Abstract:
An image sensor senses object information and converts the sensed object information into an electrical signal. An image processing apparatus uses the image sensor. The image sensor includes a column signal line connected to output terminals of a plurality of pixel sensors, a comparator circuit configured to output a signal corresponding to a comparison result of a signal output to the column signal line and a reference signal, an ADC circuit configured to convert an analog signal corresponding to an optical signal sensed by the pixel sensor selected from the plurality of pixel sensors connected to the column signal line into digital data based on the signal output from the comparator circuit and, a load circuit connected in series to the comparator circuit between the column signal line and a ground terminal, wherein the load circuit is configured as a common load device of the plurality of pixel sensors connected to the column signal line and the comparator circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a divisional application of U.S. patent application Ser. No. 13/760,556, filed on Feb. 6, 2013, which claims the benefit of Korean Patent Application No. 10-2012-0011798, filed on Feb. 6, 2012, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in their entireties by reference. 
     
    
     BACKGROUND 
       [0002]    The inventive concept relates to a sensor for sensing a signal, and, more particularly, to an image sensor for sensing object information and converting the sensed object information into an electrical signal and an image processing apparatus using the image sensor. 
         [0003]    An example of an image sensor is a complementary metal oxide semiconductor (CMOS) image sensor. The CMOS image sensor is a device for converting an optical image into an electrical signal, and may be applied to electronic products, such as a digital camera, a cellular phone, and the like. As electronic products become slimmer, research into minimization of the CMOS image sensor and reduction of power consumption is needed. 
       SUMMARY 
       [0004]    The inventive concept may provide an image sensor for reducing a size of a product and power consumption thereof. 
         [0005]    The inventive concept may also provide an image processing apparatus using an image sensor for reducing a size of a product and power consumption thereof. 
         [0006]    According to an aspect of the inventive concept, there is provided an image sensor including: a column signal line connected to output terminals of a plurality of pixel sensors; a comparator circuit configured to output a signal corresponding to a comparison result of a signal output to the column signal line and a reference signal; an analog/digital conversion (ADC) circuit configured to convert an analog signal corresponding to an optical signal sensed by the pixel sensor selected from the plurality of pixel sensors connected to the column signal line into digital data based on the signal output from the comparator circuit; and a load circuit connected in series to the comparator circuit between the column signal line and a ground terminal, wherein the load circuit is configured as a common load device of the plurality of pixel sensors connected to the column signal line and the comparator circuit. 
         [0007]    The comparator circuit may include a transistor, wherein the reference signal is applied to a gate terminal of the transistor, the column signal line is connected to a first terminal of the transistor, and the load circuit is connected to a second terminal of the transistor. 
         [0008]    The transistor may include a PMOS transistor. 
         [0009]    The comparator circuit may include a transistor, a capacitor, and a switch, wherein the reference signal is applied to a first terminal of the capacitor, a gate terminal of the transistor and a first terminal of the switch are connected to a second terminal of the capacitor, the column signal line is connected to a first terminal of the transistor, and a second terminal of the switch and the load circuit are connected to a second terminal of the transistor. 
         [0010]    The switch may be turned on during a first section before a correlated double sampling (CDS) process is performed and may be turned off during sections other than the first section. 
         [0011]    The load circuit may include an active load circuit. 
         [0012]    The reference signal may include a signal having a ramp waveform. 
         [0013]    The ADC circuit may include: a counter circuit configured to generate the digital data as a counting value corresponding to a difference in a length of double sampling sections determined according to the signal output from the comparator circuit based on the CDS process. 
         [0014]    The counter circuit may generate the digital data by performing up-counting during one of the double sampling sections and performing down-counting during another double sampling section, or performing up-counting or down-counting during the double sampling sections and changing a digital data code of one of the double sampling sections through bit-inversion between the double sampling sections. 
         [0015]    The image sensor may further include an amplification circuit between an output terminal of the comparator circuit and the ADC circuit. 
         [0016]    The amplification circuit may include an inverter or an amplifier. 
         [0017]    Each of the plurality of pixel sensors may include: a photoelectric conversion device configured to generate charges corresponding to an incident light; and a signal transfer circuit configured to transfer an electrical signal corresponding to the charges generated by the photoelectric conversion device to the column signal line. 
         [0018]    The signal transfer circuit may include: a first transistor connected between the photoelectric conversion device and the first node and configured to transmit the charges accumulated in the photoelectric conversion device to the first node according to a first driving signal; a second transistor connected between the first node and a power voltage and configured to reset the charges charged in the first node according to a second driving signal; a third transistor connected between the first node and a second node and configured to transfer a signal sensed by the first node to the second node; and a fourth transistor connected between the second node and the column signal line and configured to transfer a signal of the second node to the column signal line according to a third driving signal. 
         [0019]    According to another aspect of the inventive concept, there is provided an image processing apparatus including: an image sensor configured to convert an incident image signal into an electrical signal; and a processor configured to control an operation of the image sensor and post-processing a signal output from the image sensor, wherein the image sensor includes: a column signal line connected to output terminals of a plurality of pixel sensors; a comparator circuit configured to output a signal corresponding to a comparison result of a signal output to the column signal line and a reference signal; an analog/digital conversion (ADC) circuit configured to convert an analog signal corresponding to an optical signal sensed by the pixel sensor selected from the plurality of pixel sensors connected to the column signal line into digital data based on the signal output from the comparator circuit; and a load circuit connected in series to the comparator circuit between the column signal line and a ground terminal, wherein the load circuit is configured to operate as a common load device of the plurality of pixel sensors connected to the column signal line and the comparator circuit. 
         [0020]    The comparator circuit may include a transistor, wherein a reference signal having a ramp waveform is applied to a gate terminal of the transistor, the column signal line is connected to a first terminal of the transistor, and the load circuit is connected to a second terminal of the transistor. 
         [0021]    According to another aspect of the inventive concept, an image sensor comprises a column signal line connected to output terminals of a plurality of pixel sensors and configured to generate a column signal line output signal, a comparator circuit configured to output a comparison result signal corresponding to a comparison of the column signal line output signal and a reference signal, and a load circuit connected to the comparator circuit between the column signal line and a ground terminal and configured as a common load device of the plurality of pixel sensors connected to the column signal line and the comparator circuit. 
         [0022]    The comparator circuit comprises a transistor, a gate terminal of the transistor receives the reference signal, the column signal line is connected to a first terminal of the transistor, and the load circuit is connected to a second terminal of the transistor. 
         [0023]    The transistor may comprise a PMOS transistor. 
         [0024]    The comparator circuit may comprise a transistor, a capacitor, and a switch, a first terminal of the capacitor receives the reference signal, a gate terminal of the transistor and a first terminal of the switch are connected to a second terminal of the capacitor, the column signal line is connected to a first terminal of the transistor, and a second terminal of the switch and the load circuit are connected to a second terminal of the transistor. 
         [0025]    The switch may be in a turned on state during a first section before a correlated double sampling (CDS) process is performed and may be in a turned off during sections other than the first section. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    Exemplary embodiments of the inventive concept will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0027]      FIG. 1  is a block diagram of an image sensor, according to an embodiment of the inventive concept; 
           [0028]      FIGS. 2A through 2C  are exemplary circuit diagrams of pixel sensors of  FIG. 1 ; 
           [0029]      FIG. 3  is a circuit diagram of a comparator circuit connected to one column signal line of a pixel sensor of  FIG. 1 , according to an embodiment of the inventive concept; 
           [0030]      FIG. 4  is a circuit diagram of a comparator circuit connected to one column signal line of a pixel sensor of  FIG. 1 , according to another embodiment of the inventive concept; 
           [0031]      FIG. 5  is a circuit diagram of an amplification circuit added to a comparator circuit connected to one column signal line of the pixel sensor of  FIG. 3 , according to an embodiment of the inventive concept; 
           [0032]      FIGS. 6A and 6B  are circuit diagrams of the amplification circuit of  FIG. 5 , according to embodiments of the inventive concept; 
           [0033]      FIG. 7  is a circuit diagram of an amplification circuit added to a comparator circuit connected to one column signal line of the pixel sensor of  FIG. 4 , according to another embodiment of the inventive concept; 
           [0034]      FIGS. 8A and 8B  are circuit diagrams of the amplification circuit of  FIG. 7 , according to embodiments of the inventive concept; 
           [0035]      FIG. 9  is a circuit diagram of a load circuit of  FIG. 1 , according to an embodiment of the inventive concept; 
           [0036]      FIG. 10  is a circuit diagram of a load circuit of  FIG. 1 , according to another embodiment of the inventive concept; 
           [0037]      FIG. 11  is a timing diagram of main signals generated by an image sensor, according to an embodiment of the inventive concept; 
           [0038]      FIG. 12  is a timing diagram of main signals generated by an image sensor, according to another embodiment of the inventive concept; and 
           [0039]      FIG. 13  is a block diagram of an image processing apparatus according to embodiments of the inventive concept. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0040]    Hereinafter, the inventive concept will be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the inventive concept are shown. The present inventive concept may, however, be embodied in many different forms and should not be construed as limited to the example embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those of ordinary skill in the art. As the inventive concept allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in detail in the written description. However, this is not intended to limit the inventive concept to particular modes of practice, and it is to be appreciated that all changes, equivalents, and substitutes that do not depart from the spirit and technical scope of the inventive concept are encompassed in the inventive concept. It will be understood that when an element or layer is referred to as being “on,” “connected to” or “coupled with” another element or layer, it can be directly on, connected or coupled with the other element or layer or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly connected to” or “directly coupled with” another element or layer, there are no intervening elements or layers present. In the drawings, like reference numerals denote like elements and the sizes or thicknesses of elements may be exaggerated for clarity of explanation. 
         [0041]    The terms used in the present specification are merely used to describe particular embodiments, and are not intended to limit the inventive concept. An expression used in the singular encompasses the expression in the plural, unless it has a clearly different meaning in the context. In the present specification, it is to be understood that the terms such as “including” or “having,” etc., are intended to indicate the existence of the features, numbers, steps, actions, components, parts, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that one or more other features, numbers, steps, actions, components, parts, or combinations thereof may exist or may be added. 
         [0042]    Unless defined differently, all terms used in the description including technical and scientific terms have the same meaning as generally understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and this specification and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. 
         [0043]      FIG. 1  is a block diagram of an image sensor  100 , according to an embodiment of the inventive concept. 
         [0044]    Referring to  FIG. 1 , the image sensor  100  includes a pixel sensor array  10 , a comparator circuit block  20 , an analog/digital conversion (ADC) circuit block  30 , a load circuit block  40 , a buffer memory block  50 , a timing controller TCON  60 , a row driver  70 , and a ramp signal generator  80 . 
         [0045]    The pixel sensor array  10  includes a plurality of pixel sensors P  11  that are respectively connected to a plurality of column signal lines  12 - 1 ˜ 12 -m in a matrix shape. The comparator circuit block  20  includes a plurality of comparator circuits C  21  that are respectively connected to the column signal lines  12 - 1 ˜ 12 -m. The ADC circuit block  30  includes a plurality of ADC circuits  31 . The ADC circuit block  30  includes at least one of the ADCs  31  for each of the column signal lines  12 - 1 ˜ 12 -m. The load circuit block  40  includes a plurality of load circuits L  41 . The load circuit block  40  includes the single common load circuit L  41  for each of the column signal lines  12 - 1 ˜ 12 -m. 
         [0046]    The detailed construction and operation of the image sensor  100 , according to some embodiments of the inventive concept, will now be described below. 
         [0047]    The pixel sensor array  10  is briefly referred to as a pixel array. The pixel sensor array  10  includes the plurality of pixel sensors P  11 . The pixel sensors P  11  may include a plurality of color pixel sensors, for example, at least one red pixel sensor, at least one green pixel sensor, and at least one blue pixel sensor. 
         [0048]    If the image sensor  100  is implemented as a 3D image sensor, the pixel sensors P  11  may further include at least one depth pixel sensor in addition to the color pixel sensors. The depth pixel sensor may generate optical charges corresponding to wavelengths of an infrared region. 
         [0049]    The pixel sensor array  10  may include the plurality of column signal lines  12 - 1 ˜ 12 -m (m is a natural number). The pixel sensors P  11  that are arranged in a column direction may be respectively connected to the column signal lines  12 - 1 ˜ 12 -m. 
         [0050]      FIGS. 2A through 2C  are exemplary circuit diagrams of the pixel sensors P  11  of  FIG. 1 . 
         [0051]    Referring to  FIG. 2A , a pixel sensor  11   a  according to an embodiment of the inventive concept may be implemented as one photoelectric conversion device PD and four transistors M 1 ˜M 4 . 
         [0052]    The photoelectric conversion device PD that is an optical sensing device may be implemented as a photo diode, a photo transistor, a photo gate, or a pinned photo diode. 
         [0053]    The photoelectric conversion device PD is connected between a floating diffusion node FD and a ground terminal and generates charges corresponding to an incident optical signal. 
         [0054]    The transistor M 2  is connected between a power voltage terminal VDD and the floating diffusion node FD and functions to emit charges stored in the floating diffusion node FD in response to a driving signal RG. 
         [0055]    The transistor M 1  is connected between an output terminal of the power voltage terminal VDD and the floating diffusion node FD and functions to transmit the optical charges generated by the photoelectric conversion device PD to the floating diffusion node FD in response to another driving signal TG. 
         [0056]    The transistor M 3  functions as a source follower buffer amplifier and may perform a buffering operation in response to the charges stored in the floating diffusion node FD. 
         [0057]    A drain terminal of the transistor M 4  is connected to a source terminal of the transistor M 3 , and a source terminal thereof is connected to a node P of the column signal line  12 -i. Another driving signal SL is applied to a gate terminal of the transistor M 4 . 
         [0058]    Accordingly, the transistor M 4  may output a pixel signal PIX_OUT output from the transistor M 3  to the column signal line  12 -i in response to the driving signal SL. 
         [0059]    Referring to  FIG. 2B , a pixel sensor  11   b  according to another embodiment of the inventive concept may be implemented as the photoelectric conversion device PD and the three transistors M 2 ˜M 4 . 
         [0060]    Referring to  FIGS. 2A and 2B , the pixel sensor  11   b  of  FIG. 2B  has a structure in which the transistor M 1  functioning as a transmission transistor is deleted. 
         [0061]    Referring to  FIG. 2C , a pixel sensor  11   c  according to another embodiment of the inventive concept may be implemented as the photoelectric conversion device PD and five transistors M 1 ˜M 5 . The driving signal TG for controlling an operation of the transistor M 1  functioning as the transmission transistor is supplied to a gate of the transistor M 1  through the transistor M 5  that is turned on/off in response to the driving signal SL. 
         [0062]    Referring to  FIG. 1 , the timing controller  60  generates control signals necessary for selecting the pixel sensors P  11  or outputting image signals sensed by the pixel sensors P  11 . The timing controller  60  may control generation timing of a ramp signal necessary for performing a correlated double sampling (CDS) process and control output of data stored in the buffer memory block  50 . 
         [0063]    The row driver  70  outputs a plurality of driving signals necessary for controlling photoelectric conversion operations of the pixel sensors P  11  arranged in a row direction to the pixel sensor array  10  in response to the control signals. In this regard, the plurality of driving signals may include, for example, the driving signals RG, TG, and SL of  FIG. 11 . The plurality of driving signals may further include, for example, a driving signal AZP of  FIG. 12 . The driving signal AZP will be described in more detail below. 
         [0064]    The ramp signal generator  80  generates a ramp signal RAMP in response to the control signals and outputs the ramp signal RAMP to the comparator circuit block  20 . As shown in  FIG. 11  or  12 , the ramp signal generator  80  generates a signal having one ramp waveform before a pulse of the driving signal TO is generated, and generates a signal having one ramp waveform after the pulse of the driving signal TG is generated so as to perform CDS. 
         [0065]    The comparator circuits  21  are respectively connected to the column signal lines  12 - 1   12 -m. Various embodiments of the comparator circuit  21  will be described with reference to  FIGS. 3 and 4  below. 
         [0066]      FIG. 3  is a circuit diagram of a comparator circuit  21   a  connected to one column signal line  12 -i of the pixel sensor  11  of  FIG. 1 , according to an embodiment of the inventive concept. 
         [0067]    The image sensor  100  including the comparator circuit  21   a  of  FIG. 3  may generate, for example, the driving signals SL, RG, and TG at the timings of  FIG. 11 . 
         [0068]    Referring to  FIG. 3 , the comparator circuit  21   a  may be implemented as a transistor M 6 . For example, the transistor M 6  may be implemented as a PMOS transistor. 
         [0069]    The ramp signal RAMP output from the ramp signal generator  80  is applied to a gate terminal of the transistor M 6 , the column signal line  12 -i is connected to a source terminal of the transistor M 6 , and a first terminal of the load circuit  41  is connected to a drain terminal of the transistor M 6 . A second terminal of the load circuit  41  is connected to a ground terminal. An output terminal of the pixel sensor  11  is connected to the node P of the column signal line  12 -i. 
         [0070]    A signal COMP_OUT output from a node Q disposed in a signal line that is connected to the first terminal of the load circuit  41  and the drain terminal of the transistor M 6  is applied to the ADC circuit  31 . 
         [0071]    Referring to  FIGS. 2A to 2C , in a case where the transistor M 4  is turned on by the driving signal SL applied to the pixel sensors  11   a  to  11   c,  an output signal of the source follower transistor M 3  is applied to the column signal line  12 -i through the node P. The source follower transistor M 3  needs a load circuit. 
         [0072]    The transistor M 6  constituting the comparator circuit  21   a  also needs a load circuit. 
         [0073]    Referring to  FIG. 3 , the load circuit  41  has a circuit structure in which the load circuit  41  is connected in series to the comparator circuit  21   a  between the column signal line  12 -i and the ground terminal. Accordingly, the load circuit  41  operates as a load device of a pixel sensor connected to the column signal line  12 -i, and operates as a load device of the comparator circuit  21   a.  In other words, the load circuit  41  operates as a common load device of the source follower transistor M 3  included in the pixel sensor  11  and the transistor M 6  constituting the comparator circuit  21   a.    
         [0074]    In this regard, the load circuit  41  may be implemented as an active load circuit. An example of the active load circuit is shown in  FIG. 9 . 
         [0075]    Referring to  FIG. 9 , an active load circuit  41  a may be implemented as a transistor M 7 . For example, the transistor M 7  may be implemented as an NMOS transistor. More specifically, a drain terminal of the transistor M 7  is connected to the node Q, a source terminal of the transistor M 7  is connected to a ground terminal, and a load bias voltage (or current) is applied to a gate terminal of the transistor M 7 . A drain-source current of the transistor M 7  varies with the load bias voltage (or current). That is, the transistor M 7  enables a load value between the node Q and the ground terminal to vary with the load bias voltage (or current). Accordingly, the transistor M 7  operates as an active load. 
         [0076]      FIG. 10  shows a detailed example of an active load circuit  41   b  including a circuit for generating a load bias. 
         [0077]    Referring to  FIG. 10 , for example, transistors M 7  and M 8  may be implemented as NMOS transistors. More specifically, a drain terminal of the transistor M 7  is connected to the node Q, a source terminal of the transistor M 7  is connected to a ground terminal, and a gate terminal of the transistor M 7  is connected to a node R. A gate terminal and a drain terminal of the transistor M 8  are connected to the node R, and a source terminal of the transistor M 8  is connected to the ground terminal. A first terminal of a current source I 1  is connected to a power voltage terminal, and a second terminal thereof is connected to the node R. 
         [0078]    A gate-source voltage of the transistor M 7  is the same as a gate-source voltage of the transistor M 8 , and thus a drain-source current of the transistor M 7  is the same as a drain-source current of the transistor M 8 . That is, the transistor M 7  operates a current mirror circuit. 
         [0079]    Accordingly, the drain-source current of the transistor M 7  varies with a variation of a current value of the current source I 1 . The transistor M 7  enables a load value between the node Q and the ground terminal to vary with the current value of the current source I 1 . Accordingly, the transistor M 7  operates as an active load. 
         [0080]    Referring to  FIG. 3 , it is assumed that the pixel sensor  11  is implemented as, for example, the circuit of  FIG. 2A , and the driving signals SL, RG, and TG are generated at the timings of  FIG. 11 . It is also assumed that the ramp signal generator  80  generates the ramp signal RAMP at the timing of  FIG. 11 . 
         [0081]    Then, a voltage of the floating diffusion node FD of  FIG. 2A  is as shown in  FIG. 11 . The output signal PIX_OUT of the pixel sensor  11  disposed in the node P of the column signal line  12 -i has a waveform shown in  FIG. 11 . 
         [0082]    Referring to  FIG. 3 , if the ramp signal RAMP shown in  FIG. 11  is applied to the gate terminal of the transistor M 6  of the comparator circuit M 6 , the node Q connected to the drain terminal of the transistor M 6  generates the output signal COMP_OUT of the comparator circuit  21   a.  The PMOS transistor M 6  is turned on if a voltage applied to the gate terminal of the PMOS transistor M 6  is lower than a voltage obtained by subtracting a threshold voltage Vth from a voltage applied to the source terminal thereof, and is turned off if the voltage applied to the gate terminal of the PMOS transistor M 6  is not lower than the obtained voltage. 
         [0083]    Accordingly, as shown in  FIG. 11 , the output signal COMP_OUT of the comparator circuit  21   a  is in a logic high state HIGH in a case where a voltage of the ramp signal RAMP applied to the gate terminal of the transistor M 6  is lower than a voltage obtained by subtracting the threshold voltage Vth from a voltage of the pixel signal PIX_OUT applied to the drain terminal of the transistor M 6 , and is in a logic low state LOW if the voltage of the ramp signal RAMP applied to the gate terminal of the transistor M 6  is not lower than the obtained voltage. That is, it may be understood that the transistor M 6  operates as a comparator circuit for comparing the ramp signal RAMP with the output signal PIX_OUT of the pixel sensor  11 . 
         [0084]    The above-described output signal COMP_OUT of the comparator circuit  21   a  is applied to the ADC circuits  31 . 
         [0085]    Referring to  FIG. 1 , the ADC circuits  31  may be implemented as, for example, counter circuits for generating digital data with respect to a corresponding pixel as a counting value corresponding to a difference in a length between double sampling sections determined according to the output signal COMP_OUT of the comparator signal  21  based on a CDS process. 
         [0086]    For example, referring to  FIG. 11 , before a pulse of the driving signal TG is generated, the ramp signal generator  80  starts a up-counting operation at a time T 1  at which a signal having a ramp waveform is generated, and stops the up-counting operation at a time T 2  at which the output signal COMP_OUT is in a logic high state. After the pulse of the driving signal TG is generated, the ramp signal generator  80  starts a down-counting operation at a time T 3  at which the signal having the ramp waveform is generated, and stops the down-counting operation at a time T 4  at which the output signal COMP_OUT is in the logic high state. In this way, the digital data may be generated by operating the counter circuits as described above. 
         [0087]    For another example, referring to  FIG. 11 , before the pulse of the driving signal TG is generated, the ramp signal generator  80  starts the down-counting operation at the time T 1  at which the signal having the ramp waveform is generated, and stops the down-counting operation at the time T 2  at which the output signal COMP_OUT is in the logic high state. After the pulse of the driving signal TG is generated, the ramp signal generator  80  starts the up-counting operation at the time T 3  at which the signal having the ramp waveform is generated, and stops the up-counting operation at the time T 4  at which the output signal COMP_OUT is in the logic high state. In this way, the digital data may be generated by operating the counter circuits as described above. 
         [0088]    For another example, referring to  FIG. 11 , the digital data may be generated with respect to a corresponding pixel by performing the up- or down-counting operation on the two sampling sections T 1 -T 2  and T 3 -T 4  of the double sampling section, and changing a digital data code of one of the two sampling sections through bit inversion between the two sampling sections. 
         [0089]    The counter circuits that implement the ADC circuits  31  may be reset, for example, at a time at which a pulse of the driving signal RF is generated. 
         [0090]    Referring to  FIG. 1 , a plurality of pieces of pixel data generated by the ADC circuits  31  included in the ADC circuit block  30  are stored in the buffer memory block  50 . 
         [0091]    The pixel data stored in the buffer memory block  50  may be output to an image processor (not shown) under control of the timing controller  60 . 
         [0092]      FIG. 4  is a circuit diagram of a comparator circuit  21   b  connected to the column signal line  12 -i of the pixel sensor  11  of  FIG. 1 , according to another embodiment of the inventive concept. 
         [0093]    The image sensor  100  including the comparator circuit  21   b  of  FIG. 4  may generate the driving signals SL, RG, TG, and AZP, for example, at the timing shown in  FIG. 12 . 
         [0094]    Referring to  FIG. 4 , the comparator circuit  21   b  may be implemented as the transistor M 6 , a capacitor C 1 , and a switch SW 1 . For example, the transistor M 6  may be a PMOS transistor. 
         [0095]    The gate terminal of the transistor M 6  is connected to a node T, the source terminal of the transistor M 6  is connected to the node P of the column signal line  12 -i, and the drain terminal of the transistor M 6  is connected to the node Q. A first terminal of the switch SW 1  is connected to the node T, a second terminal thereof is connected to the node Q, and the driving signal AZP is applied to a control terminal of the switch SW 1 . A first terminal of the capacitor C 1  is connected to the node T, and the ramp signal RAMP output from the ramp signal generator  80  is applied to a second terminal of the capacitor C 1 . In this regard, the driving signal AZP is generated before, for example, a CDS process is performed. For example, as shown in  FIG. 12 , before the ramp signal generator  80  generates a signal having a ramp waveform, the row driver  70  may generate the driving signal AZP under control of the timing controller  60 . 
         [0096]    A first terminal of the load circuit  41  is connected to the node Q, and a second terminal thereof is connected to a ground terminal. An output terminal of the pixel sensor  11  is connected to the node P of the column signal line  12 -i. 
         [0097]    The pixel sensor  11  connected to the node P may be implemented as, for example, the circuits shown in  FIGS. 2A to 2C . 
         [0098]    The pixel sensor  11  and the load circuit  41  are described in detail with reference to  FIG. 3 , and, thus, redundant descriptions thereof will not be repeated here. 
         [0099]    As described with reference to  FIG. 3 , the load circuit  41  of  FIG. 4  operates as a common load device of the source follower transistor M 3  included in the pixel sensor  11  and the transistor M 6  constituting the comparator circuit  21   b.    
         [0100]    The comparator circuit  21   b  of  FIG. 4  further includes the capacitor C 1  and the switch SW 1  compared to the comparator circuit  21  of  FIG. 3 . 
         [0101]    Referring to  FIG. 4 , the capacitor C 1  and the switch SW 1  function to remove an offset of the transistor M 6  constituting the comparator circuit  21   b.  That is, the switch SW 1  is turned on during a section in which the driving signal AZP is in a logic high state HIGH. A voltage reflecting an offset of a threshold voltage of the transistor M 6  is applied to both terminals of the capacitor C 1  during the section in which the switch SW 1  is turned on. Thereafter, if the switch SW 1  is turned off, the voltage reflecting the offset of the threshold voltage of the transistor M 6  is applied to both terminals of the capacitor C 1 , and, thus, the offset of the transistor M 6  is removed. 
         [0102]      FIG. 5  is a circuit diagram of an amplification circuit  22  added to the comparator circuit  21   a  connected to one column signal line of the pixel sensor  11  of  FIG. 3 , according to an embodiment of the inventive concept. 
         [0103]    The pixel sensor  11 , the comparator circuit  21   a,  and the load circuit  41  of  FIG. 5  are described in detail with reference to  FIG. 3 , and, thus, redundant descriptions thereof will not be repeated here. 
         [0104]    Referring to  FIG. 5 , an input terminal of the amplification circuit  22  is connected to the node Q corresponding to an output terminal of the comparator circuit  21   a,  and an output terminal of the amplification circuit  22  is connected to the ADC circuits  31 . Accordingly, an output signal COMP_OUT 1  of the comparator circuit  21   a  output to the node Q is amplified by the amplification circuit  22 . A signal COMP_OUT 2  amplified by the amplification circuit  22  is applied to the ADC circuits  31 . 
         [0105]      FIGS. 6A and 6B  are circuit diagrams of the amplification circuit  22  of  FIG. 5 , according to embodiments of the inventive concept. 
         [0106]    Referring to  FIG. 6A , the amplification circuit  22  may be implemented as, for example, an amplifier  22   a,  such as an operational amplifier OP AMP. A first input terminal of the amplifier  22   a  is connected to the node Q corresponding to the output terminal of the comparator circuit  21   a,  and a reference voltage REF is applied to a second input terminal of the amplifier  22   a.  For example, the first input terminal of the amplifier  22   a  is set as a positive terminal+, and the second input terminal of the amplifier  22   a  may be set as a negative terminal −. For another example, the first input terminal of the amplifier  22   a  is set as the negative terminal −, and the second input terminal of the amplifier  22   a  may be set as the positive terminal +. The output terminal of the amplifier  22   a  is connected to the ADC circuits  31 . 
         [0107]    Referring to  FIG. 6B , the amplification circuit  22  may be implemented as, for example, an inverter  22   b.  An input terminal of the inverter  22   b  is connected to the node Q corresponding to the output terminal of the comparator circuit  21   a,  and an output terminal of the inverter  22   b  is connected to the ADC circuits  31 . 
         [0108]      FIG. 7  is a circuit diagram of the amplification circuit  22  added to the comparator circuit  21   b  connected to one column signal line of the pixel sensor  11  of  FIG. 4 , according to another embodiment of the inventive concept. 
         [0109]    The pixel sensor  11 , the comparator circuit  21   b,  and the load circuit  41  of  FIG. 7  are described in detail with reference to  FIG. 4 , and, thus, redundant descriptions thereof will not be repeated here. 
         [0110]    Referring to  FIG. 7 , an input terminal of the amplification circuit  22  is connected to the node Q corresponding to an output terminal of the comparator circuit  21   b,  and an output terminal of the amplification circuit  22  is connected to the ADC circuits  31 . Accordingly, the output signal COMP_OUT 1  of the comparator circuit  21   b  output to the node Q is amplified by the amplification circuit  22 . The signal COMP_OUT 2  amplified by the amplification circuit  22  is applied to the ADC circuits  31 . 
         [0111]      FIGS. 8A and 8B  are circuit diagrams of the amplification circuit of  FIG. 7 , according to embodiments of the inventive concept. 
         [0112]    Referring to  FIG. 8A , the amplification circuit  22  may be implemented as, for example, the amplifier  22   a,  such as the operational amplifier OP AMP. A first input terminal of the amplifier  22   a  is connected to the node Q corresponding to the output terminal of the comparator circuit  21   b,  and the reference voltage REF is applied to a second input terminal of the amplifier  22   a.  For example, the first input terminal of the amplifier  22   a  is set as a positive terminal +, and the second input terminal of the amplifier  22   a  may be set as a negative terminal −. For another example, the first input terminal of the amplifier  22   a  is set as the negative terminal −, and the second input terminal of the amplifier  22   a  may be set as the positive terminal +. The output terminal of the amplifier  22   a  is connected to the ADC circuits  31 . 
         [0113]    Referring to  FIG. 8B , the amplification circuit  22  may be implemented as, for example, the inverter  22   b.  An input terminal of the inverter  22   b  is connected to the node Q corresponding to the output terminal of the comparator circuit  21   b,  and an output terminal of the inverter  22   b  is connected to the ADC circuits  31 . 
         [0114]      FIG. 13  is a block diagram of an image processing apparatus  1000  according to embodiments of the inventive concept. For example, the image processing apparatus  1000  may be included in a computer apparatus, a camera apparatus, a cellular phone apparatus, a scanner apparatus, a navigation apparatus, a security system, and the like. 
         [0115]    Referring to  FIG. 13 , the image processing apparatus  1000  may include the image sensor  100 , a processor  200 , a non-volatile memory device  300 , a random access memory (RAM)  400 , an input/output device I/O  500 , and a bus  600 . 
         [0116]    The image sensor  100  of  FIG. 1  may be applied to the image sensor  100  of  FIG. 13 . The embodiments of the image sensor  100  of  FIGS. 2 through 10  may be applied to the image sensor  100  of  FIG. 13 . 
         [0117]    The processor  200  controls an operation of the image sensor  100  and performs signal post-processing on a signal output from the image sensor  100 . The processor  200  may transmit or receive data to or from elements connected through the bus  600 . 
         [0118]    The non-volatile memory device  300  may store image data on which post-processing is performed by the processor  200  or a program and data necessary for controlling the image processing apparatus  1000 . The non-volatile memory device  300  may be implemented as a non-volatile semiconductor memory device, for example, phase change RAM (FRAM), ferroelectric RAM (FRAM), magnetic RAM (MRAM), and the like. 
         [0119]    The RAM  400  may temporarily store data used in the image processing apparatus  1000 . 
         [0120]    An input device included in the input/output device  500  may be implemented as a keyboard, a mouse, a keypad, and the like, and an output device included therein may be implemented as a display, a printer, and the like. 
         [0121]    While the inventive concept has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood that various changes in form and details may be made therein without departing from the spirit and scope of the following claims.