switch

C _SHS . . . SHS capacitor

DARK_AVG. . . Average switch

T1-t5. . . Timing point of timing signal

The first figure shows a conventional digital black level correction system.

The second figure shows an analog black level averaging circuit of the image sensor of the embodiment of the present invention.

The third A diagram shows the timing diagram of the correlation signal of the second figure.

The third B diagram shows the steps of the analog black level correction method of the embodiment of the present invention.

10. . . Analog black level averaging circuit

100. . . Average circuit

SHR. . . Sample-and-hold-reset signal (SHR) switch

C _SHR . . . SHR capacitor

SHS. . . Sample-and-hold-image signal (SHS) switch

C _SHS . . . SHS capacitor

DARK_AVG. . . Average switch

Claims (7)

An analog black level averaging circuit comprising: a plurality of sub-circuits, each of the sub-circuits inputting a signal from a pixel circuit of the image sensor, wherein each of the sub-circuits comprises an associated double sampling (CDS) a circuit having at least one capacitor, each of the sub-circuits sampling and maintaining a reset signal of the black-order pixel during the reset period, and each of the sub-circuits samples and maintains the image signal of the black-order pixel during the integration period; An average switch for controlling the coupling between the capacitors of the secondary circuit to average the reset signal between the black-order pixels and averaging the image signals between the black-order pixels; wherein the output of the average switch is used to perform black Order calibration. An analog black level averaging circuit of the image sensor according to claim 1, wherein the CDS circuit comprises: a sample-and-hold-reset signal (SHR) switch controlled by a SHR control signal; a hold-image signal (SHS) switch controlled by an SHS control signal; and an SHR capacitor and an SHS capacitor coupled to the SHR switch and the SHS switch, respectively. The analog black level averaging circuit of the image sensor according to claim 2, wherein the contact of the SHR switch and the SHR capacitor in the primary circuit can be connected to another circuit by the control of the average switch. The SHR switch is connected to the SHR capacitor. The analog black level averaging circuit of the image sensor according to claim 3, wherein the contact of the SHS switch and the SHS capacitor in the primary circuit can be connected to another circuit by the control of the average switch. The junction of the SHS switch and the SHS capacitor. An analog black level averaging circuit comprising: a plurality of sub-circuits, each of the sub-circuits inputting a signal from a pixel circuit of the image sensor, wherein each of the sub-circuits comprises: a sample-and-hold-weight a signal (SHR) switch controlled by an SHR control signal; a sample-and-hold-image signal (SHS) switch controlled by an SHS control signal; and an SHR capacitor and an SHS capacitor coupled to the SHR, respectively a switch and the SHS switch; and an average switch for controlling coupling between the SHR capacitor and the SHS capacitor of the secondary circuit; Each of the secondary circuits samples and maintains a reset signal of the black-order pixels during the reset period, and each of the secondary circuits samples and maintains the image signal of the black-order pixels during the integration period; wherein the average switch averages the black level A reset signal between the pixels, and averaging the image signals between the black level pixels; wherein the output of the average switch is used to perform black level calibration. An analog black level averaging circuit of the image sensor according to claim 5, wherein the contact of the SHR switch and the SHR capacitor in the primary circuit is connectable to another circuit by the control of the average switch The SHR switch is connected to the SHR capacitor; and by the control of the average switch, the contact of the SHS switch and the SHS capacitor in the primary circuit can be connected to the contact of the SHS switch and the SHS capacitor in another circuit. An analog black level averaging method for image sensor includes: sampling and maintaining a reset signal of a black level pixel, wherein the reset signal is subjected to a sample-and-hold-reset signal (SHR) switch during a reset period Sampled and held by an SHR capacitor; samples and maintains the image signal of the black-order pixel. When in the integration period, the image signal is sampled by a sample-and-hold image signal (SHS) switch and is sampled by a SHS. The capacitor is maintained; and The reset signal between the black-order pixels is averaged, and the image signals between the black-order pixels are averaged.