Patent Publication Number: US-8994866-B2

Title: Analog-to-digital converter, photoelectric conversion device, and imaging system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to an analog-to-digital (A/D) converter, a photoelectric conversion device, and an imaging system. 
     2. Description of the Related Art 
     Japanese Patent Application Laid-Open No. 2008-259228 discusses a photoelectric conversion device used in digital cameras, digital camcorders, and the like. The photoelectric conversion device includes a column analog-to-digital converter (ADC), as one of exemplary systems thereof, in which each column of a pixel array includes an ADC. Among the systems used in the column ADC, a ramp type ADC is widely used. In the ramp type ADC, an analog signal and a reference signal which varies with time are compared in magnitude to each other in order to measure a time period after the reference signal starts varying before the magnitude relation therebetween is reversed. 
     In the ramp type ADC, the time period after the reference signal starts varying before the magnitude relation between the reference signal and the analog signal is reversed is measured by using a counter circuit. The counter circuit is controlled as to whether the counter circuit counts the clock signal by a count actuating signal. In other words, where the counter circuit receives a clock signal and a count actuating signal, however, if timing for inputting both signals is not managed, a count start time and a count completion time may be shifted from target timing upon performing the analog-to-digital (A/D) conversion. More specifically, in the photoelectric conversion device, even signals generated based on the same amount of incident light may be converted into different digital values. As a result, an image obtained therefrom may be degraded. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present disclosure, an analog-to-digital (A/D) converter includes a plurality of comparators each configured to compare a reference signal and an analog signal, a reference signal supply unit configured to supply the reference signal, which varies with time, to the plurality of comparators, and a counter circuit configured to count a first clock signal to output a count signal thereof. The A/D converter further includes a second clock signal generation unit configured to generate a second clock signal based on the first clock signal, and a clock synchronization unit configured to output a count start signal in synchronization with the second clock signal, wherein the counter circuit performs a counting operation in response to the count start signal synchronized with the second clock signal. 
     Further features and aspects of the present disclosure will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments, features, and aspects of the invention and, together with the description, serve to explain the principles as disclosed herein. 
         FIG. 1  illustrates an example of a configuration of a photoelectric conversion device according to a first exemplary embodiment. 
         FIG. 2  illustrates an example of a configuration of a count signal generation unit according to the first exemplary embodiment. 
         FIG. 3  illustrates an example of an operation of the photoelectric conversion device according to the first exemplary embodiment. 
         FIG. 4  illustrates an example of an operation of the count signal generation unit according to the first exemplary embodiment. 
         FIGS. 5A and 5B  illustrate an example of a possible operation in a case where the count signal generation unit according to the first exemplary embodiment is not used. 
         FIG. 6  illustrates an example of a configuration of each of the second clock signal generation unit and the clock synchronization unit according to the first exemplary embodiment. 
         FIG. 7  illustrates another example of a configuration of each of the second clock signal generation unit and the clock synchronization unit according to the first exemplary embodiment. 
         FIG. 8  illustrates yet another example of a configuration of each of the second clock signal generation unit and the clock synchronization unit according to the first exemplary embodiment. 
         FIG. 9  illustrates an example of a configuration of a photoelectric conversion device according to a second exemplary embodiment. 
         FIG. 10  illustrates an example of an operation of the photoelectric conversion device according to the second exemplary embodiment. 
         FIG. 11  illustrates an example of a configuration of a count signal generation unit according to the second exemplary embodiment. 
         FIG. 12  illustrates an example of an operation of the count signal generation unit according to the second exemplary embodiment. 
         FIG. 13  is a block diagram illustrating an example of a configuration of an imaging system according to a third exemplary embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Various exemplary embodiments, features, and aspects of the disclosure will be described in detail below with reference to the drawings. 
     In the present exemplary embodiment, a photoelectric conversion device in which an ADC is used as a column ADC is exemplified. 
       FIG. 1  illustrates an example of a configuration of a photoelectric conversion device according to the present exemplary embodiment. A photoelectric conversion device  100  includes a pixel array  1 , column read-out units  2 , a timing signal generation unit  5 , a reference signal supply unit  6 , a first clock signal generation unit  7 , a count signal generation unit  8 , and a signal processing unit  9 . 
     The pixel array  1  includes a plurality of columns of pixels. Each of the column read-out units  2  is provided to each column of the pixels in a corresponding manner. Each of the column read-out units  2  has a function of an ADC for converting an analog signal from the corresponding column of the pixels into a digital signal and includes a comparator  3  and a storage unit  4 . In the present exemplary embodiment, each storage unit  4  includes a first storage unit  4   n  and a second storage unit  4   s . Each column read-out unit  2  further includes an amplifier AMP for amplifying a signal output from the pixel array  1 . The amplifier AMP may be either one of an inverting amplifier or a variable gain amplifier. 
     The timing generation unit  5  generates various signals for controlling an operation of the photoelectric conversion device  100 . The timing generation unit  5  according to the present exemplary embodiment supplies a ramp control signal RMP_EN to the reference signal supply unit  6 . The timing generation unit  5  further supplies a count start signal CNT_EN and a reset signal RST to the count signal generation unit  8 . 
     The reference signal supply unit  6  supplies a ramp signal as a reference signal to a plurality of comparators  3 . A signal level of the ramp signal monotonically varies with the passage of time. The “monotonically varying” means that, for example, the signal level decreases, but not increases, with the passage of time in a case of a monotonic decrease and further means that the signal level decreases step by step. In the reference signal supply unit  6 , a start and an end of the varying of the reference signal are controlled in response to the ramp control signal RMP_EN supplied from the timing generation unit  5 . 
     The first clock signal generation unit  7  applies a first clock signal CLK 1  to the count signal generation unit  8 . 
     The count signal generation unit  8  supplies a count signal CNT_OUT to the storage unit  4  in response to the count start signal CNT_EN, a reset signal RST, and the ramp control signal RMP_EN. 
     The signal processing unit  9  is a circuit for processing a digital signal output from the storage unit  4 . Exemplified as the processing performed by the signal processing unit  9  is differential processing provided to two signals output from the first storage unit  4   n  and the second storage unit  4   s.    
     In  FIG. 1 , a vertical scanning unit for selecting a row of the pixel array and a horizontal scanning unit for selecting the storage unit  4  are omitted. 
     The count signal generation unit  8  is described below with reference to  FIG. 2 . The count signal generation unit  8  includes a second clock signal generation unit  31 , a clock synchronization unit  32 , and a counter circuit  33 . 
     The second clock signal generation unit  31  receives the first clock signal CLK 1  and the reset signal RST. The second clock signal generation unit  31  outputs a second clock signal CLK 2  based on the first clock signal CLK 1 . The second clock signal CLK 2  causes the first clock signal CLK 1  to delay a phase thereof and to vary frequency thereof. The second clock signal generation unit  31  outputs the second clock signal CLK 2  fixed at level H or level L in a period in which the reset signal RST is at level H. The clock synchronization unit  32  outputs an internal count start signal CNT_ENI obtained such that the count start signal CNT_EN is brought into synchronization with the second clock signal CLK 2  in response to the count start signal CNT_EN and the second clock signal CLK 2 . The counter circuit  33  performs a counting operation for counting the first clock signal CLK 1  in a period in which the internal count start signal CNT_ENI at level H is input in response to the first clock signal CLK 1  and the internal count start signal CNT_ENI. 
     An A/D conversion operation performed by the photoelectric conversion device  100  is summarized below with reference to  FIG. 3 .  FIG. 3  selectively illustrates an operation of the A/D conversion provided to a signal from the pixels of a certain row. 
     In  FIG. 3 , a signal MSEL_N determines whether an output of the comparator  3  causes the count signal to be stored in the first storage unit  4   n . If the output of the comparator  3  reaches the level H in a period in which the signal MSEL_N is also at the level H, a count signal at a point of time at which the output of the comparator  3  is shifted to level H is stored in the first storage unit  4   n . Similarly, a signal MSEL_S determines whether the output of the comparator  3  causes the count signal to be stored in the second storage unit  4   s . In a case where the output of the comparator  3  reaches level H in a period in which the signal MSEL_S is at level H, a count signal at a point of time at which the output of the comparator  3  is shifted to level H is stored in the second storage unit  4   s . A potential Vin is a potential of an input terminal on a side whereat an analog signal of the comparator  3  is input. 
     In a case where the reset signal RST temporally reaches level H from time t 10 , the second clock signal generation unit  31  is reset. 
     Thereafter, the signal CNT_EN reaches level Hat time t 11  whereat the A/D conversion period for converting an N signal is started. During the N signal A/D conversion period, the signal MSEL_N is at level H and the signal MSEL_S is at level L, respectively. The N signal is generated due to a reset of a pixel. The N signal is composed of a noise and an offset. For example, in a case of the pixel including a pixel amplifier for outputting a signal corresponding to a charge amount generated in a photoelectric conversion unit, the N signal may include components generated due to the reset of an input unit of the pixel amplifier. In a case of a configuration including the amplifier AMP, the N signal may include an offset generated by the amplifier AMP. During the N signal A/D conversion period, the ramp control signal RAMP_EN reaches level H and thus the potential of the reference signal varies with the passage of time. 
     In a case where the magnitude relation between the potential Vin and the ramp signal is reversed at time t 12  during the N signal A/D conversion period, an output of the counter circuit  33  is stored in the first storage unit  4   n  in response to the reversal. 
     When the N signal A/D conversion period ends at time t 13 , an S signal A/D conversion period starts from time t 14 . In the S signal A/D conversion period, the signal MSEL_S is at level H and the signal MSEL_N is at level L. The S signal is obtained based on an electrical charge generated in the photoelectric conversion unit. Signal amplitude of a pixel including the above exemplified pixel amplifier becomes larger by an amount corresponding to the charge amount generated in the photoelectric conversion unit with respect to the N signal. More specifically, the S signal includes the N signal here. Therefore, by taking a difference between the S signal and the N signal, the noise and the offset can be decreased. In the configuration illustrated in  FIG. 1 , the noise and the offset can be decreased by subjecting the signals held in the first storage unit  4   n  and the second storage unit  4   s  to the differential processing in the signal processing unit  9 . 
     In a case where the magnitude relation between the potential Vin and the ramp signal is reversed at time t 15  during the S signal A/D conversion period, the output of the counter circuit  33  is stored in the second storage unit  4   s  in response to the reversal. According to the above, the N signal and the S signal are subjected to the A/D conversion to be stored in the storage unit  4 . Thereafter, a horizontal scanning unit (not illustrated) transmits the signal held in the storage unit  4  to the signal processing unit  9 . Then, an operation for 1 row is ended. A repetition of the above operation completes the conversion of 1 frame. 
     An operation of the count signal generation unit  8  is described below with reference to  FIG. 4 . A case where the second clock signal generation unit  32  divides a frequency of the first clock signal into halves, i.e., into 2 stages, thereby generating a second clock signal CLK 2 , is exemplified. A clock signal obtained such that the frequency of the first clock signal is divided into halves is referred to as a clock signal CLKIN. In an initial state, the output of the counter circuit  33  is 0. The counter performs a counting operation by n-bit digital signals. The counter may be either one of the binary counter and a gray counter. Furthermore, the counter may be any other counter. In  FIG. 4 , a count value is indicated by using a decimal digit for the sake of an easy understanding. 
     At time t 0 , the reset signal RST is shifted to level H. Since the second clock signal generation unit  31  does not output the second clock signal CLK 2  during a period in which the reset signal RST is at level H, both of the clock signal CLKIN and the second clock signal CLK 2  are at level L. The reset signal RST is not necessarily synchronized with the first clock signal CLK 1 . At the time t 0 , since a level of the count start signal CNT_EN is low, the internal count start signal CNT_ENI is also at level L. Therefore, the counter circuit  33  keeps the output of its own to 0 without performing the counting operation. 
     After the reset signal RST is shifted to level L, the second clock signal generation unit  31  starts an operation in synchronization with a rise of the first clock signal CLK 1  at time t 1 . A level of the intermediate clock signal CLKIN is shifted in synchronization with the rise of the first clock signal CLK 1 . A level of the second clock signal CLK 2  is shifted in synchronization with a fall of the intermediate clock signal CLKIN. 
     At time t 2 , the count start signal CNT_EN is shifted to level H. The clock synchronization unit  32  causes the count start signal CNT_EN to be output as the internal count start signal CNT_ENI synchronized with the second clock signal CLK 2 . Therefore, the counter circuit  33  starts its counting operation from time t 4  later than time t 3 . 
     The counter circuit  33  performs its counting operation in synchronization with the first clock signal CLK 1 , so that the counter circuit  33  continues, also on and after time t 4 , its operation at the same frequency as that of the first clock signal CLK 1  during a period in which the internal count start signal CNT_ENI is at level H. 
     The operation of the counter circuit  33  is summarized above. To describe an effect of the present exemplary embodiment, a case where a start of the counting operation of the counter circuit  33  is controlled by the count start signal CNT_EN is considered below. In this case, the counting operation of the counter circuit  33  is controlled by the count start signal CNT_EN that is asynchronous with any of the clock signals. 
     In  FIG. 4 , the second clock signal generation unit  31  is reset every time a row of the pixel array is selected. In other words, the second counter circuit is reset once in a horizontal synchronization period. However, the second counter circuit is not reset in each horizontal synchronization period but the second counter circuit may be reset at least once in a frame, i.e., in a vertical synchronization period. A horizontal synchronization signal for defining the horizontal synchronization period and a vertical synchronization signal for defining the vertical synchronization period are generated in the timing signal generation unit  5  or provided from an external device. 
     Possible problems raised in this configuration are described below with reference to  FIGS. 5A and 5B . The count start signal CNT_EN and the “comparator output” are common to both of  FIGS. 5A and 5B . An example of  FIG. 5A  differs from an example of  FIG. 5B  in that a phase difference of the first clock signal CLK 1  with respect to the rise of the count start signal CNT_EN differs to each other. In a case of  FIG. 5A , the first clock signal CLK 1  is at level H at time tA at which the count start signal CNT_EN rises. A count signal CNT_OUT output by the counter circuit  33  is shifted from 0 to 1 at time tC. In a case of  FIG. 5B , the first clock signal CLK 1  is at level L at time tA at which the count start signal CNT_EN rises. Since a phase of the count signal CNT_OUT output by the counter circuit  33  delays with respect to the first clock signal CLK 1  of  FIG. 5A , the count signal CNT_OUT is shifted from 0 to 1 at time tB earlier than time tC. 
     If it is provided that the output of the comparator  3  for causing the count signal CNT_OUT to be stored in the storage unit  4  varies at time tD, different values are written in the storage unit  4  in a case of  FIG. 5A  and a case of  FIG. 5B  even in a case where time periods between the time tA and the time tD are the same in both cases. In other words, the digital signals obtained therefrom become different values despite that the analog signals to be subjected to the A/D conversion are the same. In the photoelectric conversion device controlled row by row as illustrated in  FIG. 1 , even when uniform light is irradiated to the rows, the analog signals are converted into digital signals having different values per each row. Therefore, a resulting image includes a horizontal stripe. When compared with a randomly occurring noise, a noise appearing in the form of a line is visible with ease, resulting in showing a remarkable degradation of an image quality. 
     Contrary to the above described configuration, in the present exemplary embodiment, since the counting operation of the counter circuit  33  is controlled by using the internal count start signal CNT_ENI obtained such that the second clock signal CLK 2  generated based on the first clock signal CLK 1  is brought into synchronization with the count start signal CNT_EN, shifting up to 4 cycles is allowed with respect to the first clock signal CLK 1  in a case exemplified in  FIG. 2 . In other words, regardless of what timing the count start signal CNT_ENI rises during a period between the time t 5  and the time t 3 , the internal count start signal CNT_ENI rises at the time t 3 . Therefore, the operation of the counter circuit  33  can be controlled with high accuracy. 
       FIG. 6  illustrates an example of a configuration of each of the second clock signal generation unit  31  and the clock synchronization unit  32 . The second clock signal generation unit  31  operates as the second counter circuit. The second clock signal generation unit  31  is a synchronous counter circuit composed of two flip-flops  63 - 1  and  63 - 2  and an exclusive OR circuit (XOR circuit)  61 . An inverter circuit  62  inverts the reset signal RST to input the inverted reset signal RST into the flip-flop  63  in the form of a signal RSTb. A Q-terminal of the flip-flop  63 - 1  is connected to one of input terminals of the XOR circuit  61 . A QB-terminal for outputting an inversion signal output via the Q-terminal is connected to a D-terminal. A Q-terminal of a flip-flop  63 - 2  is connected to the other one of the input terminals of the XOR circuit  61  as well as connected to a CK-terminal of a flip-flop  63 - 3 . An output terminal of the XOR circuit  61  is connected to a D-terminal of the flip-flop  63 - 2 . The configuration enables generation of the second clock signal CLK 2  which is brought into synchronization with the first clock signal CLK 1  and of which frequency is divided into quarter of the first clock signal CLK 1 . The clock synchronization unit  32  can realize the above described operation by the single-stage flip-flop  63 - 3 . When the flip-flop  63 - 3  receives the second clock signal CLK 2  and the count start signal CNT_EN, the clock synchronization unit  32  outputs a count actuating signal CNT_ENI synchronized with the second clock signal CLK 2 . The reset signal RST may be applied to the flip-flop  63 - 3  of the clock synchronization unit  32  once at the time of startup of the photoelectric conversion device. Alternatively, the inverted reset signal RSTb to be input into the flip-flop  63  of the second clock signal generation unit  31  may be applied to the flip-flop  63 - 3  of the clock synchronization unit  32 . 
     The second clock signal generation unit  31  and the clock synchronization unit  32  may be configured in a manner as illustrated in  FIG. 7  in addition to the example illustrated in  FIG. 6 . In the configuration of  FIG. 7 , each of the flip-flop  63 - 1  and the flip-flop  63 - 2  receives the first clock signal CLK 1  via an inverter  62 ′. In this case, in the timing diagram of  FIG. 4 , the clock signal CLKIN and the second clock signal CLK 2 , of which frequencies are divided in synchronization with the fall of the first clock signal CLK 1 , rise. 
       FIG. 6  illustrates a case where the second clock signal generation unit  31  is configured by using a synchronous counter circuit. However, the second clock signal generation unit  31  may also be configured by using asynchronous counter circuit as illustrated in  FIG. 8 . The configuration of  FIG. 8  differs from the configuration of  FIG. 6  in that the output of the Q-terminal of the flip-flop  63 - 1  is connected to a CK-input terminal of the flip-flop  63 - 2  and in that the output of the QB-terminal as an inverted output of the flip-flop  63 - 2  is connected to a D-input terminal of the flip-flop  63 - 2 . More specifically, both flip-flops  63 - 1  and  63 - 2  operate in synchronization with the first clock signal CLK 1  in the exemplary configuration of  FIG. 6 , whereas only the flip-flop  63 - 1  operates in synchronization with the first clock signal CLK 1  in the exemplary configuration of  FIG. 8 . 
     In the above description, a configuration of a frequency divider in which a frequency of the first clock signal CLK 1  is divided into quarter to generate the second clock signal CLK 2  is exemplified. The frequency of the first clock signal CLK 1  may be divided into (1/N)-fold, provided that N is a natural number. 
     As described above, in the present exemplary embodiment, the counting operation of the counter circuit  33  is controlled by the internal count start signal CNT_ENI obtained such that the second clock signal CLK 2  generated based on the first clock signal CLK 1  is brought into synchronization with the count start signal CNT_EN. The configuration enables decrease of the shifting of the count start time of the counter circuit  33 . As a result, the operation of the counter circuit  33  can be controlled with high accuracy. In the photoelectric conversion device, a stripe-shaped noise possibly generated on an image to be obtained can be decreased. 
     A second exemplary embodiment of the present invention is described below.  FIG. 9  illustrates an example of a configuration of a photoelectric conversion device according to the present exemplary embodiment. Components similar to those of the photoelectric conversion device according to the first exemplary embodiment are provided with the similar reference numbers and descriptions thereof are omitted here. 
     A photoelectric conversion device  100 ′ differs from the photoelectric conversion device  100  in that the photoelectric conversion device  100 ′ includes storage units  40  instead of the storage units  4  and includes a clock signal supply unit  80  instead of the count signal generation unit  8 . 
     Each storage unit  40  includes an up-down counter and receives a count clock signal CNT_CLK from the clock signal supply unit  80  and an up-down selection signal UD_SEL and a counter reset signal CNT_RST from a timing signal generation unit  5 . Further, the storage unit  40  receives the output of the comparator  3 . The up-down selection signal UD_SEL is a signal for making a selection, when the up-down counter counts the count clock signal, as to whether a count-up for increasing a count value is to be performed or whether a count-down for decreasing the count value is to be performed. The signals are shared by a plurality of storage unit  40 . 
     The clock signal supply unit  80  receives the first clock signal CLK 1  from the first clock signal generation unit  7  and the count start signal CNT_EN and the reset signal RST from the timing signal generation unit  5 . 
     The present exemplary embodiment differs from the first exemplary embodiment in that the counter circuit  33  included in the count signal generation unit  8  is shared by the storage units  4  of the plurality of columns in the first exemplary embodiment, whereas a counter circuit is provided to each column of the pixel array  1  in the present exemplary embodiment. 
     An operation of the photoelectric conversion device  100 ′ according to the present exemplary embodiment is described below with reference to  FIG. 10 . As similar to a case of the first exemplary embodiment, operations performed in the N signal A/D conversion period and the S signal A/D conversion period are illustrated. The count clock signal CNT_CLK performs a counting operation during a period in which the count start signal CNT_EN is at level H. 
     In the operation illustrated in  FIG. 10 , the up-down counter performs the count-down when the up-down selection signal UD_SEL reaches level H during the N signal A/D conversion period, whereas, the up-down counter performs the count-down when the up-down selection signal UD_SEL reaches level L in the S signal A/D conversion period. The up-down counter stops its counting operation in response to the shifting of the output of the comparator  3  from level L to level H in each A/D conversion period. Since the up-down counter is not reset during a period between the N signal A/D conversion period and the S signal A/D conversion period, the up-down counter of each column stores digital data corresponding to a difference between the S signal and the N signal after the end of the S signal A/D conversion period. The configuration enables obtainment of the difference between the S signal and the N signal even without providing the first storage unit  4   n  and the second storage unit  4   s  to each column of the pixel array  1 . 
       FIG. 11  illustrates an example of a configuration of a clock signal supply unit  80 . The clock signal supply unit  80  includes a second clock signal generation unit  31 , a clock synchronization unit  32 , and an AND circuit  91 . The clock signal supply unit  80  differs from the count signal supply unit  8  of  FIG. 3  in that the AND circuit  91  is provided instead of the counter circuit  33 . Also, in the configuration, as similar to the case of the counter signal supply unit  8 , the second clock signal generation unit  31  generates an internal count start signal CNT_ENI such that the second clock signal CLK 2  generated based on the first clock signal CLK 1  is brought into synchronization with the count start signal CNT_EN. An AND operation between the first clock signal CLK 1  and the internal count signal CNT_ENI is supplied to the plurality of storage units  40  as the count clock signal CNT_CLK. 
     An operation of the clock signal supply unit  80  is described below with reference to  FIG. 12 . In  FIG. 12 , the input terminal to which the first clock signal CLK 1  is applied among the input terminals of the second clock signal generation unit  31  is provided with an inverter. 
       FIG. 12  illustrates, in addition to the first clock signal CLK 1 , the reset signal RST, and the count start signal CNT_EN all to be supplied to the clock signal supply unit  80 , an internal count signal CLKIN and a second clock signal CLK 2  both generated in the second clock signal generation unit  31 , an internal count actuating signal CNT_ENI generated in the clock synchronization unit  32 , and a count clock signal CNT_CLK output from the clock signal supply unit  80 . In the present exemplary embodiment, the second clock signal generation unit  31  generates the internal count signal CLKIN obtained by dividing the frequency of the first clock signal CLK 1  into halves, and further generates the second clock signal obtained by dividing the frequency of the internal count signal CLKIN into halves. 
     When the second clock signal generation unit  31  receives the reset signal RST at level H at time t 0 , the second clock signal generation unit  31  stops the generation of the second clock signal CLK 2 . 
     After the reset signal RST is shifted to level L, the second clock signal generation unit  31  starts the counting operation from time t 1  at which the first clock signal CLK 1  is shifted to level L. On and after the time t 1 , the internal clock signal CLKIN rises in synchronization with the fall of the first clock signal CLK 1 , and the second clock signal CLK 2  rises in synchronization with the fall of the internal clock signal CLKIN. 
     At the time t 2 , the count start signal CNT_EN reaches level H. The clock synchronization unit  32  outputs the internal count start signal CNT_ENI obtained such that the count start signal CNT_EN is brought into synchronization with the second clock signal CLK 2 . Therefore, the internal count start signal CNT_ENI is shifted to level H in synchronization with the rise of the second clock signal CLK 2  at the time t 3 . 
     The AND circuit  91  brings the internal count start signal CNT_ENI into synchronization with the first clock signal CLK 1 . Therefore, during the period in which the internal count signal CNT_ENI is at level H, the AND circuit  91  outputs the count clock signal CNT_CLK in synchronization with the rise of the first clock signal CLK 1 . In response to thus generated count clock signal CNT_CLK and the first clock signal CLK 1 , the counter circuit of each column performs the counting operation. 
     The second clock signal generation unit  31  and the clock synchronization unit  32  in the clock signal supply unit  80  can be realized with the configuration of  FIG. 7 . 
     As described above, in the present exemplary embodiment, the counter circuit is operated by using the internal count start signal CNT_ENI obtained such that the count start signal CNT_EN is brought into synchronization with the second clock signal CLK 2  generated based on the first clock signal CLK 1 . The configuration enables decrease of the shifting of the time for starting the counting operation performed by the counter circuit. As a result, the operation of the counter circuit can be controlled with high accuracy. Specifically, in the photoelectric conversion device, generation of the stripe-shaped noise can be decreased. 
     An imaging system according to a third exemplary embodiment is summarized below with reference to  FIG. 13 . 
     An imaging system  800  includes, for example, an optical unit  810 , a photoelectric conversion device  1000 , a video signal image processing circuit unit  830 , a recording and communicating unit  840 , a timing control circuit unit  850 , a system control circuit unit  860 , and a reproducing and displaying unit  870 . The photoelectric conversion device described in each of the above exemplary embodiments is used as the photoelectric conversion device  1000 . A case where the timing signal generation unit  5  illustrated in  FIGS. 1 and 9  is included not in the photoelectric conversion device but in the timing control circuit unit  850  is illustrated. 
     The optical unit  810  as an optical system, e.g., lens, causes light from an object to be formed into an image on the pixel array composed of a plurality of two-dimensionally arranged pixels of a photoelectric conversion device  1000 , thereby forming the image of the object. The photoelectric conversion device  1000  outputs a signal according to the light formed into the image on the pixel array at timing of receiving a signal from the timing control circuit unit  850 . 
     A video signal image processing circuit unit  830  as a video signal processing unit receives the signal output from the photoelectric conversion device  1000 . Then, the video signal image processing circuit unit  830  performs processing such as a noise reduction and a gain adjustment to the signal according to a method defined by a program. The recording and communicating unit  840  receives the signal obtained by the processing of the video signal image processing circuit unit  830  in the form of image data. The recording and communicating unit  840  transmits a signal for forming an image to the reproducing and displaying unit  870  to cause the reproducing and displaying unit  870  to reproduce and display a moving image or a still image. The recording and communicating unit  840  communicates with a system control circuit unit  860  in response to a signal from the video signal image processing circuit unit  830 . The recording and communicating unit  840  also records a signal for forming an image on a recording medium (not illustrated). 
     The system control circuit unit  860  generally controls an operation of the imaging system and controls driving of the optical unit  810 , the timing control circuit unit  850 , the recording and communicating unit  840 , and the reproducing and displaying unit  870 . The system control circuit unit  860  includes, for example, a storage device (not illustrated) as a recording medium and a program required for controlling the operation of the image system recorded therein. The system control circuit unit  860  supplies a signal for switching a driving mode according to, for example, an operation of the user within the imaging system. Specific examples thereof include a change of a row to be read out and a row to be reset, a change of an angle of view corresponding to an electrical zooming, and shifting of the angle of view corresponding to an electronic image stabilization. 
     The timing control circuit unit  850  controls driving timings of the photoelectric conversion device  1000  and the video signal image processing circuit unit  830  based on a control by the system control circuit unit  860  as a control unit. 
     The video signal image processing circuit unit  830  holds the correction coefficient described in each of the above described exemplary embodiments and performs correction processing to the signal output from the photoelectric conversion device  1000 . 
     While the present disclosure has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation to encompass all modifications, equivalent structures, and functions. 
     This application claims priority from Japanese Patent Application No. 2011-223296 filed Oct. 7, 2011, which is hereby incorporated by reference herein in its entirety.