Patent Document

FIELD OF THE INVENTION 
     The present invention relates to an image sensing device that senses an object image using a photoelectric converter and a control method therefor. 
     BACKGROUND OF THE INVENTION 
     Conventionally, implementing an electronic shutter operation by executing a reset scan by pixel or by line to remove unneeded charges accumulated at the pixels and then carrying out a scan by pixel or by line to output the signal charge after a predetermined period of time has elapsed for each pixel or for each line is known. 
     A description is now given of an electronic shutter operation in an image sensing apparatus that uses an image sensing device that employs a conventional XY address-type scan method. Specifically, using  FIG. 11  and  FIG. 12 , a description is given of the structure of the conventional image sensing device and the drive method called rolling electronic shutter operation among the electronic shutter operations. 
       FIG. 11  is a schematic diagram showing the structure of an image sensing device employing the conventional XY address-type scan method. 
     Reference numeral  101  designates a unit pixel, with multiple unit pixels  101  arranged in a matrix. Reference numeral  102  designates a photodiode (hereinafter “PD”) that converts light of an object image into a signal charge. Reference numeral  104  designates an area that temporarily stores the signal charge (that is, a floating diffusion part, hereinafter referred to as “FD”). Reference numeral  103  designates a transfer switch that transfers the signal charge generated at the PD  102  to the FD  104  using a transfer pulse φTX. Reference numeral  105  designates a MOS amplifier that functions as a source follower. Reference numeral  106  designates a selection switch that selects a unit pixel  101  using a selection pulse φSEL. Reference numeral  107  designates a reset switch that resets the FD  104  to a predetermined potential (V DD ) using a reset pulse φRES. A floating diffusion amplifier is composed of the FD  104 , the MOS amplifier  105  and a constant current source  109  that is described below. The signal charge of the unit pixel  101  selected by the selection switch  106  is converted into voltage and output to an output circuit  111  over a signal line  108 . Reference numeral  109  designates the constant current source that becomes a load of the MOS amplifier  105 . 
     By the driving of a horizontal scan circuit  110 , the output from the pixels  101  are output from the output circuit  111  to an output line  112 . In addition, reference numeral  113  designates a vertical scan circuit that controls the driving of the pixels by supplying the respective drive signals φTX, φSEL and φRES to the switches  103 ,  106  and  107 , respectively. In φTX, φSEL and φRES, respectively, the drive signals supplied to an nth scan line selected for scanning by the vertical scan circuit  113  are written as φTXn, φSELn and φRESn. 
       FIG. 12  is a schematic diagram showing drive pulse and sequence of a rolling electronic shutter operation. It should be noted that  FIG. 11  describes an example of the driving of from a line n to a line n+3 by the vertical scan circuit  113 . 
     With the rolling electronic shutter operation, in line n, first, between a time t 31  and a time t 32 , pulses are applied to φRESn and φTXn and the transfer switch  103  and the reset switch are turned on, removing the unneeded charges accumulated in PD  102  and FD  104  of each pixel on the line n and resetting them to a predetermined potential. At time t 32 , the transfer switch  103  is turned off and the light charge generated at the PD  102  begins to be accumulated. The charge generated at the PD  102  by photoelectric conversion is called “light charge”, hereinafter. 
     Next, at a time t 34 , a pulse is applied to φTXn and the transfer switch  103  is turned on, transferring the light charge accumulated in the PD  102  to the FD  104 . It should be noted that the reset switch  107  is turned off prior to the transfer. From time t 32  to a time t 35 , when φTXn becomes low and transfer ends, is a charge accumulation time. After the transfer in the line n ends, a pulse is applied to φSELn and the selection switch  106  is turned on, converting the light charge held in the FD  104  to voltage and outputting it to the output circuit  111 . The output circuit  111  is driven by the horizontal scan circuit  110 , and the signals temporarily held at the output circuit  111  are output in succession from a time t 36 . From the start of transfer at time t 34  to the end of output at time t 37  is T 3 read, and the time from time t 31  to time t 33  is T 3 wait. The process is the same for the remaining lines, with the time from the start of transfer to the end of output being T 3 read and the time from the start of reset of one line to the start of reset of the next line being T 3 wait. 
     A problem with the rolling electronic shutter shown in  FIG. 12  is that the charge accumulation timing shifts between the top part of the screen and the bottom part of the screen time by the length of time required to scan the screen. This is because the time T 3 wait from the start of reset of one line to the start of reset of the next line must be set to a duration that is greater than the time T 3 read from the start of transfer to the end of output. If T 3 wait is shorter than T 3 read, the following problem occurs when attempting to make the charge accumulation time the same for all lines: Specifically, before output of the signals of line n temporarily held in the output circuit  111  ends, the signals of the next line are transferred to the output circuit  111  while the signals of the pixels of line n still remain in the output circuit  111 . This not only makes it impossible to output the signals of the pixels of a part of the line n, but the remaining signals of the line n are also added to the signals of line n+1, leading to the wrong signals being output as the signals of the line n+1. In addition, if the signal output cannot be scanned from the output circuit  111  at high speed, then, particularly in the case of a large number of pixels, the shift in the charge accumulation timing (that is, the image sensing timing) from the top of the image to the bottom of the image increases. 
     Moreover, as is described in Japanese Patent Application Laid-Open No. 2003-17677, there is also a MOS-type image sensing device that performs reset and transfer of charges collectively. The sequence of operations of this sort of operation is shown in  FIG. 13 . In  FIG. 13 , all the lines are reset simultaneously from a time t 41  to a time t 42 . In addition, between a time t 43  and a time t 44  transfer of charges is also performed simultaneously. Hereinafter, this type of electronic shutter is referred to as a collective transfer electronic shutter. With a collective transfer electronic shutter, the charge accumulation time for all lines is from t 42  to t 44 , achieving an electronic shutter with no shift in charge accumulation timing from the top to the bottom of the image. 
     In addition, in order to carry out reset and transfer of charges at high speed, there is also a MOS-type image sensing device that performs the sequence of operations shown in  FIG. 14  designed to increase the speed of reset and transfer of charges. A description is given of this sequence using  FIG. 11  and  FIG. 14 . 
     First, by applying pulses to φRESn and φTXn from a time t 51  to a time t 52 , the reset switch  107  and the transfer switch  103  of each pixel on the line n are turned on, resetting the PD  102  and FD  104  of each pixel on the line n. From time t 52  the charge accumulation that generates a light charge on the PD  102  of each pixel on the line n starts, and at a time t 53  the resetting of all the lines up to and including the last line ends. From a time t 54  to a time t 55  a pulse is applied to φTXn, turning on the transfer switch  103  of each pixel on the line n and transferring the light charge accumulated in PD  102  to the FD  104  of each pixel on the line n. The time from time t 52  to time t 55  is the charge accumulation time for line n. Transfers for line n+1 and all lines thereafter are carried out in succession from time t 55 , with transfers for all lines completed at a time t 57 . 
     After transfer of the light charge of each pixel on the line n ends, a pulse is applied to φSEL at a time t 55 , turning switch  106  on. This causes the charge held in the FD  104  to be converted into voltage by a source follower circuit composed of the MOS amplifier  105  and the constant current source  109  and output to the output circuit  111 . The signals temporarily held in the output circuit  111  are output in succession to the output line  112  from time t 56  by the horizontal scan circuit  110  control. Output of line n+1 is carried out from a time t 58 , after all the signals of line n have been output from the output circuit  111 . The sequence shown in  FIG. 14  enables the time from the start of scanning of one line to the start of scanning of the next line to be determined independently of the output time, thus enabling distortion of an image by the rolling electronic shutter to be reduced. This sort of electronic shutter is referred to as a rolling transfer electronic shutter. With the collective transfer electronic shutter, the rolling electronic shutter and the rolling transfer electronic shutter there is no need to use a mechanical shutter, and thus these shutters are optimal for moving image applications. 
     At the same time, a structure is known that removes noise unique to the pixels from the light signals output from the pixels. As one example thereof, the light signals output from the pixels on one line are temporarily held in capacitors, respectively, the noise signals of the same pixels are temporarily held in other capacitors prior to or after light signal output, and the noise signals are then subtracted from the light signals at each of the pixels. This operation is performed sequentially for all of the lines. Configuring and controlling an apparatus in the foregoing manner enables the noise component to be reduced. 
     However, because there are slight differences between the capacitors that holds the light signals and the capacitors that holds the noise signals, to further reduce the noise component accurately a structure is known in which clamping circuits are used. With a structure that uses clamping circuits, in the case of a rolling electronic shutter, first, the noise signals of the pixels of the output line are output and made reference levels of the clamping circuits, and at the same time the noise signals are output and held in capacitors. Thereafter, the light signals are output and held in other capacitors after being clamped by the clamping circuit. Therefore, the light signals are clamped by the noise signals, enabling light signals minus noise signals to be obtained from the clamping circuits. 
     By contrast, in the case of a collective transfer electronic shutter or a rolling transfer electronic shutter, the transfer of the light charge from the PD to the FD is carried out prior to output. As a result, it is not possible to realize a sequence of operations in which the noise signal is output and then the light signal is output after the noise signal is clamped during readout of the light signal. The light charge has already been transferred to the FD when the FD is initially scanned, and therefore the voltage of the FD which is output first becomes the reference level of the clamping circuit. If the noise signal is then output, then the output from the clamping circuit becomes the noise signal minus the light signal, which would be outside the operating range of the amplifier used in the later stage of the clamping circuit. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in light of the above-described situation, and has as its object to keep the post-clamping output levels within a predetermined range when using clamping circuits. 
     According to the present invention, the foregoing object is attained by providing an image sensing apparatus comprising: a plurality of pixels arranged two dimensionally, each pixel containing a photoelectric converter that outputs a photoelectrically converted signal in response to a quantity of received light; an output unit containing a clamping circuit; a signal supply circuit that outputs a reference signal to the clamping circuit; a control unit that controls to clamp the reference signal prior to outputting the photoelectrically converted signal from the pixel to the clamping circuit, output the photoelectrically converted signal to the clamping circuit, and then output a noise signal from the pixel to the clamping circuit; and a differential circuit that subtracts the noise signal from the photoelectrically converted signal processed by the clamping circuit. 
     According to the present invention, the foregoing object is also attained by providing an image sensing apparatus capable of being driven by multiple scan methods including a first scan method and a second scan method, the image sensing apparatus comprising: a plurality of pixels arranged two dimensionally, each pixel containing a photoelectric converter that outputs a photoelectrically converted signal in response to a quantity of received light; an output unit containing a clamping circuit; a signal supply circuit that outputs a reference signal to the clamping circuit; a control unit that with a first scan method clamps a noise signal from the pixels with the clamping circuit and then outputs the photoelectrically converted signal to the clamping circuit, and with a second scan method controls to clamp the reference signal prior to outputting the photoelectrically converted signal from the pixel to the clamping circuit, output the photoelectrically converted signal to the clamping circuit, and then output a noise signal from the pixel to the clamping circuit; and a differential circuit that subtracts the noise signal from the photoelectrically converted signal processed by the clamping circuit. 
     Further, the foregoing object is also attained by providing a control method for an image sensing apparatus comprising a plurality of pixels arranged two dimensionally, each pixel containing a photoelectric converter that outputs a photoelectrically converted signal in response to a quantity of received light, an output unit containing a clamping circuit, and a signal supply circuit that outputs a reference signal to the clamping circuit, the method comprising: clamping the reference signal prior to outputting the photoelectrically converted signal from the pixel to the clamping circuit; outputting the photoelectrically converted signal to the clamping circuit; outputting a noise signal from the pixel to the clamping circuit; and subtracting the noise signal from the photoelectrically converted signal processed by the clamping circuit. 
     Furthermore, the foregoing object is also attained by providing an image sensing apparatus comprising: a plurality of pixels arranged two dimensionally, each pixel containing a photoelectric converter that outputs a photoelectrically converted signal in response to an amount of received light, a storage unit that stores a signal from the photoelectric converter, and a transfer switch that transfers a signal from the photoelectric converter to the storage unit; a mechanical shutter, and a switching unit that switches operations between a first scan sequence for still image sensing and a second scan sequence for moving image sensing, wherein in the first scan sequence, transference of the photoelectrically converted signals from the photoelectric converters to the storage units and output of the photoelectrically converted signals from the pixels are sequentially performed by a predetermined number of line/lines, and in the second scan sequence, photoelectrically converted signals are collectively transferred from the photoelectric converters contained in the plurality of pixels to the storage units, and then the photoelectrically converted signals are sequentially output from the plurality of pixels by a predetermined number of line/lines, or photoelectrically converted signals are sequentially transferred from the photoelectric converters to the storage units by a predetermined number of line/lines at a first period, and then the photoelectrically converted signals are sequentially output from the plurality of pixels by a predetermined number of line/lines at a second period, with the first period being shorter than the second period. 
     Other features and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a block diagram showing a schematic structure of an image sensing apparatus according to an embodiment of the present invention; 
         FIG. 2  is a diagram showing a schematic structure of an image sensing device of the image sensing apparatus according to the embodiment of the present invention; 
         FIGS. 3A ,  3 B and  3 C are schematic diagrams showing examples of general structures of a dummy pixel of according to the embodiment of the present invention; 
         FIG. 4  is a schematic diagram showing an example of the general structure of an output circuit shown in  FIG. 1 ; 
         FIG. 5  is a timing chart showing a first scan sequence of the image sensing device according to the embodiment of the present invention; 
         FIG. 6  is a timing chart showing a second scan sequence of the image sensing device according to the embodiment of the present invention; 
         FIG. 7  is a schematic diagram showing scanning with a rolling electronic shutter according to the embodiment of the present invention; 
         FIG. 8  is a schematic diagram showing scanning with a rolling electronic shutter using a mechanical shutter according to the embodiment of the present invention; 
         FIG. 9  is a schematic diagram showing scanning with a rolling transfer electronic shutter according to the embodiment of the present invention; 
         FIG. 10  is a schematic diagram showing scanning with a collective transfer electronic shutter according to the embodiment of the present invention; 
         FIG. 11  is a schematic diagram showing the structure of an image sensing device employing a conventional XY address-type scan method 
         FIG. 12  is a schematic diagram showing drive pulse and sequence of operations during rolling electronic shutter operation; 
         FIG. 13  is a diagram for the purpose of illustrating scanning with a collective transfer electronic shutter; and 
         FIG. 14  is a diagram for the purpose of illustrating scanning with a rolling transfer sensor electronic shutter. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will now be described in detail in accordance with the accompanying drawings. 
       FIG. 1  is a block diagram showing the schematic structure of an image sensing apparatus of an embodiment of the present invention. 
     In  FIG. 1 , reference numeral  201  designates a lens unit for focusing an optical image of an object on an image sensing device  205 , with lens zoom, focus and aperture control performed by a lens drive device  202 . Reference numeral  203  designates a shutter, which is controlled by a shutter drive device  204 . Reference numeral  205  designates the image sensing device that converts the optical image of the object focused by the lens unit  201  into an electrical image signal. Reference numeral  206  designates an image sensing signal processing circuit with the capability to amplify the image signal output from the image sensing device  205 , to perform analog-to-digital (A/D) conversion on the image signal, and to carry out a variety of corrections on the A/D converted image signal, and the capability to compress the signal. Reference numeral  207  designates a timing generator that outputs a variety of timing signals to the solid-state image sensing device  205  and to the image sensing signal processing circuit  206 . Reference numeral  208  designates a memory for temporarily storing image data. Reference numeral  209  designates a control circuit for controlling a variety of calculations as well as the image sensing apparatus as a whole. Reference numeral  210  designates an interface for recording on and reading from a recording medium (referred to as recording medium control I/F, hereinafter). Reference numeral  211  designates a detachable recording medium such as a semiconductor memory which storing and providing image data. Reference numeral  212  designates an interface for communicating with an external computer or the like. Reference numeral  213  designates a photometric device, which measures light and determines the brightness of the object. The control circuit  209  adjusts the aperture of the lens unit  201  according to the results of the determination made by the photometric device  213  using the lens drive device  202 . Reference numeral  214  designates a distance measurement device, which measures the distance to the object and detects a focus state. The control circuit  209  adjusts the focus lens of the lens unit  201  according to the detected focus state using the lens drive device  202 . 
       FIG. 2  is a diagram showing the schematic structure of an image sensing device of the image sensing apparatus of an embodiment of the present invention. 
     In  FIG. 2 , reference numeral  801  designates a unit pixel, with multiple unit pixels  801  arranged in a matrix. Reference numeral  802  designates a photodiode (hereinafter “PD”) that converts light of an image of an object into a signal charge. Reference numeral  804  designates an area that temporarily stores the signal charge (that is, a floating diffusion part, hereinafter referred to as “FD”). Reference numeral  803  designates a transfer switch that transfers the charge generated at the PD  802  to the FD  804  in response to a transfer pulse φTX. Reference numeral  805  designates a MOS amplifier that functions as a source follower. Reference numeral  806  designates a selection switch that selects a unit pixel  801  using a selection pulse φSEL. Reference numeral  807  designates a reset switch that resets the FD  804  to a predetermined potential (V DD ) using a reset pulse φRES. A floating diffusion amplifier is composed of the FD  804 , the MOS amplifier  805  and a constant current source  809  that is described below. The charge of the unit pixel  801  selected by the selection switch  806  is converted into voltage and output to an output circuit  811  over a signal output line  808 . Reference numeral  809  designates the constant current source that becomes a load of the MOS amplifier  805 . Reference numeral  814  designates a dummy pixel, which outputs a signal to the signal output line  808  when a dummy pixel selection circuit switch  905  described below is turned on. 
     The output of the pixels  801  is output from the output circuit  811  to an output line  812  by the driving of the horizontal scan circuit  810 . In addition, reference numeral  813  designates a vertical scan circuit that controls the driving of the pixels by supplying the respective drive signals φTX, φSEL and φRES to the switches  803 ,  806  and  807 , respectively. In φTX, φSEL and φRES, respectively, the drive signals supplied to an nth scan line selected for scanning by the vertical scan circuit  813  are written as φTXn, φSELn and φRESn. 
       FIG. 3A  is a schematic diagram showing an example of the general structure of the dummy pixel  814 . 
     In  FIG. 3A , reference numeral  901  designates a PD that converts the light of the object image into an electric charge. Reference numeral  903  designates an FD, and  902  designates a transfer switch that transfers a signal charge generated at the PD  901  in response to a transfer pulse φTX. Reference numeral  904  designates a MOS amplifier that functions as a source follower. Reference numeral  905  designates a selection switch that selects the dummy pixel using a selection pulse φSELD. Reference numeral  906  designates a reset switch that resets the FD  903  to a predetermined potential (V DD ) in response to a reset pulse φRESD. A floating diffusion amplifier is composed of the FD  903 , the MOS amplifier  904  and the constant current source  809  described above. When the selection pulse φSELD is high, the selection switch  905  is turned on, the dummy pixel is selected, and the charge in the FD  903  of the dummy pixel is converted into voltage and output to the output circuit  811  over the signal output line  808 . It should be noted that the dummy pixel exists in order to determine the clamping reference, and therefore the transfer switch  902  is always turned off so that no light charge is output from the dummy pixel. Alternatively, a dummy signal can be obtained even with the transfer switch  902  turned on by shielding the dummy pixel from light so that no light charge is output therefrom. 
     The dummy pixel shown in  FIG. 3B  may be used in place of the dummy pixel shown in  FIG. 3A . This dummy pixel employs a structure that eliminates the PD that takes up a large amount of surface area in an ordinary pixel. Such a structure enable the dummy pixel surface area to be reduced. 
     In addition, the dummy pixel shown in  FIG. 3C  may be used in place of the dummy pixel shown in  FIG. 3A . This dummy pixel employs a structure in which a constant voltage is always supplied to the FD  903 . Such a structure enables the surface area of the transfer switch and the reset switch to be reduced. 
       FIG. 4  is a diagram showing one example of the schematic structure of the output circuit  811  shown in  FIG. 2 , and provided for each signal output line  808  (i.e., for each column). In the structure shown in  FIG. 4 , however, the differential amplifier  612  is not provided for each column but on the output circuit  811  and shared by all columns. 
     The output circuit  811  shown in  FIG. 4  has a clamping circuit and an amplifier for each line of unit pixels  801  arranged in a matrix, and clamps the output at a level suitable for the operating range of the differential amplifier  612  provided in the downstream of the output circuit  811 . Reference numeral  808  designates a signal output line as in  FIG. 2 , and  809  designates the constant current source as in  FIG. 2 . Reference numeral  603  designates a clamping capacitor. Reference numeral  605  designates a clamping switch and  604  designates an amplifier. Reference numeral  606  designates a switch that selects and transfers a light signal corresponding to the electric charge converted by the PD  802  of the pixel  801  in response to a pulse φCS. Reference numeral  607  designates a capacitor that keeps the voltage proportional to the light signal. Reference numeral  608  designates a switch that selects the reset level (noise signal) of each pixel in response to a pulse φCN. Reference numeral  609  designates a capacitor that holds the reset level (noise signal). Reference numerals  610 ,  611  designate switches that transfer the output to the differential amplifier  612  in response to a pulse φCH applied by the horizontal scan circuit  810 . Reference numeral  613  designates a switch that changes the gain of the amplifier  604 . Reference numerals  614 ,  615  designate capacitances parasitic on the wirings. 
     The image sensing device having the structure shown in  FIGS. 2 through 4  described above can be driven by either a rolling electronic shutter, a collective transfer electronic shutter like that shown in  FIG. 13 , or a rolling transfer electronic shutter like that shown in  FIG. 14 . 
       FIG. 5  is a timing chart showing a first scan sequence of the image sensing apparatus according to the present embodiment. The first scan sequence is carried out where the image sensing device is driven using a rolling electronic shutter, and is executed by the control circuit  209 . The description given here is of the readout of a single line. 
     First, at a time t 71 , the φSEL and the φClamp are set to high, turning the clamping switch  605  and the selection switch  806  on and outputting the voltage of the FD  804  (the noise signal), which is then made the reference level for the clamping capacitor  603 . At this time, the light charge is not yet transferred to the FD  804  from the PD  802  and the FD  804  is reset to a reset voltage. Thereafter, at a time t 72 , φClamp is set to low, turning the clamping switch  605  off. At a tine t 73 , φCN is set to high, turning the switch  608  on and holding the noise signal amplified by the amplifier  604  at the capacitor  609 . 
     Next, at a time t 74 , φTX is set to high, turning the transfer switch  803  and transferring the light charge in the PD  802  to the FD  804 . Thereafter, at a time t 75 , φCS is set to high, turning the switch  606  on, which causes the light signal to be clamped by the noise signal by the clamping circuit (i.e., light signal minus noise signal) and amplified by the amplifier  604  and held in the capacitor  607 . When the light signal is held in the capacitor  607 , at a time t 76  φSEL is set to low. 
     Thereafter, from a time t 77 , the output circuit  811  sets φCH to high for each column in turn, thereby turning on the switches  610 ,  611  for each column in turn and causing the differential amplifier  612  to subtract the noise signal from the clamped light signal, resulting in the output of a low-noise light signal. 
     Thus, as described above, using a clamping circuit and reading a signal with a first scan sequence enables noise to be reduced more effectively than a case in which a clamping circuit is not used. 
       FIG. 6  is a timing chart showing a second scan sequence of the image sensing apparatus of the present embodiment. When the image sensing device is driven by either the collective transfer electronic shutter shown in  FIG. 13  or the rolling electronic shutter shown in  FIG. 14 , the second scan sequence is carried out while a pulse is being applied to the φSEL, and executed by the control circuit  209 . The description here is of the readout of a single line. 
     As described above, with the collective transfer electronic shutter and the rolling transfer electronic shutter, after a predetermined period of time for charge accumulation, the light charges of the PD  802  of all the pixels are transferred simultaneously or at high speed to the FD  804 , after which the transferred light charges are sequentially output by line. Therefore, dark signal can not be read prior to light signal like that of the first scan sequence described above. In addition, if the output of the light signal and the output of the noise signal are reversed, the noise signal will be clamped by the clamping circuit at the light signal, resulting in the output of noise signal minus light signal. In this case, if the operating range of the amplifier  604  is not wide enough, the output will fall outside the operating range of the amplifier  604 . In order to ensure that the output does not fall outside the operating range of the amplifier  604 , the amplifier  604  must be given an operating range, (maximum light signal) to (−maximum light signal), in which the polarity is substantially reversed, which is impractical in terms of making the image sensing apparatus inexpensive and compact. Therefore, performing scanning with the second scan sequence described below enables pixel signals to be output even during operation of the collective transfer electronic shutter or the rolling transfer electronic shutter using the output circuit shown in  FIG. 4  without expanding the operating range of the amplifier  604 . It should be noted that, in the following description, the dummy pixel  814  has the structure shown in  FIG. 3A  or  FIG. 3B . 
     When scanning starts, first, between a time t 1001  after charge accumulation ends and a time t 1002  pulses are applied to φCLAMP, φRESD and φSELD, and the reset level for the dummy pixel becomes the reference level for the clamping circuit. 
     Then, at a time t 1003 , the line for which the voltage of the FD  804  of the unit pixel  801  is to be output is selected by φSEL and between a time t 1004  to a time t 1005  a pulse is applied to φCS. During this time, the light charge transferred to the FD  804  is output from the pixel  801  through the source follower, clamped by the clamping circuit at the reference potentials and amplified by the amplifier  604 , after which it is temporarily held in the capacitor  607 . 
     Next, between a time t 1006  and a time t 1007 , pulses are applied to φRES and φCN, causing the reset potential of the FD  804  from each pixel  801  on the selected line to be temporarily held in the capacitor  609  as a noise signal. 
     Thereafter, at a time t 1008  φSEL is set to low, and φCH is set to high for each column in turn from a time t 1009 , thereby turning on the switches  610 ,  611  for each column in turn, causing the differential amplifier  612  to subtract the noise signal from the clamped light signal, resulting in the output of a light signal with less noise. 
     Thus, as described above, with the collective transfer electronic shutter and the rolling transfer electronic shutter, the reset signal of the dummy pixel  814  is clamped as the reference level, causing the clamping circuit to output the light signal minus the reference level when the light charge is output. As a result, the signal values from the clamping circuit can be kept within the operating range of the amplifier  604 . 
     It should be noted that, if the dummy pixel  814  has the structure shown in  FIG. 3C , there is no need to supply the φRESD between t 1001  and t 1002 . Other than this, the image sensing device can be driven with a timing like that shown in  FIG. 6 . 
     With the second scan sequence, the light signal and the reset signal from the same pixel  801  are temporarily held in the capacitors  607  and  609 , respectively, and the light signal from which the reset signal is subtracted is output as the final signal. Therefore, it is effective in removing fixed pattern noise from the circuit. However, because the reset is carried out and the switch  807  is operated while the light signal and the noise signal are being output, the reset noise of the reset switch  807  is added to the light signal and the noise signal. 
     Accordingly, in the present embodiment, in a moving image mode the collective transfer electronic shutter or the rolling transfer electronic shutter operation is carried out and scanning accomplished with the second scan sequence. By contrast, in a still image mode, in which a premium is placed on picture quality, a rolling electronic shutter operation is carried out and scanning accomplished with the first scan sequence. Thus, using different scan sequences for the moving image mode and for the still image mode as described enables an image of low distortion to be obtained without the use of a mechanical shutter in the moving image mode, and enables an image with a superior SN ratio in which the reset noise of the reset switch  807  is removed to be obtained in the still image mode. It is also possible to change the exposure time by using a mechanical shutter together with the electronic shutter in the still image mode. 
     It should be noted that the foregoing description is of a structure in which a dummy pixel is provided inside the image sensing device, the dummy pixel signal is taken as the reference signal, and the reference signal is clamped by a clamping circuit. However, the present invention is not limited thereto, provided that there is a signal supply circuit (independent of the dummy pixel; for example, a voltage conversion circuit or the like that takes a voltage supplied from outside the image sensing device and makes it the same voltage as a voltage generated by the dummy pixel) that outputs a reference signal to the clamping circuit. 
     Next, a description is given of the operation of the image sensing apparatus shown in  FIG. 1  during image sensing. 
     When a main power supply is turned on, the control system power is turned on, and further, power to the image sensing signal processing circuit  206  and other image sensing system circuits is turned on. 
     If a still image mode is selected by the user and a release-button, not shown, is pressed, the control circuit  209  causes the photometric device  213  to measure the amount of light and determine the brightness of the object, and the lens drive device  202  adjusts the aperture of the lens unit  201  according to the results of that determination. 
     Next, a high-frequency component is extracted based on a signal output from the distance measurement device  214  and the control circuit  209  calculates the distance to the object. After that, the lens drive device  202  drives the lens unit  201 , measures the distance to the object and determines whether the object is in focus or not. If not, then the lens drive device  202  again drives the lens unit  201  and measures the distance to the object. 
     Then, after it is confirmed that the object is in focus, image sensing starts. 
     Next, a description is given of the scan method of the image sensing device during still image sensing based on  FIG. 7 . The following image sensing device scanning is controlled by the control circuit  209 . As described above, during still image sensing, the image sensing device is driven by rolling electronic shutter. 
     First, the resetting of the image sensing device  205  starts. When reset starts, a reset scan is sequentially carried out by line of the image sensing device  205 . Then, after a fixed charge accumulation time T 1101 , reading of charges is carried out with the first scan sequence. 
     Next, a description is given of a scan method in a case in which a mechanical shutter is used when the image sensing device is driven with a rolling electronic shutter, based on  FIG. 8 . 
     First, the resetting of the image sensing device  205  starts. When reset starts, a reset scan is sequentially carried out by line of the image sensing device  205 . Then, when the mechanical shutter is opened, image sensing device  205  exposure starts. After a charge accumulation time T 1201 , the mechanical shutter is closed, thereby ending the exposure of the image sensing device  205 . Thereafter, reading of charges is carried out with the first scan sequence. In this case, the charge accumulation time of the image sensing device  205  is a time T 1202 , but the exposure time of the image sensing device  205  is T 1201  by the mechanical shutter. Thus, using a mechanical shutter enables the difference in image sensing timing between the top and the bottom of the screen to be decreased. 
     The signals output from the image sensing device  205  by the scans shown in  FIG. 7  or  FIG. 8  described above are amplified and processed (A/D converted and the like) by the image sensing signal processing circuit  206 , and written to the memory  208  by the control circuit  209 . 
     The data stored in the memory  208  is recorded on the semiconductor memory or other detachable recording medium  211  through the recording medium control I/F  210  under control of the control circuit  209 . 
     In addition, alternatively, the image may be input directly to a computer or the like through the external I/F  212  and then processed. 
     By contrast, if the moving image mode is selected by the user and the release button, not shown, is pressed, the control circuit  209  causes the photometric device  213  to measure the amount of light and determine the brightness of the object, and the lens drive device  202  adjusts the aperture of the lens unit  201  according to the results of that determination. 
     Next, a high-frequency component is extracted based on a signal output from the distance measurement device  214  and the control circuit  209  calculates the distance to the object. After that, the lens drive device  202  drives the lens unit  201 , measures the distance to the object and determines whether the object is in focus or not. If not, then the lens drive device  202  again drives the lens unit  201  and measures the distance to the object. 
     Then, after it is confirmed that the object is in focus, image sensing starts. It should be noted that adjustment of the exposure light amount and focus adjustment is carried out at predetermined time intervals until there is an instruction to end moving image sensing. 
     Next, a description is given of the scan method of the image sensing device during moving image sensing with reference to  FIG. 9  and  FIG. 10 . 
       FIG. 9  is a diagram illustrating scanning with a rolling transfer electronic shutter. 
     First, the resetting of the image sensing device  205  starts. When reset starts, a reset scan is sequentially carried out by line of the image sensing device  205 . Then, after the reset scan, and after a predetermined charge accumulation time T 1301 , signal charges are transferred by line of the image sensing device  205 , after which charges are read out with the second scan sequence. 
       FIG. 10  is a diagram illustrating scanning with a collective transfer electronic shutter. 
     First, the resetting of the image sensing device  205  starts. When reset starts, a reset scan is carried out for all the pixels of the image sensing device  205  simultaneously. Then, after a predetermined charge accumulation time T 1401 , the signal charges of all the pixels are transferred collectively, after which charges are read out with the second scan sequence. 
     The signals output from the image sensing device  205  by the scans shown in  FIG. 9  and  FIG. 10  described above are amplified and processed (A/D converted and the like) by the image sensing signal processing circuit  206 , and written to the memory  208  by the control circuit  209 . 
     The data stored in the memory  208  is recorded on the semiconductor memory or other detachable recording medium  211  through the recording medium control I/F  210 . 
     In addition, alternatively, the image may be input directly to a computer or the like through the external I/F  212  and then processed. 
     The above-described embodiment enables to output signals of the same polarity from the clamping circuit when the image sensing device is driven using collective transfer electronic shutter and rolling transfer electronic shutter as when the image sensing device is driven using the rolling electronic shutter. In other words, the clamping circuit output range can be limited, and the output from the clamping circuit can be prevented from falling outside the operating range of the amplifier provided in the downstream of the clamping circuit. In addition, using the clamping circuit enables more accurate noise removal when the image sensing device is driven using rolling electronic shutter. 
     It should be noted that in the above embodiment, when light charge is sequentially transferred from PD to FD and light charge is sequentially read out from FD, the transfer operation and the read out operation are performed by line. However, the present invention is not limited thereto. If the image sensing apparatus has a plurality of output circuits such as the output circuits  811 , the transfer operation and the read out operation can be performed by a plurality of lines. 
     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims. 
     This application claims the benefit of Japanese Application Nos. 2005-144539 filed on May 17, 2005 and 2006-122532 filed on Apr. 26, 2006, which are hereby incorporated by reference herein in its entirety.

Technology Category: 5