Image sensor, operation method thereof, and analog-to-digital converter

Abstract

The present application relates to an image sensor, an operation method thereof, and an analog-to-digital converter. An image sensor includes: a pixel of a first label; a pixel of a second label; an operational amplifier, which is suitable for comparing a pixel signal of the first label and a pixel signal of the second label with each other in a comparison operation to generate a comparison result, receiving a ramp voltage and a selected pixel signal selected based on the comparison result between the pixel signal of the first label and the pixel signal of the second label in an auto-zero operation, and receiving a ramp voltage and an unselected pixel signal that is not selected in the pixel signal of the first label and the pixel signal of the second label in an analog-to-digital conversion operation; and a counter circuit, which is suitable for generating a digital code based on the output of the operational amplifier.

Description

Technical Field

[0001] An exemplary embodiment of the present invention relates to an image sensor. Background Art

[0002] Recently, image sensors are being developed that provide three-dimensional (3D) distance images by measuring a specific range of distances simultaneously. Acquiring such distance images is based on time-of-flight (TOF) technology. According to this technology, distance can be measured by irradiating light from a light source near the image sensor and measuring the time it takes for the light to be reflected by an object and return.

[0003] TOF technology is roughly divided into two methods. One is a direct method and the other is an indirect method. The direct method is to irradiate pulsed light, measure the time taken to receive the reflected light, and convert the measured time into distance. In the direct method, the accuracy can be improved by making the pulse width as small as possible in consideration of the light flux. In addition, the direct method requires very precise time measurement.

[0004] In addition, the indirect method does not directly measure TOF, but the indirect method illuminates the object with modulated light, measures the phase difference with the reflected light, and extracts the distance. Specifically, according to the indirect method, the distance to the object can be measured by detecting the reflected light using the pixels of the tag A and the pixels of the tag B enabled at different times and obtaining the difference between the value obtained by analog-to-digital conversion of the pixel signal of the tag A and the value obtained by analog-to-digital conversion of the pixel signal of the tag B.

[0005] Since the indirect method of TOF requires an analog-to-digital converter for analog-to-digital conversion of a pixel signal of tag A and an analog-to-digital converter for analog-to-digital conversion of a pixel signal of tag B, a burden may increase in terms of area. Summary of the invention

[0006] Embodiments of the present invention are directed to an image sensor in which the area of ​​the image sensor can be reduced by performing analog-to-digital conversion on a voltage difference between pixel signals of two tags.

[0007] According to one embodiment of the present invention, an image sensor includes: pixels of a first label; pixels of a second label; an operational amplifier, which is suitable for comparing the pixel signal of the first label and the pixel signal of the second label with each other to generate a comparison result in a comparison operation, receiving a ramp voltage and a selected pixel signal selected based on the comparison result between the pixel signal of the first label and the pixel signal of the second label in an automatic zeroing operation, and receiving a ramp voltage and an unselected pixel signal that is not selected in the pixel signal of the first label and the pixel signal of the second label in an analog-to-digital conversion operation; and a counter circuit, which is suitable for generating a digital code based on the output of the operational amplifier.

[0008] According to another embodiment of the present invention, a method for operating an image sensor includes the following steps: comparing the level of a pixel signal of a first tag and the level of a pixel signal of a second tag with each other; performing an automatic zeroing operation of an operational amplifier based on a ramp voltage and a pixel signal selected as a result of comparing the level of the pixel signal of the first tag and the level of the pixel signal of the second tag with each other; performing a comparison operation including comparing an unselected pixel signal among the pixel signal of the first tag and the pixel signal of the second tag with the ramp voltage; and generating a digital code based on the result of performing the comparison operation.

[0009] According to another embodiment of the present invention, an analog-to-digital converter includes: an operational amplifier, which is suitable for comparing a first voltage and a second voltage with each other to generate a comparison result in a comparison operation, receiving a ramp voltage and a selected voltage selected based on the comparison result between the first voltage and the second voltage in an automatic zeroing operation, and receiving the ramp voltage and an unselected voltage that is not selected between the first voltage and the second voltage in an analog-to-digital conversion operation; and a counter circuit, which is suitable for generating a digital code based on the output of the operational amplifier. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 is a block diagram showing an image sensor 100 according to an embodiment of the present invention.

[0011] Figure 2 It is shown Figure 1 FIG. 1 is a timing diagram of the operation of the comparison operation section of the image sensor 100 shown.

[0012] Figure 3 It is shown Figure 1 The illustrated timing diagram is a diagram showing the operation of the auto-zero operation section AZ and the operation of the analog-to-digital conversion section ADC of the image sensor 100 . DETAILED DESCRIPTION

[0013] The exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. However, the present invention can be specifically implemented in different forms and should not be construed as being limited to the embodiments set forth herein. On the contrary, these embodiments are provided so that the present disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. Throughout the present disclosure, the same reference numerals throughout the various drawings and embodiments of the present invention represent the same parts.

[0014] As used herein, unless the context clearly indicates otherwise, the singular forms may also include the plural forms and vice versa.

[0015] It will be further understood that when used in this specification, the terms "include" and "comprising" specify the presence of the elements described and do not exclude the presence or addition of one or more other elements. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0016] Figure 1 is a block diagram showing an image sensor 100 according to an embodiment of the present invention.

[0017] Reference Figure 1 , the image sensor 100 may include a pixel 110 of a first tag, a pixel 120 of a second tag, a first pixel signal output unit 130, a second pixel signal output unit 140, an operational amplifier 150, capacitors 151 and 152, switches 153 to 159, a counter circuit 160, and a switch control circuit 170.

[0018] The pixel 110 of the first tag may detect light in a section where the signal MIXA is enabled. The pixel 110 of the first tag may include a first photodetector 111, a first reset transistor 112, a first transfer transistor 113, a first capacitor 114, a first drive transistor 115, and a first selection transistor 116. The first photodetector 111 may perform a photoelectric conversion function. The first photodetector 111 may be implemented using at least one of a photodiode, a phototransistor, a photogate, a pinned photodiode, and a combination thereof. The first photodetector 111 may be enabled in a section where the signal is enabled to a high level under the control of the signal MIXA. The first reset transistor 112 may transmit a power supply voltage to the node A in response to the reset signal RX. The first transfer transistor 113 may electrically connect the node A and the first floating diffusion node FDA to each other in response to the transfer signal TX. The first floating diffusion node FDA may be a node that accumulates charges corresponding to the light detected by the first photodetector 111 or charges corresponding to the initialization voltage. The first capacitor 114 may be coupled to the first floating diffusion node FDA. The first drive transistor 115 may include a gate coupled to the first floating diffusion node FDA, and include a drain and a source coupled between a power supply voltage terminal and the first selection transistor 116. The first drive transistor 115 may amplify the voltage of the first floating diffusion node FDA. The first selection transistor 116 may transmit the current transmitted from the first drive transistor 115 to a node outputting a pixel signal VA_PXL of a first tag associated with the pixel 110 in response to a selection signal SX.

[0019] The pixel 120 of the second tag may detect light in a section where the signal MIXB is enabled. The signal MIXA and the signal MIXB may be periodic waves with the same frequency but different phases. The pixel 120 of the second tag may include a second photodetector 121, a second reset transistor 122, a second transfer transistor 123, a second capacitor 124, a second drive transistor 125, and a second selection transistor 126. The second photodetector 121 may perform a photoelectric conversion function. The second photodetector 121 may be implemented using at least one of a photodiode, a phototransistor, a photogate, a pinned photodiode, and a combination thereof. The second photodetector 121 may be enabled in a section where the signal is enabled to a high level under the control of the signal MIXB. The second reset transistor 122 may transmit a power supply voltage to the node B in response to the reset signal RX. The second transfer transistor 123 may electrically connect the node B and the second floating diffusion node FDB to each other in response to the transfer signal TX. The second floating diffusion node FDB may be a node that accumulates charges corresponding to the light detected by the second photodetector 121 or charges corresponding to the initialization voltage. The second capacitor 124 may be coupled to the second floating diffusion node FDB. The second drive transistor 125 may include a gate coupled to the second floating diffusion node FDB and a drain and a source coupled between a power supply voltage terminal and a second selection transistor 126. The second drive transistor 125 may amplify the voltage of the second floating diffusion node FDB. The second selection transistor 126 may transmit the current transmitted from the second drive transistor 125 to a node outputting a pixel signal VB_PXL of a second tag associated with the pixel 120 in response to a selection signal SX.

[0020] The first pixel signal output unit 130 may sink a constant current from a node outputting the pixel signal VA_PXL of the first tag. The voltage level of the pixel signal VA_PXL of the first tag may be determined by the amount of current provided (sourced) from the first selection transistor 116 and the amount of current absorbed by the first pixel signal output unit 130. The amount of current sinking of the first pixel signal output unit 130 may be constant, and the amount of current sourced by the first selection transistor 116 may increase as the voltage level of the first floating diffusion node FDA increases. In one embodiment, the voltage level of the pixel signal VA_PXL of the first tag may increase as the voltage level of the first floating diffusion node FDA increases. The first pixel signal output unit 130 may include a current source.

[0021] The second pixel signal output unit 140 may sink a constant current from a node outputting the pixel signal VB_PXL of the second label. The voltage level of the pixel signal VB_PXL of the second label is determined by the amount of current provided from the second selection transistor 126 and the amount of current absorbed by the second pixel signal output unit 140. The amount of current sinking of the second pixel signal output unit 140 may be constant, and the amount of current sourcing of the second selection transistor 126 may increase as the voltage level of the second floating diffusion node FDB increases. In one embodiment, the voltage level of the pixel signal VB_PXL of the second label may increase as the voltage level of the second floating diffusion node FDB increases. The second pixel signal output unit 140 may include a current source.

[0022] The operational amplifier 150 may receive a signal selected from one or more of the pixel signal VA_PXL of the first tag, the pixel signal VB_PXL of the second tag, and the ramp voltage VRAMP through switches 153 to 157 at input terminals INN and INP through capacitors 151 and 152. When the voltage level of the input terminal INP of one of the input terminals INN and INP is high, the operational amplifier 150 may generate an output signal OUTP of a high level, and when the voltage level of the input terminal INN of one of the input terminals INN and INP is high, the operational amplifier 150 may generate an output signal OUTP of a low level. When switches 158 and 159 are turned on, the input terminal INN and the output terminal OUTP of the operational amplifier 150 may be short-circuited, and the input terminal INP and the output terminal OUTN of the operational amplifier 150 may be short-circuited to automatically return the operational amplifier 150 to zero. The input terminal INN may be a negative (-) input terminal, the input terminal INP may be a positive (+) input terminal, the output terminal OUTP may be a positive (+) output terminal, and the output terminal OUTN may be a negative (-) output terminal. As will be described later, an auto-zero operation of the operational amplifier 150 may be performed while a pixel signal having a higher voltage level among the pixel signals VA_PXL and VB_PXL is applied to the operational amplifier 150, and thereafter, a comparison operation with the ramp voltage VRAMP may be performed while a pixel signal having a lower voltage level of one of the pixel signals VA_PXL and VB_PXL is applied to the operational amplifier 150, so that an analog-to-digital conversion operation may be performed on a voltage corresponding to a voltage difference between the two pixel signals VA_PXL and VB_PXL.

[0023] The counter circuit 160 may generate a digital code DOUT by counting the count clock CNT_CLK in response to a signal from the output terminal OUTP of the operational amplifier 150. The counter circuit 160 and the operational amplifier 150 may be referred to as an analog-to-digital converter. In other words, the analog-to-digital converter may include the operational amplifier 150 and the counter circuit 160 to perform an analog-to-digital conversion operation and a digital code generation operation.

[0024] The switch control circuit 170 may generate control signals PS1 to PS5 and RS for controlling the switches 153 to 159. The switch control circuit 170 may use the control signals PS1 to PS5 and RS to control on / off of the switches 153 to 159. The switch control circuit 170 may generate the control signals PS1 to PS5 and RS using a signal of the output terminal OUTP.

[0025] The operation section of the image sensor 100 can be functionally divided into a comparison operation section, an auto-zero operation section, and an analog-to-digital conversion section. Hereinafter, the operation of the image sensor 100 in each operation section will be described.

[0026] Figure 2 It is shown Figure 1 FIG. 1 is a timing diagram of an operation in a comparison operation section of the image sensor 100 . In the comparison operation section, the voltage levels of the two pixel signals VA_PXL and VB_PXL may be compared with each other using the operational amplifier 150 .

[0027] Reference Figure 2 , first, during segment "201", the switch control circuit 170 may enable the control signals PS1, PS2 and RS to a high level to turn on switches 153, 154, 158 and 159. Since switches 153 and 154 are turned on, the pixel signal VA_PXL of the first label may be applied to the input terminals INN and INP through capacitors 151 and 152. In addition, since switches 158 and 159 are turned on, the input terminal INN and the output terminal OUTP are short-circuited, and the input terminal INP and the output terminal OUTN are short-circuited, so that the operational amplifier 150 may be automatically zeroed. The automatic zeroing operation may enable the voltage levels of the two input terminals INN and INP of the operational amplifier 150 to be the same. Figure 2 In one embodiment, the section “ 201 ” may auto-zero the operational amplifier 150 before comparing the voltage levels of the two pixel signals VA_PXL and VB_PXL with each other for accurate operation.

[0028] exist Figure 2In the section "203" shown, the switch control circuit 170 may enable the control signals PS1 and PS4 to a high level to turn on the switches 153 and 156. In the section "203", since the input terminal INN continues to receive the pixel signal VA_PXL of the first label through the capacitor 151 as in the previous section "201", the voltage level of the input terminal INN may be maintained at the level of VREF1. In addition, since the input terminal INP receives the pixel signal VA_PXL of the first label through the capacitor 151 in the section "201" and then receives the pixel signal VB_PXL of the second label in the section "203", the level of the input terminal INP may change. When the voltage level of the pixel signal VB_PXL of the second label associated with the pixel 120 is higher than the voltage level of the pixel signal VA_PXL of the first label associated with the pixel 110, the voltage level of the input terminal INP is higher than VREF1. When the voltage level of the pixel signal VB_PXL of the second tag is lower than the voltage level of the pixel signal VA_PXL of the first tag, the voltage level of the input terminal INP is lower than VREF1. An embodiment in which the voltage level of the pixel signal VB_PXL of the second tag is lower than the voltage level of the pixel signal VA_PXL of the first tag and thus the voltage level of the input terminal INP becomes lower than VREF1 is described herein.

[0029] exist Figure 2 In the section "203" shown in FIG. 1 , since the voltage level of the input terminal INP is lower than the voltage level of the input terminal INN, the output signal OUTP is generated at a low level. When the output signal OUTP is at a low level, it may mean that the voltage level of the pixel signal VA_PXL of the first tag is higher than the voltage level of the pixel signal VB_PXL of the second tag. Figure 2 Unlike the example of , when the voltage level of the pixel signal VB_PXL of the second tag is higher than the voltage level of the pixel signal VA_PXL of the first tag, the output signal OUTP here may be generated at a high level. The output signal OUTP may be transmitted as a switch control signal, and the switch control circuit 170 may perform a voltage level comparison between the pixel signal VA_PXL of the first tag associated with the pixel 110 and the pixel signal VB_PXL of the second tag associated with the pixel 120.

[0030] The voltage level of the pixel signal VA_PXL of the first tag and the voltage level of the pixel signal VB_PXL of the second tag can be obtained by Figure 2 The operations of the above-mentioned comparison operation sections described in are compared with each other.

[0031] Hereinafter, a pixel signal determined to have a higher voltage level through the operation of the comparison operation section among the pixel signal VA_PXL of the first label and the pixel signal VB_PXL of the second label may be referred to as a selected pixel signal, and a pixel signal determined to have a lower voltage level may be referred to as an unselected pixel signal.

[0032] Figure 3 It is shown Figure 1 The illustrated timing diagram is a diagram showing the operation of the auto-zero operation section AZ and the operation of the analog-to-digital conversion section ADC of the image sensor 100 . Figure 3 The operation of the auto-zero operation section AZ and the analog-to-digital conversion section ADC shown can be performed Figure 2 The operation of the comparison operation section is then performed using the operational amplifier 150 .

[0033] exist Figure 3 In the Figure 2 As a result of the operation of the comparison operation section shown, the pixel signal VA_PXL of the first tag becomes the selected pixel signal and the pixel signal VB_PXL of the second tag becomes the unselected pixel signal. In other words, it can be assumed that the voltage level of the pixel signal VA_PXL of the first tag is higher than the voltage level of the pixel signal VB_PXL of the second tag.

[0034] In the auto-zero operation section AZ, when the selected pixel signal VA_PXL and the ramp voltage VRAMP are applied to the operational amplifier 150, the operational amplifier 150 can be automatically zeroed. In the auto-zero operation section AZ, the switch control circuit 170 can enable the control signals PS1, PS5 and RS to a high level to turn on switches 153, 157, 158 and 159. As switch 153 is turned on, the selected pixel signal VA_PXL can be applied to the input terminal INN through the capacitor 151, and as switch 157 is turned on, the ramp voltage VRAMP can be applied to the input terminal INP through the capacitor 152. In addition, as switches 158 and 159 are turned on, the input terminal INN and the output terminal OUTP are short-circuited, and the input terminal INP and the output terminal OUTN are short-circuited, so the operational amplifier 150 can be automatically zeroed. The auto-zero operation can make the voltage levels of the two input terminals INN and INP of the operational amplifier 150 the same. Figure 3 In the example, this voltage level is indicated by VREF2.

[0035] In the analog-to-digital conversion section ADC, an analog-to-digital conversion operation may be performed while the unselected pixel signal VB_PXL and the ramp voltage VRAMP are applied to the operational amplifier 150. In this section, an analog voltage corresponding to a voltage difference between the pixel signal VA_PXL of the first tag and the pixel signal VB_PXL of the second tag may be converted into a digital code DOUT.

[0036] exist Figure 3 At the time 301 shown, the control signal PS1 may be disabled and the control signal PS3 may be enabled. This may turn on the switch 155 to apply the unselected pixel signal VB_PXL to the input terminal INN through the capacitor 151. Since the voltage level at one end of the capacitor 151 changes from VA_PXL to VB_PXL, that is, since the voltage level drops by (VA_PXL-VB_PXL), the voltage level of the input terminal INN at the other end of the capacitor 151 may drop by (VA_PXL-VB_PXL). In one embodiment, the voltage level of the input terminal INN may be VREF2-(VA_PXL-VB_PXL).

[0037] From time 303, a ramp operation in which the level of the ramp voltage VRAMP rises and then falls may be performed. Therefore, the voltage level of the input terminal INP may rise and then fall in the same form as the ramp voltage VRAMP. From time 305 when the voltage level of the input terminal INP starts to fall to time 307 when the voltage of the input terminal INP becomes equal to the voltage of the input terminal INN (i.e., the output signal OUTP of the operational amplifier 150 changes from high to low), the counter circuit 160 may generate a digital code DOUT by counting the number of times the count clock CNT_CLK is enabled. The digital code DOUT may be a value obtained by analog-to-digital conversion of the value of (VA_PXL-VB_PXL).

[0038] In such Figure 3 In one embodiment shown, the auto-zero operation of the operational amplifier 150 may be performed based on the selected pixel signal (the pixel signal with a higher voltage level among the pixel signal VA_PXL of the first label and the pixel signal VB_PXL of the second label). By performing an analog-to-digital conversion operation while the pixel signal with a lower voltage level among the pixel signal VA_PXL of the first label and the pixel signal VB_PXL of the second label is applied to the operational amplifier 150, a value corresponding to the voltage difference between the two pixel signals VA_PXL and VB_PXL may be analog-to-digital converted by a single analog-to-digital conversion operation to generate a digital code DOUT. Using the digital code DOUT, a distance may be measured. In some proposed methods, a digital code corresponding to the voltage difference between two pixels may be generated by performing analog-to-digital conversion on the voltages of the two pixels using two analog-to-digital converters to generate two digital codes so as to perform analog-to-digital conversion on the voltage difference between the two pixels, and calculating the two digital codes. Various embodiments of the present invention may make it possible to convert the voltage difference between two pixels into analog-to-digital at one time using one analog-to-digital converter.

[0039] Figure 3An example is shown in which the selected pixel signal is the pixel signal VA_PXL of the first tag. However, when the selected pixel signal is the pixel signal VB_PXL of the second tag, the switch control circuit 170 may enable the control signal PS3 instead of the control signal PS1 to be high in the auto-zero operation section AZ, and enable the control signal PS1 instead of the control signal PS3 to be high in the analog-to-digital conversion section ADC. In this example, the operational amplifier 150 may be auto-zeroed while the pixel signal VB_PXL of the second tag and the ramp voltage VRAMP are applied to the operational amplifier 150, and an analog-to-digital conversion operation may be performed while the pixel signal of the first tag and the ramp voltage are applied to the operational amplifier 150, so that the difference between the two voltages (VB_PXL-VA_PXL) may be converted into a digital code DOUT.

[0040] exist Figure 1 In the image sensor 100, constituent elements other than constituent elements 110, 120, 130, and 140 related to pixels are elements for converting analog voltages output from pixels into digital codes. Therefore, the technology of the present invention can be applied not only to image sensors but also to general analog-to-digital conversion circuits for converting analog voltages into digital codes. Specifically, the technology of the present invention can be used for analog-to-digital conversion of the difference between two voltages.

[0041] According to various embodiments of the present invention, the voltage difference between the pixel signals of two tags may be analog-to-digital converted.

[0042] While the present invention has been described with respect to the specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the claims.

[0043] CROSS-REFERENCE TO RELATED APPLICATIONS

[0044] This application claims priority to Korean Patent Application No. 10-2021-0017664, filed on February 8, 2021, which is hereby incorporated by reference in its entirety.

Claims

1. An image sensor, comprising: Pixels of the first label; pixels of the second label; an operational amplifier that compares the pixel signal of the first tag and the pixel signal of the second tag with each other to generate a comparison result in a comparison operation, receives a ramp voltage and a selected pixel signal selected based on the comparison result between the pixel signal of the first tag and the pixel signal of the second tag in an auto-zero operation, and receives the ramp voltage and an unselected pixel signal that is not selected among the pixel signal of the first tag and the pixel signal of the second tag in an analog-to-digital conversion operation; as well as a counter circuit that generates a digital code based on the output of the operational amplifier, Wherein, in the comparison operation of the operational amplifier, When the pixel signal of the first tag is applied to the positive input terminal and the negative input terminal of the operational amplifier, the operational amplifier is automatically zeroed.

2. The image sensor according to claim 1, wherein: In the comparison operation of the operational amplifier, After the operational amplifier is automatically zeroed, as the pixel signal of the first tag is applied to the negative input terminal and the pixel signal of the second tag is applied to the positive input terminal, a voltage level of the pixel signal of the first tag and a voltage level of the pixel signal of the second tag are compared with each other.

3. The image sensor according to claim 1, wherein: In the auto-zero operation of the operational amplifier, The operational amplifier is auto-zeroed when a) the selected pixel signal is applied to a negative input terminal of the operational amplifier and b) the ramp voltage is applied to a positive input terminal of the operational amplifier.

4. The image sensor according to claim 1, wherein: In the analog-to-digital conversion operation of the operational amplifier, The unselected pixel signal is applied to the negative input terminal of the operational amplifier, and The ramp voltage is applied to the positive input terminal of the operational amplifier.

5. The image sensor according to claim 4, wherein: In the analog-to-digital conversion operation of the operational amplifier, a ramp operation of changing a voltage level of the ramp voltage is performed.

6. The image sensor according to claim 1, wherein: The pixels of the first tag and the pixels of the second tag are exposed in different time periods of the image sensor.

7. The image sensor according to claim 1, wherein: The operational amplifier receives a voltage through a first capacitor coupled to a negative input terminal and a second capacitor coupled to a positive input terminal.

8. The image sensor according to claim 1, wherein: The pixels of the first label include: a first photodetector coupled to the first node; a first reset transistor that resets the first node in response to a reset signal; a first transfer transistor electrically connecting the first node and a first floating diffusion node to each other in response to a transfer signal; a first capacitor coupled to the first floating diffusion node; a first driving transistor that amplifies a voltage of the first floating diffusion node; and A first selection transistor outputs a voltage amplified by the first drive transistor to a first tag output line in response to a first selection signal.

9. The image sensor according to claim 8, wherein: The pixels of the second label include: a second photodetector coupled to the second node; a second reset transistor that resets the second node in response to a reset signal; a second transfer transistor electrically connecting the second node and a second floating diffusion node to each other in response to a transfer signal; a second capacitor coupled to the second floating diffusion node; a second driving transistor that amplifies a voltage of the second floating diffusion node; and a second selection transistor that outputs the voltage amplified by the second drive transistor to a second tag output line in response to a second selection signal.

10. The image sensor according to claim 1, further comprising: A plurality of switches selectively apply the pixel signal of the first tag, the pixel signal of the second tag, and the ramp voltage to the positive input terminal and the negative input terminal of the operational amplifier.

11. A method for operating an image sensor, the method comprising the steps of: comparing a level of a pixel signal of a first tag and a level of a pixel signal of a second tag with each other; performing an auto-zero operation of the operational amplifier based on a ramp voltage and a selected pixel signal selected as a result of comparing a level of the pixel signal of the first tag and a level of the pixel signal of the second tag with each other; performing a comparison operation, the comparison operation comprising comparing an unselected pixel signal of the pixel signal of the first tag and the pixel signal of the second tag with the ramp voltage; as well as generating a digital code based on a result of performing the comparison operation, wherein, in the step of comparing the level of the pixel signal of the first tag and the level of the pixel signal of the second tag with each other, When the pixel signal of the first tag is applied to the positive input terminal and the negative input terminal of the operational amplifier, the operational amplifier is automatically zeroed.

12. The method according to claim 11, wherein: In the step of comparing the level of the pixel signal of the first tag and the level of the pixel signal of the second tag with each other, After the operational amplifier is automatically zeroed, as the pixel signal of the first tag is applied to the negative input terminal and the pixel signal of the second tag is applied to the positive input terminal, a voltage level of the pixel signal of the first tag and a voltage level of the pixel signal of the second tag are compared with each other.

13. The method according to claim 11, wherein: In the step of performing the auto-zero operation of the operational amplifier, The operational amplifier is auto-zeroed when a) the selected pixel signal is applied to a negative input terminal of the operational amplifier and b) the ramp voltage is applied to a positive input terminal of the operational amplifier.

14. The method according to claim 11, wherein: In the step of performing the comparison operation, The unselected pixel signal is applied to the negative input terminal of the operational amplifier, and The ramp voltage is applied to the positive input terminal of the operational amplifier.

15. The method according to claim 14, wherein: In the step of performing the comparison operation, A ramp operation of changing a voltage level of the ramp voltage is performed.

16. An analog-to-digital converter, the analog-to-digital converter comprising: an operational amplifier that compares a first voltage and a second voltage with each other to generate a comparison result in a comparison operation, receives a ramp voltage and a selected voltage selected based on the comparison result between the first voltage and the second voltage in an auto-zero operation, and receives the ramp voltage and an unselected voltage that is not selected between the first voltage and the second voltage in an analog-to-digital conversion operation; as well as a counter circuit that generates a digital code based on the output of the operational amplifier, Wherein, in the comparison operation of the operational amplifier, When the first voltage is applied to the positive input terminal and the negative input terminal of the operational amplifier, the operational amplifier is auto-zeroed.

17. The analog-to-digital converter according to claim 16, wherein: In the comparison operation of the operational amplifier, After the operational amplifier is auto-zeroed, as the first voltage is applied to the negative input terminal and the second voltage is applied to the positive input terminal, a voltage level of the first voltage and a voltage level of the second voltage are compared with each other.

18. The analog-to-digital converter according to claim 16, wherein: In the auto-zero operation of the operational amplifier, The operational amplifier is auto-zeroed when a) the selected voltage is applied to a negative input terminal of the operational amplifier and b) the ramp voltage is applied to a positive input terminal of the operational amplifier.

19. The analog-to-digital converter according to claim 16, wherein: In the analog-to-digital conversion operation of the operational amplifier, The unselected voltage is applied to the negative input terminal of the operational amplifier, and The ramp voltage is applied to the positive input terminal of the operational amplifier.

20. The analog-to-digital converter according to claim 19, wherein: In the analog-to-digital conversion operation of the operational amplifier, A ramp operation of changing a voltage level of the ramp voltage is performed.

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