Patent Publication Number: US-8994575-B2

Title: Time detection circuit, ad converter, and solid state image pickup device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application based on a PCT Patent Application No. PCT/JP2011/062519, filed May 31, 2011, whose priority is claimed on Japanese Patent Application No. 2010-177756, filed on Aug. 6, 2010, the entire content of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a time detection circuit, an analog-to-digital (AD) converter, and a solid state image pickup device. 
     BACKGROUND ART 
     2. Description of the Related Art 
     As an example of a time detection circuit of the related art, a configuration illustrated in  FIG. 8  (for example, see Japanese Unexamined Patent Application, First Publication, No. 2009-38726 and Japanese Unexamined Patent Application, First Publication, No. 2009-38781) is well known. First, a configuration and operation of the time detection circuit of  FIG. 8  will be described. 
       FIG. 8  illustrates the time detection circuit in accordance with an example of the related art. The time detection circuit illustrated in  FIG. 8  includes a delay unit  30 , a comparison unit  31 , a latch unit  33 , and a count unit  34 . The delay unit  30  includes a plurality of delay units DU[ 0 ] to DU[ 7 ], each of which delays and outputs an input signal. A start pulse (=StartP) is input to the leading delay unit DU[ 0 ]. The comparison unit  31  includes a voltage comparator, which receives an analog signal Signal serving as a time detection target and a ramp wave Ramp that decreases with the passage of time and outputs a signal indicating a result obtained by comparing the analog signal Signal with the ramp wave Ramp. The latch unit  33  includes latch circuits D_ 0  to D_ 7  that latch logic states of outputs CK 0  to CK 7  of the delay unit  30 . The count unit  34  includes a counter circuit that performs a count operation based on the output CK 7  from the delay unit  30 . 
     In the comparison unit  31 , a time interval (a magnitude of a time-axis direction) corresponding to an amplitude of the analog signal Signal is generated. A buffer circuit is an inverting buffer circuit that inverts an input signal and outputs the inverted input signal. Here, a configuration of the inverting buffer circuit is provided for a better understanding of the description of the specification. 
     The latch circuits D_ 0  to D_ 7  constituting the latch unit  33  are in an enable (valid) state when an output Hold of the buffer circuit is high, and directly output the outputs CK 0  to CK 7  of the delay units DU[ 0 ] to DU[ 7 ]. In addition, the latch circuits D_ 0  to D_ 7  are in a disable (invalid) state when the output Hold of the buffer circuit transitions from a high level to a low level. At this time, the latch circuits D_ 0  to D_ 7  latch logic states corresponding to the outputs CK 0  to CK 7  of the delay units DU[ 0 ] to DU[ 7 ]. 
     A control signal RST is a signal for performing a reset operation of the counter circuit constituting the count unit  34 . Further, although a count latch circuit that latches a logic state of a count result of the count unit  34  is not explicitly illustrated, a counter circuit having a latch function is used and hence the counter circuit also includes the count latch circuit. 
     Next, an operation of the example of the related art will be described.  FIG. 9  illustrates the operation of the time detection circuit in accordance with the related art. 
     First, at a timing (first timing) relating to a comparison start in the comparison unit  31 , a clock having a cycle approximately consistent with a delay time of the delay unit  30  is input as the start pulse (=StartP) to the delay unit  30 . Thereby, the operation of the delay unit  30  is started. The delay unit DU[ 0 ] constituting the delay unit  30  outputs the output CK 0  by inverting and delaying the start pulse (=StartP), and the delay units DU[ 1 ] to DU[ 7 ] constituting the delay unit  30  output the outputs CK 1  to CK 7  by inverting and delaying the outputs of previous-stage delay units, respectively. The outputs CK 0  to CK 7  of the delay units DU[ 0 ] to DU[ 7 ] are input to the latch circuits D_ 0  to D_ 7  of the latch unit  33 . Because the output Hold of the buffer circuit is high, the latch circuits D_ 0  to D_ 7  are in the enable state, and directly output the outputs CK 0  to CK 7  of the delay units DU[ 0 ] to DU[ 7 ]. 
     The count unit  34  performs a count operation based on the output CK 7  of the delay unit  30  output as an output Q 7  of the latch circuit D_ 7  of the latch unit  33 . In this count operation, a count value is increased or decreased by the rising or falling of the output CK 7 . At a timing (second timing) at which the analog signal Signal is approximately consistent with the ramp wave Ramp, an output CO is inverted. After the output CO of the comparison unit  31  has been buffered by the buffer circuit (a third timing), the output Hold of the buffer circuit becomes low. 
     Thereby, the latch circuits D_ 0  to D_ 7  are in the disable state. At this time, the logic states corresponding to the outputs CK 0  to CK 7  of the delay units DU[ 0 ] to DU[ 7 ] are latched in the latch circuits D_ 0  to D_ 7 . 
     The latch circuit D_ 7  is disabled, and hence the count unit  34  latches a count value. Data corresponding to the analog signal Signal is obtained according to the logic state latched by the latch unit  33  and the count value latched by the count unit  34 . 
     According to the time detection circuit in accordance with the above-described example of the related art, it is possible to obtain data corresponding to a time interval. That is, it is possible to detect a time corresponding to the time interval. 
     It is possible to configure an AD converter that converts an analog signal into a digital signal using the above-described time detection circuit. 
     In the above-described time detection circuit, the latch circuits D_ 0  to D_ 6  constituting the latch unit  33  operate in a period of a time interval. Accordingly, a value of current consumption by the latch unit  33  is increased and it is difficult to reduce current consumption of the time detection circuit. 
     In the time detection circuit of the example of the related art, the latch circuits D_ 0  to D_ 6  constituting the latch unit  33  constantly operate in a period from the first timing to the third timing. Because the outputs CK 0  to CK 7  of the delay unit  30  generally have a high frequency, it is difficult to implement low current consumption of the time detection circuit itself according to the current consumption by the latch circuits D_ 0  to D_ 6  constituting the latch unit  33 . 
     Here, an imager for use in a digital still camera (DSC) or the like is considered as an example of a specific device using the time detection circuit of the example of the related art. Specifically, specs in which the number of pixels is 2000×10 4  and a frame rate is 60 frames/sec are assumed. Further, the AD converter is arranged for every pixel column. Assuming that a pixel array of 2000×10 4  pixels is designated as 4000 rows×5000 columns in length and width for ease of description and a blanking period is absent for further simplicity, the number of lines from which pixel signals are read per second is as follows.
 
60 frames/sec×4000 rows/frame=240 Klines/sec
 
     That is, a read rate of one row becomes 240 KHz. For example, when AD conversion of 10 bits is configured by seven higher-order bits (a count value of the count unit  34 ) and three lower-order bits (data of the latch circuits D_ 0  to D_ 7  constituting the latch unit  33 ), the clocks CK 0  to CK 7  have to be output from the delay unit  30  at about 30 MHz that is greater  128  (=2 7 ) times than the read rate of one row. Here, assuming that a current consumption value per latch circuit constituting the latch unit  33  is 1 μA/piece, a current consumption value in the latch circuits D_ 0  to D_ 6  per column becomes 1 μA/piece×7 pieces=7 μA. Further, the output of the latch circuit D_ 7  is not included in a calculation for use as a count clock of the counter circuit constituting the count unit  34 . 
     That is, a current consumption value of 5000 columns becomes 35 mA. In this calculation, a period in which a comparison operation serving as AD conversion is not performed such as a standby period until the AD converter receives data from a pixel is not considered. In addition, because a period in which a pixel signal is read from an optical black (OB) pixel rather than the above-described pixel or a blanking period is excluded, a frequency may be higher than 30 MHz estimated as described above. 
     SUMMARY 
     The present invention provides a time detection circuit, an AD converter, and a solid state image pickup device capable of reducing current consumption. 
     A time detection circuit in accordance with a preferred embodiment of the present invention may include: a delay unit configured to have a plurality of delay units, each of which delays and outputs an input signal, and start an operation at a first timing relating to an input of a first pulse; a latch unit configured to latch logic states of the plurality of delay units; a count unit configured to perform a count operation based on a clock output from one of the plurality of delay units; a count latch unit configured to latch a state of the count unit; and a latch control unit configured to enable the latch unit at a second timing relating to an input of a second pulse and cause the latch unit and the count latch unit to execute a latch at a third timing at which a predetermined time has elapsed from the second timing. 
     Preferably, the delay unit is an annular delay circuit in which the plurality of delay units are connected in an annular shape. 
     Preferably, the time detection circuit may further include: a comparison unit configured to receive an analog signal and a reference signal that increases or decreases with the passage of time and output a comparison signal when the reference signal has satisfied a predetermined condition with respect to the analog signal. The comparison signal may be input to the latch control unit, the first timing may relate to a timing at which the analog signal is input to the comparison unit, and the second timing may relate to a timing at which the comparison signal is input to the latch control unit. 
     An analog-to-digital (AD) converter in accordance with a preferred embodiment of the present invention may include: the above time detection circuit; a reference signal generation unit configured to generate a reference signal; and a calculation unit configured to generate a digital signal based on logic states latched in a latch unit and a state latched by a count latch unit. 
     A solid state image pickup device in accordance with a preferred embodiment of the present invention may include: an image capturing unit in which a plurality of pixels, each of which outputs a pixel signal according to an amount of an incident electromagnetic wave, are arranged; and the above AD converter configured to receive an analog signal corresponding to the pixel signal. The comparison unit, the latch unit, the count unit, the count latch unit, and the latch control unit may be provided for every one or more columns of the pixels constituting the image capturing unit. 
     According to the present invention, current consumption can be reduced because an operation time of a latch unit is shortened by enabling the latch unit at a second timing relating to an input of a second pulse and causing the latch unit and a count latch unit to perform a latch at a third timing at which a predetermined time has elapsed from the second timing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram illustrating a configuration of a time detection circuit in accordance with a first preferred embodiment of the present invention. 
         FIG. 2  is a timing chart illustrating an operation of the time detection circuit in accordance with the first preferred embodiment of the present invention. 
         FIG. 3  is a circuit diagram illustrating a configuration of a time detection circuit in accordance with a second preferred embodiment of the present invention. 
         FIG. 4  is a timing chart illustrating an operation of the time detection circuit in accordance with the second preferred embodiment of the present invention. 
         FIG. 5  is a circuit diagram illustrating a configuration of a time detection circuit in accordance with a third preferred embodiment of the present invention. 
         FIG. 6  is a timing chart illustrating an operation of the time detection circuit in accordance with the third preferred embodiment of the present invention. 
         FIG. 7  is a block diagram illustrating a configuration of a solid state image pickup device in accordance with a fourth preferred embodiment of the present invention. 
         FIG. 8  is a circuit diagram illustrating a configuration of a time detection circuit in accordance with the related art. 
         FIG. 9  is a timing chart illustrating an operation of the time detection circuit in accordance with the related art. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. 
     First Preferred Embodiment 
     First, a first preferred embodiment of the present invention will be described.  FIG. 1  illustrates an example of a configuration of a time detection circuit in accordance with the first preferred embodiment. Hereinafter, the configuration of this example will be described. The time detection circuit illustrated in  FIG. 1  includes a delay unit  30 , a signal generation unit  32 , a latch unit  33 , and a count unit  34 . 
     The delay unit  30  includes a plurality of delay units DU[ 0 ] to DU[ 7 ], each of which delays an input signal and outputs the delayed signal. A start pulse (=StartP) is input to the leading delay unit DU[ 0 ]. 
     The signal generation unit  32  generates a control signal to control operations of the latch unit  33  and the count unit  34 . The signal generation unit  32  includes an inverting delay circuit DLY, which inverts and delays an analog signal Signal serving as a time detection target, and an AND circuit, which outputs a signal obtained by performing an AND operation on an input LO (=Signal) of the inverting delay circuit DLY and an output xLO_D of the inverting delay circuit DLY. Details will be described later. According to the above configuration, the signal generation unit  32  generates a control signal for setting latch circuits D_ 0  to D_ 6  of the latch unit  33  at a second timing after the start pulse (=StartP) has been input at a first timing to an enable (valid) state and causing the latch circuits D_ 0  to D_ 6  and the count unit  34  to execute a latch at a third timing at which a predetermined time has elapsed from the second timing. 
     The latch unit  33  includes latch circuits D_ 0  to D_ 7 , which latch logic states of outputs CK 0  to CK 7  of the delay unit  30 . In addition, the latch unit  33  includes an AND circuit, which outputs a signal Hold_C obtained by performing an AND operation on the output xLO_D of the inverting delay circuit DLY of the signal generation unit  32  and a control signal Enable to the latch circuit D_ 7 . The count unit  34  includes a counter circuit that performs a count operation based on the output CK 7  from the delay unit  30 . 
     The latch circuits D_ 0  to D_ 6  constituting the latch unit  33  are in the enable (valid) state when an output Hold_L of the AND circuit of the signal generation unit  32  is high, and directly output the outputs CK 0  to CK 6  of the delay units DU[ 0 ] to DU[ 6 ]. In addition, the latch circuits D_ 0  to D_ 6  are in a disable (invalid) state when the output Hold_L of the AND circuit of the signal generation unit  32  transitions from the high level to the low level. At this time, the latch circuits D_ 0  to D_ 6  latch the logical states corresponding to the outputs CK 0  to CK 6  of the delay units DU[ 0 ] to DU[ 6 ]. 
     On the other hand, the latch circuit D_ 7  constituting the latch unit  33  is in the enable (valid) state when the output Hold_C of the AND circuit of the latch unit  33  is high, and directly outputs the output CK 7  of the delay unit DU[ 7 ]. In addition, the latch circuit D_ 7  is in the disable (invalid) state when the output Hold_C of the AND circuit of latch unit  33  transitions from the high level to the low level. At this time, the latch circuit D_ 7  latches the logical state corresponding to the output CK 7  of the delay unit DU[ 7 ]. 
     The control signal Enable is a signal for controlling the AND circuit of the latch unit  33 . A control signal RST is a signal for performing a reset operation of the counter circuit constituting the count unit  34 . Although a count latch circuit that latches the logic state of a count result of the count unit  34  is not explicitly illustrated in this drawing, a counter circuit having a latch function is used and hence the counter circuit also includes the count latch circuit. Further, this configuration is only an example, and the present invention is not limited thereto. 
     Next, an operation of this example will be described.  FIG. 2  illustrates the operation of the time detection circuit in accordance with the first preferred embodiment of the present invention. 
     First, a clock of a cycle approximately consistent with a delay time of the delay unit  30  is input as the start pulse (=StartP) (the first timing). Thereby, the operation of the delay unit  30  is started. The delay unit DU[ 0 ] constituting the delay unit  30  outputs the output CK 0  by inverting and delaying the start pulse (=StartP), and the delay units DU[ 1 ] to DU[ 7 ] constituting the delay unit  30  output the outputs CK 1  to CK 7  by inverting and delaying the outputs of previous-stage delay units. The outputs CK 0  to CK 7  of the delay units DU[ 0 ] to DU[ 7 ] are input to the latch circuits D_ 0  to D_ 7  of the latch unit  33 . Because the output Hold_L of the AND circuit of the signal generation unit  32  is low, the latch circuits D_ 0  to D_ 6  are disabled in the disable state. 
     On the other hand, because the output Hold_C of the AND circuit of the latch unit  33  is high, the latch circuit D_ 7  is in the enable state and directly outputs the output CK 7  of the delay unit DU[ 7 ]. The count unit  34  performs a count operation based on the output CK 7  of the delay unit  30  output as an output Q 7  of the latch circuit D_ 7 . In the count operation, a count value is increased or decreased by the rising or falling of the output CK 7 . 
     After a “time to be detected” serving as a detection target has elapsed from the first timing, the input LO (=Signal) of the inverting delay circuit DLY of the signal generation unit  32  is inverted and hence the output Hold_L of the AND circuit of the signal generation unit  32  becomes high. Thereby, the latch circuits D_ 0  to D_ 6  are in the enable state. After a time consistent with a delay time of the inverting delay circuit DLY of the signal generation unit  32  has elapsed from the second timing (the third timing), the output xLO_D of the inverting delay circuit DLY of the signal generation unit  32  is inverted and the output Hold_L of the AND circuit of the signal generation unit  32  becomes low. Thereby, the latch circuits D_ 0  to D_ 6  are in the disable state. At this time, the logic states corresponding to the outputs CK 0  to CK 6  of the delay units DU[ 0 ] to DU[ 6 ] are latched in the latch circuits D_ 0  to D_ 6  of the latch unit  33 . 
     In addition, because the output Hold_C of the AND circuit of the latch unit  33  becomes low at the above-described third timing, the latch circuit D_ 7  is in the disable state, and the logic state corresponding to the output CK 7  of the delay unit DU[ 7 ] is latched in the latch circuit D_ 7  of the latch unit  33 . The latch circuit D_ 7  is disabled and hence the count unit  34  latches the count value. Data corresponding to the “time to be detected” is obtained according to the logic state latched by the latch unit  33  and the count value latched by the count unit  34 . The latched data, for example, is output to a subsequent-stage calculation unit (not illustrated), and a process of binarization or the like is performed. 
     In the above-described operation, it is possible to reduce current consumption in the latch unit  33  because the latch circuits D_ 0  to D_ 6  operate only in a period from the second timing to the third timing. Therefore, it is possible to reduce current consumption of the time detection circuit. 
     Further, although a configuration implementing low power consumption by controlling operations of the latch circuits D_ 0  to D_ 6  constituting the latch unit  33  is made in this example, for example, a configuration in which the latch circuits D_ 1  to D_ 5  are controlled may be made. In addition, the present invention is not limited thereto. 
     Second Preferred Embodiment 
     Next, a second preferred embodiment of the present invention will be described.  FIG. 3  illustrates a configuration of a time detection circuit in accordance with the second preferred embodiment of the present invention. Hereinafter, a configuration diagram of this example will be described. A difference from the configuration illustrated in  FIG. 1  is a delay unit  30 . In the second preferred embodiment, an annular delay circuit is implemented by connecting a plurality of delay units DU[*] (* is 0 to 7) constituting the delay unit  30  in a ring shape. Because other components are substantially the same as in  FIG. 1 , description thereof is omitted. 
     Next, an operation of this example will be described.  FIG. 4  illustrates an operation of the time detection circuit in accordance with the second preferred embodiment. A difference from  FIG. 2  is a start pulse (=StartP). The logic state of the start pulse (=StartP) is changed from the low level to the high level, and hence the delay unit  30  starts the operation and a count operation of the count unit  34  is performed based on the output CK 7  from the delay unit  30 . Because other components are substantially the same as in  FIG. 2 , description thereof is omitted. 
     Although it is necessary to generate the start pulse (=StartP) as a synchronization clock approximately consistent with a delay time of the delay unit  30  in the first preferred embodiment, it is easy to generate the start pulse (=StartP) in the second preferred embodiment. Thus, control of the delay unit  30 , that is, control of the time detection circuit, is facilitated. 
     Third Preferred Embodiment 
     Next, a third preferred embodiment of the present invention will be described.  FIG. 5  illustrates an example of a configuration of a time detection circuit in accordance with the third preferred embodiment of the present invention. Hereinafter, a configuration diagram of this example will be described. The illustration of the delay unit  30  is omitted in  FIG. 5 . A difference from the configuration illustrated in  FIG. 3  is that a comparison unit  31  is added. The comparison unit  31  includes a voltage comparator, which receives an analog signal Signal serving as a time detection target and a ramp wave Ramp that increases or decreases with the passage of time and outputs a signal indicating a result obtained by comparing the analog signal Signal with the ramp wave Ramp. Thereby, a time interval (corresponding to the “time to be detected” in the description of  FIG. 2 ) corresponding to the analog signal Signal is generated. Because other components are substantially the same as in  FIG. 3 , description thereof is omitted. 
     Next, an operation of this example will be described. First, at a timing (first timing) relating to a comparison start in the comparison unit  31 , the logic state of a start pulse (=StartP) is changed from the low level to the high level. 
     Thereby, the delay unit  30  starts the operation. The delay unit DU[ 0 ] constituting the delay unit  30  outputs the output CK 0  by inverting and delaying the start pulse (=StartP), and the delay units DU[ 1 ] to DU[ 7 ] constituting the delay unit  30  output the outputs CK 1  to CK 7  by inverting and delaying the outputs of the previous-stage delay units. The outputs CK 0  to CK 7  of the delay units DU[ 0 ] to DU[ 7 ] are input to the latch circuits D_ 0  to D_ 7  of the latch circuit  33 . Because the input CO of the inverting delay circuit DLY is low and the output Hold_L of the AND circuit of the signal generation unit  32  is low, the latch circuits D_ 0  to D_ 6  are disabled in the disable state. 
     On the other hand, because the output Hold_C of the AND circuit of the latch unit  33  is high, the latch circuit D_ 7  is in the enable state and directly outputs the output CK 7  of the delay unit DU[ 7 ]. The count unit  34  performs a count operation based on the output CK 7  of the delay unit  30  output as the output Q 7  of the latch circuit D_ 7 . In the count operation, a count value is increased or decreased by the rising or falling of the output CK 7 . 
     At a timing (second timing) at which the analog signal Signal is approximately consistent with the ramp wave Ramp, the output CO of the comparison unit  31  is inverted and becomes high. Thereby, the latch circuits D_ 0  to D_ 6  are in the enable state. After a time consistent with a delay time of the inverting delay circuit DLY of the signal generation unit  32  has elapsed from the second timing (a third timing), an output xCO_D of the inverting delay circuit DLY of the signal generation unit  32  is inverted and the output Hold_L of the AND circuit of the signal generation unit  32  becomes low. Thereby, the latch circuits D_ 0  to D_ 6  are in the disable state. At this time, the logic states corresponding to the outputs CK 0  to CK 6  of the delay units DU[ 0 ] to DU[ 6 ] are latched in the latch circuits D_ 0  to D_ 6  of the latch unit  33 . 
     In addition, because the output Hold_C of the AND circuit of the latch unit  33  becomes low at the above-described third timing, the latch circuit D_ 7  is in the disable state and the logic state corresponding to the output CK 7  of the delay unit DU[ 7 ] is latched in the latch circuit D_ 7  of the latch unit  33 . The latch circuit D_ 7  is disabled and hence the count unit  34  latches the count value. According to the logic state latched by the latch unit  33  and the count value latched by the count unit  34 , data corresponding to a time interval from the first timing to the second timing is obtained. The latched data, for example, is output to a subsequent-stage calculation unit (not illustrated), and a process of binarization or the like is performed. 
     In the above-described operation, it is possible to reduce current consumption in the latch unit  33  because the latch circuits D_ 0  to D_ 6  operate only in a period from the second timing to the third timing. Therefore, it is possible to reduce current consumption of the time detection circuit. 
     Further, although a configuration implementing low power consumption by controlling operations of the latch circuits D_ 0  to D_ 6  constituting the latch unit  33  is made in this example, for example, a configuration in which the latch circuits D_ 1  to D_ 5  are controlled may be made. In addition, the present invention is not limited thereto. 
     Fourth Preferred Embodiment 
     Next, a fourth preferred embodiment of the present invention will be described.  FIG. 7  illustrates an example of a configuration of a solid state image pickup device in accordance with the fourth preferred embodiment of the present invention. Hereinafter, a configuration diagram of this example will be described. A solid state image pickup device  1  illustrated in  FIG. 7  includes an image capturing unit  2 , a vertical selection unit  12 , a read current source unit  5 , an analog unit  6 , a delay unit  18 , a ramp unit  19 , a column processing unit  15 , a horizontal selection unit  14 , a calculation unit  17 , and a control unit  20 . 
     In the image capturing unit  2 , a plurality of unit pixels  3 , each of which generates and outputs a signal corresponding to an amount of an incident electromagnetic wave, are arranged in a matrix. The vertical selection unit  12  selects each row of the image capturing unit  2 . The read current source unit  5  reads a signal from the image capturing unit  2  as a voltage signal. The analog unit  6  performs analog processing on the signal read from the image capturing unit  2 . The delay unit  18  corresponds to the delay unit  30  described in the second and third preferred embodiments, and has an annular delay circuit  8 . The ramp unit  19  generates a ramp wave as a reference signal that increases or decreases with the passage of time. The column processing unit  15  is connected to the ramp unit  19  via a reference signal line  119 . The horizontal selection unit  14  reads data generated by the column processing unit  15  to a horizontal signal line  117 . The calculation unit  17  is connected to the horizontal signal line  117 . The control unit  20  controls each unit. 
     Although the image capturing unit  2  including unit pixels  3  of 4 rows×6 columns has been described with reference to  FIG. 7  for simplicity, several tens to several tens of thousands of unit pixels  3  are actually arranged in each row or column of the image capturing unit  2 . Although not illustrated, the unit pixels  3  constituting the image capturing unit  2  include a photoelectric conversion element such as a photodiode, a photogate, or a phototransistor and a transistor circuit. 
     In this system configuration, a peripheral driving system and a peripheral signal processing system for controlling driving of each of the unit pixels  3  of the image capturing unit  2 , that is, peripheral circuits such as the vertical selection unit  12 , the horizontal selection unit  14 , the column processing unit  15 , the calculation unit  17 , the delay unit  18 , the ramp unit  19 , and the control unit  20 , are integrally formed in a semiconductor area such as single crystal silicon or the like along with the image capturing unit  2  using technology similar to technology for manufacturing a semiconductor integrated circuit. 
     Hereinafter, the units will be described in further detail. In the image capturing unit  2 , the unit pixels  3  are two-dimensionally arranged only in 4 rows and 6 columns, and a row control line  11  is wired for every row with respect to a pixel array of 4 rows and 6 columns. One end of the row control line  11  is connected to an output end corresponding to each row of the vertical selection unit  12 . The vertical selection unit  12  includes a shift register or a decoder. The vertical selection unit  12  controls row addressing or row scanning of the image capturing unit  2  via the row control line  11  when each unit pixel  3  of the image capturing unit  2  is driven. In addition, a vertical signal line  13  is wired for every column with respect to the pixel array of the image capturing unit  2 . 
     The read current source unit  5 , for example, is configured using an n-channel metal-oxide-semiconductor (NMOS) transistor. The vertical signal line  13  from the image capturing unit  2  is connected to a drain terminal. A desired voltage is appropriately applied to a control terminal, and a source terminal is connected to the ground (GND). Thereby, a signal from the unit pixel  3  is output as a voltage mode. Further, although the case in which the NMOS transistor is used as a current source has been described, the present invention is not limited thereto. 
     Although not described in detail, the analog unit  6  removes a noise component referred to as fixed pattern noise (FPN), which is fixed variation for every pixel, or reset noise by processing a difference between a signal level (reset level) immediately after pixel reset and a true signal level with respect to a pixel signal of the voltage mode input via the vertical signal line  13 . Further, if necessary, an auto gain control (AGC) circuit having a signal amplification function or the like may be provided. 
     The column processing unit  15 , for example, has an AD conversion (ADC) unit  16  provided for every pixel column of the image capturing unit  2 , that is, every vertical signal line  13 . The column processing unit  15  converts an analog pixel signal read from each unit pixel  3  of the image capturing unit  2  through the vertical signal line  13  for every pixel column into digital data. Further, although ADC units  16  are arranged to have a one-to-one correspondence relationship with pixel columns of the image capturing unit  2  in this example, this is only an example and the present invention is not limited to this layout relationship. For example, one ADC unit  16  for a plurality of pixel columns can be arranged and the one ADC unit  16  can be used and configured in time division among the plurality of pixel columns. The column processing unit  15  includes an AD converter for converting an analog pixel signal read from a unit pixel  3  of a selected pixel row of the image capturing unit  2  into digital pixel data along with the ramp unit  19 , the delay unit  18 , and the calculation unit  17  as will be described later. 
     The delay unit  18  is not limited to a voltage controlled oscillator (VCO) circuit, which is a symmetric oscillation circuit as an annular delay circuit. Although the annular delay circuit itself includes an odd number of delay units as in the symmetric oscillation circuit, a so-called asymmetric oscillation circuit in which the number of outputs is equivalently an even number (particularly, a power of two) may be used. Further, the annular delay circuit itself may include an even number (particularly, a power of two) of delay units. A ring delay line (RDL) circuit or an annular delay circuit having an even number (particularly, a power of two) of outputs (terminals) of the lower-order logic state may include an even number (particularly, a power of two) of delay units. Further, a so-called fully differential oscillation circuit may be used in which the outputs of the final stage of the fully differential inversion circuit forming the delay unit are returned to the reverse side of the inputs of the initial stage. Further, although the annular delay circuit is suitable for the delay unit  18 , the present invention is not limited thereto. 
     The ramp unit  19 , for example, includes an integrating circuit. According to control of the control unit  20 , the ramp unit  19  generates a ramp wave, the level of which changes along a gradient with the passage of time, and supplies the ramp wave to one of input terminals of a voltage comparison unit  131  via the reference signal line  119 . The ramp unit  19  is not limited to the integrating circuit, and a digital-to-analog conversion (DAC) circuit may be used. However, in the case of a configuration in which a ramp wave is digitally generated using the DAC circuit, a configuration that makes the step of the ramp wave fine or a configuration equivalent thereto is necessary. 
     The horizontal selection unit  14  includes a shift register, a decoder, or the like. The horizontal selection unit  14  controls column addressing or column scanning of the ADC unit  16  of the column processing unit  15 . According to control of the horizontal selection unit  14 , digital data subjected to ADC by the ADC unit  16  is sequentially read to the horizontal signal line  117 . 
     The calculation unit  17  performs code conversion such as binarization based on digital data output to the horizontal signal line  117 , and outputs binary digital data. In addition, the calculation unit  17  may have embedded signal processing functions, for example, such as black level adjustment, column variation correction, color processing, and the like. Further, n-bit parallel digital data may be converted into serial data and the serial data may be output. 
     The control unit  20  includes a functional block of a timing generator (TG), which supplies predetermined timing pulse signals or clocks necessary for operations of units such as the ramp unit  19 , the delay unit  18 , the vertical selection unit  12 , the horizontal selection unit  14 , and the calculation unit  17 , and a functional block for communicating with the TG. Further, the control unit  20  may be provided as another semiconductor integrated circuit so as to be independent of other functional elements such as the image capturing unit  2  or the vertical selection unit  12  and the horizontal selection unit  14 . In this case, an image capturing apparatus, which is an example of a semiconductor system, is constructed by the control unit  20  and an image pickup device including the image capturing unit  2  or the vertical selection unit  12 , the horizontal selection unit  14 , and the like. The image capturing apparatus may be provided as an image capturing module embedded with a peripheral signal process, a power-supply circuit, or the like. 
     Next, a configuration of the ADC unit  16  will be described. The ADC unit  16  generates a time interval having the magnitude (pulse width) of a time-axis direction corresponding to each magnitude of the pixel signal by comparing an analog pixel signal read from each unit pixel  3  of the image capturing unit  2  via the vertical signal line  13  with a ramp wave for ADC provided from the ramp unit  19 . The ADC is performed by designating data corresponding to the time interval as digital data corresponding to the magnitude of a pixel signal. 
     Hereinafter, details of the configuration of the ADC unit  16  will be described. The ADC unit  16  is provided for every column. In  FIG. 7 , six ADC units  16  are provided. The ADC units  16  of the columns have the same configuration. The ADC unit  16  includes the voltage comparison unit  131 , a latch control unit  132 , a latch unit  133 , and a column counter  134 . 
     The voltage comparison unit  131 , which is an example of a comparison unit, converts the magnitude of the pixel signal into a time interval (pulse width), which is information of the time-axis direction, by comparing a signal voltage corresponding to an analog pixel signal output from the unit pixel  3  of the image capturing unit  2  via the vertical signal line  13  with a ramp wave supplied from the ramp unit  19 . A comparison output of the voltage comparison unit  131 , for example, has the low level when a ramp voltage is greater than a signal voltage, and has the high level when the ramp voltage is less than or equal to the signal voltage. The latch control unit  132  generates a control signal for controlling the latch unit  133  and the column counter  134  based on the comparison output of the voltage comparison unit  131 . 
     The latch unit  133  includes the latch circuits D_ 0  to D_ 6  and the latch circuit D_ 7 . The latch circuits D_ 0  to D_ 6  constituting the latch unit  133  are in the enable stat at a timing (second timing) at which the comparison output of the voltage comparison unit  131  is received and inverted. After a predetermined time has elapsed from the second timing (a third timing), the latch circuits D_ 0  to D_ 7  of the latch unit  133  are in the disable state and hence the logic state generated by the delay unit  18  is latched (held/stored). The column counter  134  performs a count operation based on an output of the latch circuit D_ 7  of the latch unit  133 . Here, the column counter  134  is assumed to have a latch function that holds the logic state of the column counter  134 . 
     Here, a lower-order data signal indicating the logic state of the latch unit  133 , for example, is 8-bit data. In addition, a higher-order data signal indicating a count result of the column counter  134 , for example, is 10-bit data. Further, 10 bits are just an example and a number of bits less than 10 bits (for example, 8 bits) or a number of bits greater than 10 bits (for example, 12 bits) may be used. 
     Next, an operation of this example will be described. Here, although the specific operation of the unit pixel  3  is omitted, the unit pixel  3  outputs a reset level and a signal level as is well known. The output reset level and signal level are output as a pixel output signal subjected to correlated double sampling (CDS) processing in the analog unit  6 . 
     The ADC is performed as follows. For example, digital data corresponding to a pixel output signal is obtained by comparing the ramp wave falling at a predetermined slope with the pixel output signal and measuring a period from a point in time (a first timing) relating to the start of the comparison process to a point in time (the third timing) at which a predetermined time has elapsed from a point in time (the second timing) at which the pixel output signal has been consistent with the ramp voltage of the ramp wave using a count based on an output (for example, CK 7 , that is, corresponding to an output Q of the latch circuit D_ 7  of the latch unit  33  illustrated in  FIG. 5 ) from the annular delay circuit and logic states of multi-phase clocks (CK 0  to CK 7 , that is, corresponding to outputs Q of the latch circuits D_ 0  to D_ 7  of the latch unit  33  illustrated in  FIG. 5 ) having a constant phase difference. Further, the ADC is performed by reading a reset level including noise of a pixel signal in a first read operation from each unit pixel  3  of a selection row of the image capturing unit  2 . Next, the ADC is performed by reading a signal level in a second read operation. Thereafter, digital data corresponding to a pixel output signal may be obtained by digitally performing a CDS operation. The present invention is not limited thereto. 
     After a pixel output signal output from the unit pixel  3  of an arbitrary pixel row to the vertical signal line  13  has been stabilized, the control unit  20  supplies control data of ramp-wave generation to the ramp unit  19 . The ramp unit  19  receiving the control data outputs a ramp wave that temporally changes in a ramp shape as a whole as a comparison voltage applied to one input terminal of the voltage comparison unit  131 . The voltage comparison unit  131  starts a comparison of the ramp wave and the pixel output signal (the first timing). In addition, the control unit  20  changes a start pulse to be output to the annular delay circuit  8  from the low level to the high level at the first timing. 
     The voltage comparison unit  131  compares the ramp wave applied from the ramp unit  19  with the pixel output signal, and outputs a comparison output when two voltages are approximately consistent (the second timing). The comparison output is further inverted or delayed and output (the third timing). At the second timing, the latch circuits D_ 0  to D_ 6  of the latch unit  133  are in the enable state based on the comparison output of the voltage comparison unit  131 . At the third timing, the latch circuits D_ 0  to D_ 7  of the latch unit  133  are in the disable state, and the logic state corresponding to the output from the delay unit  18  is latched. The latch circuit D_ 7  of the latch unit  133  is stopped and hence the column counter  134  latches a count value. Thereby, digital data (a data signal) corresponding to the pixel output signal is obtained. When a predetermined period has elapsed, the control unit  20  stops the supply of the control data to the ramp unit  19  and the output from the delay unit  18 . Thereby, the ramp unit  19  stops the generation of the ramp wave. 
     Thereafter, digital data is output by the horizontal selection unit  14  via the horizontal signal line  117 , and is transferred to the calculation unit  17 . In the calculation unit  17 , binary data is obtained by performing a binary process. Further, the calculation unit  17  may be configured to be embedded in the column processing unit  15 . 
     Because the latch circuits D_ 0  to D_ 6  operate in only a period from the second timing to the third timing in the above-described operation, it is possible to reduce current consumption in the latch unit  33 . Therefore, it is possible to reduce current consumption of an AD converter and current consumption of a solid state image pickup device. 
     Further, although a configuration implementing low power consumption by controlling operations of the latch circuits D_ 0  to D_ 6  constituting the latch unit  133  is made in this example, for example, a configuration may be made to control the latch circuits D_ 1  to D_ 5 . In addition, the present invention is not limited thereto. 
     While preferred embodiments of the present invention have been described and illustrated above, it should be understood that these are examples of the present invention and are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the scope of the present invention. 
     According to the present invention, it is possible to provide a time detection circuit, an AD converter, and a solid state image pickup device in which current consumption has been reduced by shortening an operation time of a latch unit.