CN106303311A - The output intent of pixel photosensitive value - Google Patents

Abstract

The present invention provides the output intent of a kind of pixel photosensitive value.The pixel photosensitive value output intent of the present invention, the time of exposure short exposure less than constant exposure time is added outside normal exposure, by the normal exposure of each row pixel cell and the differenceization of short exposure are exported, effectively eliminate pixel cell reading circuit and the fixed pattern noise of sequential introducing, further increase picture quality.

Description

Output method of pixel sensitivity value

technical field

The invention relates to the field of image sensors, in particular to a method for outputting light-sensing values of pixels.

Background technique

At present, image sensors mainly include CCD image sensor (Charged Coupled Device) and CMOS image sensor (CMOS Imaging Sensor, CIS). Compared with CDD image sensors, CMOS image sensors have the advantages of low power consumption, low noise, wide dynamic range, small size, and low cost. Therefore, CMOS image sensors have gradually become a research and development hotspot in this technical field.

The Analog-to-Digital Converter (ADC) is an important part of the CMOS image sensor, used to convert the analog signal generated by each pixel unit into a digital signal, and is the interface between the analog circuit and the digital circuit.

The existing CMOS image sensors mainly use three types of ADCs, namely: chip-level ADC, column-parallel ADC and pixel-level ADC. Chip-level ADC means that there is only one ADC in the entire chip, and the analog output generated by each pixel must be sequentially passed through this ADC for analog-to-digital conversion. Applications with small arrays and low requirements on CIS speed. Pixel-level ADC means that each pixel or every few pixels share an ADC. This kind of ADC has high signal-to-noise ratio, low power consumption, and low requirements on the speed of the ADC. However, the pixel fill factor is low and the layout design is complicated, so it cannot be industrialized yet. The column-parallel ADC is a compromise between the chip-level ADC and the pixel-level ADC. It uses one ADC for each column of pixels, and the ADC of each column is only responsible for processing the data of this column. The ADCs of each column work at the same time. Adopting the advantages of chip-level ADC and pixel-level ADC can greatly improve the conversion efficiency, and has a wide application prospect in the development of CIS in the future.

The wide application of high-pixel image sensors puts forward higher requirements for image quality, and fixed pattern noise (Fixed Pattern Noise, FPN) is the main factor affecting image quality. In order to eliminate the FPN introduced by the readout circuit of the pixel photosensitive unit, for image sensors using column-parallel ADC as the readout circuit, Correlated Double Sampling (CDS) technology is widely used, including analog correlated double sampling and digital correlated double sampling. sampling.

Among them, the analog correlated double sampling is to respectively sample the reset voltage and the photosensitive voltage of the pixel unit through two capacitors, and realize the digitization of the difference between the photosensitive voltage and the reset voltage of the pixel through the analog circuit ADC. However, in order to achieve a sufficiently high accuracy, a large sampling capacitor value is usually required, so the design cost is high.

The Chinese patent application with the publication number CN 103686004A discloses a method for outputting the photosensitive value of pixels using digital correlation double sampling. Through reset and normal exposure operations, the reset voltage and photosensitive voltage of the pixel unit are respectively collected, and two ramp signals are used to compare with The reset voltage is compared with the photosensitive voltage, and the difference operation between the pixel photosensitive voltage and the reset voltage is realized in the digital system, so as to obtain the pixel photosensitive value. However, although this method can eliminate the FPN introduced by the readout circuit, it still cannot effectively eliminate the FPN introduced by the timing.

Contents of the invention

The technical problem to be solved by the present invention is to provide a method for outputting the photosensitive value of a pixel, which can eliminate the fixed pattern noise introduced by the readout circuit and timing of the pixel unit, and further improve the image quality.

In order to solve the above problems, the present invention provides a method for outputting a pixel sensitivity value, comprising the following steps:

A． After the first reset, each column of pixel units in the pixel array is exposed for the first time, and the first exposure time is the normal exposure time, and the first photosensitive voltage of each column of pixel units is output;

Generating a ramp signal through a ramp generating circuit, comparing the ramp signal with the first photosensitive voltage through a comparator, and obtaining a first photosensitive count value of each row of pixel units through a counter;

B． After the second reset, each column of pixel units in the pixel array is exposed for the second time, and the second exposure time is shorter than the normal exposure time, and the second photosensitive voltage of each column of pixel units is output;

Generating a ramp signal through a ramp generating circuit, comparing the ramp signal with the second photosensitive voltage through a comparator, and obtaining a second photosensitive count value of each row of pixel units through a counter;

C． The difference between the first light-sensing count value and the second light-sensing count value of each row of pixel units is calculated to obtain the pixel light-sensing value of each row of pixel units.

Preferably, the output method of the photosensitive value of the pixel includes the following steps:

A1. After the first reset, each column of pixel units in the pixel array is exposed for the first time, and the first exposure time is the normal exposure time, and the first photosensitive voltage of each column of pixel units is output;

The ramp generating circuit generates the first ramp signal, and the counter starts counting from zero;

The comparator compares the first ramp signal voltage with the first photosensitive voltage of each column of pixel units, and when the first ramp signal voltage exceeds the first photosensitive voltage of each column of pixel units, the counter obtains the first photosensitive voltage of each column of pixel units. Photosensitive count value VSN;

B1. After the second reset, each column of pixel units in the pixel array is exposed for the second time, and the second exposure time is shorter than the normal exposure time, and the second photosensitive voltage of each column of pixel units is output;

The ramp generating circuit generates a second ramp signal, and the counter restarts counting from zero;

The comparator compares the second ramp signal voltage with the second photosensitive voltage of each column of pixel units, and when the second ramp signal voltage exceeds the second photosensitive voltage of each column of pixel units, the counter obtains the second photosensitive voltage of each column of pixel units. Photosensitive count value VSZ;

C1. The difference between the first light-sensing count value VSN and the second light-sensing count value VSZ of each column of pixel units is calculated to obtain the pixel light-sensing value VSN-VSZ of each column of pixel units.

Preferably, the output method of the photosensitive value of the pixel includes the following steps:

A2. Resetting for the first time, generating a first reset voltage for each row of pixel units;

The ramp generating circuit generates the first ramp signal, and the counter starts counting from zero;

The comparator compares the first ramp signal voltage with the first reset voltage of each column of pixel units, and when the first ramp signal voltage exceeds the first reset voltage of each column of pixel units, the counter obtains the first reset voltage of each column of pixel units. Reset count value VRN;

Each column of pixel units is exposed for the first time, and the first exposure time is the normal exposure time, and the first photosensitive voltage of each column of pixel units is output;

The ramp generating circuit generates a second ramp signal, and the counter restarts counting from zero;

The comparator compares the second ramp signal voltage with the first photosensitive voltage of each column of pixel units, and when the second ramp signal voltage exceeds the first photosensitive voltage of each column of pixel units, the counter obtains the first photosensitive voltage of each column of pixel units. Photosensitive count value VSN;

B2. Resetting for the second time, generating a second reset voltage for each row of pixel units;

The ramp generating circuit generates a third ramp signal, and the counter restarts counting from zero;

The comparator compares the third ramp signal voltage with the second reset voltage of each column of pixel units, and when the third ramp signal voltage exceeds the second reset voltage of each column of pixel units, the counter obtains the second reset voltage of each column of pixel units. Reset count value VRZ;

Each column of pixel units is exposed for the second time, the second exposure time is shorter than the normal exposure time, and the second photosensitive voltage of each column of pixel units is output;

The ramp generating circuit generates a fourth ramp signal, and the counter restarts counting from zero;

The comparator compares the fourth ramp signal voltage with the second photosensitive voltage of each column of pixel units, and when the fourth ramp signal voltage exceeds the second photosensitive voltage of each column of pixel units, the counter obtains the second photosensitive voltage of each column of pixel units. Photosensitive count value VSZ;

C2. Calculate the difference between the first photosensitive count value VSN and the first reset count value VRN of each column of pixel units to obtain the first exposure count difference VSN-VRN;

Respectively calculate the difference between the second photosensitive count value VSZ and the second reset count value VRZ of each row of pixel units to obtain the second exposure count difference VSZ-VRZ;

Calculate the difference between the first exposure count difference VSN-VRN and the second exposure count difference VSZ-VRZ of each row of pixel units to obtain the pixel sensitivity value (VSN-VRN)-(VSZ-VRZ ).

Preferably, the output method of the pixel photosensitivity value further includes: repeating the step B to obtain a plurality of photosensitivity count values whose exposure time is shorter than the normal exposure time of each row of pixel units, respectively The difference between the count value and the photosensitive count values whose exposure time is shorter than the normal exposure time is calculated to obtain the pixel photosensitive value of each row of pixel units.

Preferably, the output method of the photosensitive value of the pixel further includes: adjusting the second exposure time according to the input offset voltage of different comparators.

Preferably, the first ramp signal, the second ramp signal, the third ramp signal and the fourth ramp signal are upward ramp signals or downward ramp signals.

Preferably, the method for outputting the light-sensing value of the pixel further includes: recording the first reset count value, the second reset count value, the first light-sensing count value, and the second light-sensing count value through a memory.

The pixel photosensitivity value output method of the present invention adds a short exposure with an exposure time shorter than the normal exposure time in addition to the normal exposure, and effectively eliminates the pixel unit readout through the differential output of the normal exposure and the short exposure of each column of pixel units. The fixed pattern noise introduced by the output circuit and timing further improves the image quality.

Description of drawings

Fig. 1 is the flow chart of the pixel photosensitive value output method of the present invention;

FIG. 2 is a timing diagram of a method for outputting a pixel sensitivity value according to Embodiment 1 of the present invention;

FIG. 3 is a timing diagram of a method for outputting a pixel sensitivity value according to Embodiment 2 of the present invention.

detailed description

In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention can be implemented in many other ways different from those described here, and those skilled in the art can make similar extensions without violating the connotation of the present invention, so the present invention is not limited by the specific implementations disclosed below.

Secondly, the present invention is described in detail by means of schematic diagrams. When describing the embodiments of the present invention in detail, for convenience of explanation, the schematic diagrams are only examples, which should not limit the protection scope of the present invention.

As shown in Figure 1, the present invention provides a method for outputting a pixel sensitivity value, comprising the following steps:

A． After the first reset, each column of pixel units in the pixel array is exposed for the first time, and the first exposure time is the normal exposure time, and the first photosensitive voltage of each column of pixel units is output;

Generating a ramp signal through a ramp generating circuit, comparing the ramp signal with the first photosensitive voltage through a comparator, and obtaining a first photosensitive count value of each row of pixel units through a counter;

B． After the second reset, each column of pixel units in the pixel array is exposed for the second time, and the second exposure time is shorter than the normal exposure time, and the second photosensitive voltage of each column of pixel units is output;

Generating a ramp signal through a ramp generating circuit, comparing the ramp signal with the second photosensitive voltage through a comparator, and obtaining a second photosensitive count value of each row of pixel units through a counter;

C． The difference between the first light-sensing count value and the second light-sensing count value of each row of pixel units is calculated to obtain the pixel light-sensing value of each row of pixel units.

The pixel sensitivity value output method of the present invention will be described in detail below in conjunction with specific embodiments.

Embodiment one

FIG. 2 is a timing diagram of a pixel sensitivity value output method according to Embodiment 1 of the present invention.

As shown in FIG. 2 , the RST signal is the reset control signal of the pixel photosensitive unit, the TX signal is the exposure transmission control signal of the pixel photosensitive unit, and ramp is the ramp signal generated by the ramp generating circuit.

First, the RST signal controls the pixel units of each column to reset for the first time, and then the TX signal controls the pixel units of each column to be exposed for the first time. The first exposure time T1 is the normal exposure time, and the pixel units of each column output the first photosensitive voltage respectively. The ramp generating circuit generates a first ramp signal, and the counter starts counting from zero. As the first ramp signal gradually increases, the voltage difference between the first ramp signal and the first photosensitive voltage gradually decreases. When the When the first ramp signal exceeds the first photosensitive voltage, the comparator is triggered to reverse, and the count value of the counter at this time is the first photosensitive count value VSN of each row of pixel units. Preferably, the first photosensitive count value VSN can also be recorded by a memory.

Then, the RST signal controls the pixel units of each column to reset for the second time, after that, the TX signal controls the pixel units of each column to expose for the second time, and the second exposure time T2 is less than the normal exposure time (that is to say, the second exposure is the exposure time "short exposure" shorter than the normal exposure time), the pixel units in each column output the second photosensitive voltage respectively. Preferably, the second exposure time T2 can be adjusted according to different input offset voltages of the comparators. The ramp generating circuit generates a second ramp signal, and the counter restarts counting from zero. As the second ramp signal gradually increases, the voltage difference between the second ramp signal and the second photosensitive voltage gradually decreases. When the second ramp signal exceeds the second photosensitive voltage, the comparator is triggered to flip, and the count value of the counter at this time is the second photosensitive count value VSZ of each row of pixel units. Preferably, the second photosensitive count value VSZ can also be recorded by a memory.

Finally, calculate the difference between the first light-sensing count value VSN and the second light-sensing count value VSZ of each column of pixel units to obtain the pixel light-sensing value VSN-VSZ of each column of pixel units, thereby eliminating the fixed time introduced by the pixel unit readout circuit and timing. Pattern noise to improve image quality.

Those skilled in the art can understand that, in this embodiment, both the first ramp signal and the second ramp signal are upward ramp signals (the slope is positive), and in other embodiments not shown, the first ramp signal and the second ramp signal The second slope signal can also be a downward slope signal (the slope is negative), and the legend is only a schematic diagram.

Embodiment two

FIG. 3 is a timing diagram of a method for outputting a pixel sensitivity value according to Embodiment 2 of the present invention.

As shown in FIG. 3 , the RST signal is the reset control signal of the pixel photosensitive unit, the TX signal is the exposure transmission control signal of the pixel photosensitive unit, and ramp is the ramp signal generated by the ramp generating circuit.

Firstly, the RST signal controls the pixel units in each column to reset for the first time, and the pixel units in each column output the first reset voltage respectively. The ramp generating circuit generates a first ramp signal, and the counter starts counting from zero. As the first ramp signal gradually increases, the voltage difference between the first ramp signal and the first reset voltage gradually decreases. When the When the first ramp signal exceeds the first reset voltage, the comparator is triggered to reverse, and the count value of the counter at this time is the first reset count value VRN of each row of pixel units. Preferably, the first reset count value VRN can also be recorded by a memory.

Afterwards, the TX signal controls each column of pixel units to be exposed for the first time, and the first exposure time T1 is a normal exposure time, and each column of pixel units respectively outputs a first photosensitive voltage. The ramp generating circuit generates a second ramp signal, and the counter restarts counting from zero. As the second ramp signal gradually increases, the voltage difference between the second ramp signal and the first photosensitive voltage gradually decreases, and when the second ramp signal exceeds the first photosensitive voltage, trigger the The comparator is reversed, and the count value of the counter at this time is the first light-sensing count value VSN of each column of pixel units. Preferably, the first photosensitive count value VSN can also be recorded by a memory.

Next, the RST signal controls the pixel units in each column to reset for the second time, and the pixel units in each column output the second reset voltage respectively. The ramp generating circuit generates a third ramp signal, and the counter restarts counting from zero. As the third ramp signal gradually increases, the voltage difference between the third ramp signal and the second reset voltage gradually decreases. When the When the third ramp signal exceeds the second reset voltage, the comparator is triggered to flip, and the count value of the counter at this time is the second reset count value VRZ of each row of pixel units. Preferably, the second reset count value VRZ can also be recorded by a memory.

Afterwards, the TX signal controls the second exposure of each row of pixel units, and the second exposure time T2 is shorter than the normal exposure time (that is, the second exposure is a "short exposure" whose exposure time is shorter than the normal exposure time), and each row of pixels The units respectively output the second photosensitive voltage. Preferably, the second exposure time T2 can be adjusted by adjusting the input offset voltage of the comparator. The ramp generating circuit generates a fourth ramp signal, and the counter restarts counting from zero. As the fourth ramp signal gradually increases, the voltage difference between the fourth ramp signal and the second photosensitive voltage gradually decreases, and when the fourth ramp signal exceeds the second photosensitive voltage, trigger the The comparator is reversed, and the count value of the counter at this time is the second photosensitive count value VSZ of each column of pixel units. Preferably, the second photosensitive count value VSZ can also be recorded by a memory.

Finally, calculate the difference between the first photosensitive count value VSN and the first reset count value VRN of each column of pixel units to obtain the first exposure count difference VSN-VRN; respectively calculate the second photosensitive count value VSZ of each column of pixel units Calculate the difference with the second reset count value VRZ to obtain the second exposure count difference VSZ-VRZ; respectively calculate the first exposure count difference VSN-VRN and the second exposure count difference VSZ-VRZ of each row of pixel units Calculate the difference to obtain the pixel sensitivity value (VSN-VRN)-(VSZ-VRZ) of each column of pixel units, thereby eliminating the fixed pattern noise introduced by the pixel unit readout circuit and timing, and improving image quality.

Compared with Embodiment 1, the method of Embodiment 2 eliminates the fixed pattern noise introduced by the readout circuit more cleanly and thoroughly by increasing the difference between the photosensitive count value and the reset count value of the two exposures, which is beneficial to the improvement of image quality. Further improve.

Those skilled in the art can understand that in this embodiment, the first ramp signal, the second ramp signal, the third ramp signal, and the fourth ramp signal are all upward ramp signals (the slope is positive). In an embodiment, these ramp signals may also be downward ramp signals (the slope is negative), and the legend is only a schematic diagram.

In addition, theoretically speaking, the method of the present invention can also obtain the photosensitivity count value whose exposure time of each row of pixel units is shorter than the normal exposure time by repeating the step of short exposure multiple times, respectively for the first pixel unit of each row The difference between the photosensitive count value and multiple photosensitive count values whose exposure time is shorter than the normal exposure time is obtained to obtain the pixel photosensitive value of each column of pixel units, thereby eliminating the fixed pattern noise introduced by timing more cleanly and thoroughly. However, in practical applications, it is usually necessary to comprehensively consider the application frame rate, conversion efficiency and other requirements, and reasonably select the number of short exposures. Embodiment 1 and Embodiment 2 only take the case of one short exposure as an example, not as a limitation.

The pixel photosensitivity value output method of the present invention adds a short exposure with an exposure time shorter than the normal exposure time in addition to the normal exposure, and effectively eliminates the pixel unit readout through the differential output of the normal exposure and the short exposure of each column of pixel units. The fixed pattern noise introduced by the output circuit and timing further improves the image quality.

Although the present invention has been disclosed as above with preferred embodiments, it is not intended to limit the present invention, and any person skilled in the art can use the methods disclosed above and technical content to analyze the present invention without departing from the spirit and scope of the present invention. Possible changes and modifications are made in the technical solution. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention, which do not depart from the content of the technical solution of the present invention, all belong to the technical solution of the present invention. protected range.

Claims (7)

1. the output intent of a pixel photosensitive value, it is characterised in that comprise the following steps:

A., after resetting for the first time, each row pixel cell exposure for the first time in pel array, time of exposure is normal exposure for the first time Time, export the first photoreceptor voltage of each row pixel cell；

Produce ramp signal by slope generating circuit, compare ramp signal and the first photoreceptor voltage by comparator, by meter Number device obtains the first sensitometer numerical value of each row pixel cell；

B., after second time resets, each row pixel cell second time exposure in pel array, time of exposure exposes less than normal for the second time The light time, export the second photoreceptor voltage of each row pixel cell；

Produce ramp signal by slope generating circuit, compare ramp signal and the second photoreceptor voltage by comparator, by meter Number device obtains the second sensitometer numerical value of each row pixel cell；

The first sensitometer numerical value and the second sensitometer numerical value to each row pixel cell ask poor the most respectively, it is thus achieved that each row pixel cell Pixel photosensitive value.

2. the output intent of pixel photosensitive value as claimed in claim 1, it is characterised in that comprise the following steps:

A1., after resetting for the first time, each row pixel cell exposure for the first time in pel array, time of exposure is normal exposure for the first time Time, export the first photoreceptor voltage of each row pixel cell；

Slope generating circuit produces the first ramp signal, and enumerator starts from zero count；

The more described first ramp signal voltage of comparator and the first photoreceptor voltage of Ge Lie pixel cell, when described first slope When signal voltage exceedes the first photoreceptor voltage of each row pixel cell, enumerator obtains the first photosensitive counting of each row pixel cell Value VSN；

B1., after second time resets, each row pixel cell second time exposure in pel array, time of exposure exposes less than normal for the second time The light time, export the second photoreceptor voltage of each row pixel cell；

Slope generating circuit produces the second ramp signal, and enumerator restarts from zero count；

The more described second ramp signal voltage of comparator and the second photoreceptor voltage of Ge Lie pixel cell, when described second slope When signal voltage exceedes the second photoreceptor voltage of each row pixel cell, enumerator obtains the second photosensitive counting of each row pixel cell Value VSZ；

The most respectively the first sensitometer numerical value VSN and the second sensitometer numerical value VSZ of each row pixel cell is asked poor, it is thus achieved that respectively arrange The pixel photosensitive value VSN-VSZ of pixel cell.

3. the output intent of pixel photosensitive value as claimed in claim 1, it is characterised in that comprise the following steps:

A2. reset for the first time, produce the first resetting voltage of each row pixel cell；

Slope generating circuit produces the first ramp signal, and enumerator starts from zero count；

The more described first ramp signal voltage of comparator and the first resetting voltage of Ge Lie pixel cell, when described first slope When signal voltage exceedes the first resetting voltage of each row pixel cell, enumerator obtains the first reset count of each row pixel cell Value VRN；

The for the first time exposure of each row pixel cell, time of exposure is constant exposure time for the first time, exports the of each row pixel cell One photoreceptor voltage；

Slope generating circuit produces the second ramp signal, and enumerator restarts from zero count；

The more described second ramp signal voltage of comparator and the first photoreceptor voltage of Ge Lie pixel cell, when described second slope When signal voltage exceedes the first photoreceptor voltage of each row pixel cell, enumerator obtains the first photosensitive counting of each row pixel cell Value VSN；

B2. second time resets, and produces the second resetting voltage of each row pixel cell；

Slope generating circuit produces the 3rd ramp signal, and enumerator restarts from zero count；

The more described 3rd ramp signal voltage of comparator and the second resetting voltage of Ge Lie pixel cell, when described 3rd slope When signal voltage exceedes the second resetting voltage of each row pixel cell, enumerator obtains the second reset count of each row pixel cell Value VRZ；

The second time exposure of each row pixel cell, time of exposure is less than constant exposure time for the second time, exports each row pixel cell Second photoreceptor voltage；

Slope generating circuit produces the 4th ramp signal, and enumerator restarts from zero count；

The more described 4th ramp signal voltage of comparator and the second photoreceptor voltage of Ge Lie pixel cell, when described 4th slope When signal voltage exceedes the second photoreceptor voltage of each row pixel cell, enumerator obtains the second photosensitive counting of each row pixel cell Value VSZ；

The first sensitometer numerical value VSN and the first reset count value VRN to each row pixel cell ask poor the most respectively, obtain first Secondary exposure count difference value VSN-VRN；

The second sensitometer numerical value VSZ and the second reset count value VRZ to each row pixel cell ask poor respectively, obtain second time and expose Light count difference value VSZ-VRZ；

First time to each row pixel cell exposes count difference value VSN-VRN and second time exposure count difference value VSZ-VRZ respectively Ask poor, it is thus achieved that pixel photosensitive value (VSN-VRN)-(VSZ-VRZ) of each row pixel cell.

The output intent of pixel photosensitive value the most according to claim 1, it is characterised in that repeating said steps B, obtains each Multiple time of exposure of row pixel cell are less than the sensitometer numerical value of constant exposure time, first to each row pixel cell respectively Sensitometer numerical value asks poor with multiple time of exposure less than the sensitometer numerical value of constant exposure time, it is thus achieved that the picture of each row pixel cell Element photosensitive value.

The output intent of pixel photosensitive value the most according to claim 1, it is characterised in that defeated according to different comparators Enter offset voltage, regulation second time time of exposure.

6. according to the output intent of the pixel photosensitive value described in Claims 2 or 3, it is characterised in that described first ramp signal, Second ramp signal, the 3rd ramp signal, the 4th ramp signal are acclivity signal or downward ramp signal.

7. according to the output intent of the pixel photosensitive value described in Claims 2 or 3, it is characterised in that also include: pass through memorizer Record described first reset count value, the second reset count value, the first sensitometer numerical value, the second sensitometer numerical value.