Abstract:
Provided is an inexpensive photoelectric converter having a function of switching resolutions. In a photoelectric converter including plural photoelectric conversion circuits, an amplifier circuit connected to outputs of the photoelectric conversion circuits, and a reset circuit connected to outputs of part of the photoelectric conversion circuits, there is arranged a connection circuit between outputs of adjacent photoelectric conversion circuits of the photoelectric conversion circuits.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a photoelectric converter for converting a received optical signal to an electric signal, an image sensor provided with the photoelectric converter and used in an image reader, and a signal reading circuit.  
         [0003]     2. Description of the Related Art  
         [0004]      FIG. 9  is a circuit block diagram of an image sensor used in a conventional image reader.  FIG. 10  is a circuit block diagram of a shift register and light receiving elements for the image sensor.  FIG. 11  is an equivalent circuit of the light receiving elements.  FIG. 12  is an operation timing chart of the image sensor.  
         [0005]     As shown in  FIG. 10 , the image sensor changes resolutions by switching connections of the shift register based on a MODE signal. Further, as shown in  FIGS. 11 and 12 , at a low resolution mode, the image sensor simultaneously reads plural pixel signals into a common signal line. When the resolution is ½ for example, the image sensor simultaneously reads two pixel signals into the common signal line (see JP 11-234473 A: pages 7 to 11 and FIGS. 1 to 4).  
         [0006]     However, such a conventional image reader requires many elements to form a switching circuit for switching data signals and has a problem of increase in chip area. In particular, when three or more kinds of resolutions are switched, still more elements are needed for switching data signals. Also, when signals of plural light receiving elements are simultaneously read into the common signal line, offsets of amplifiers for amplifying an output of each light receiving element differ from each other, so levels of signals to be read vary depending on resolutions. Thus, a problem occurs in that a circuit for correcting the variation is necessary.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention has been made to solve such conventional problems and has an object to provide an inexpensive photoelectric converter having a function of switching resolutions, and an image sensor.  
         [0008]     According to an aspect of the present invention, there is provided a photoelectric converter, including: plural photoelectric conversion circuits; an amplifier circuit connected to outputs of the photoelectric conversion circuits; a reset circuit connected to outputs of part of the photoelectric conversion circuits; and a connection circuit connected between outputs of adjacent photoelectric conversion circuits of the photoelectric conversion circuits.  
         [0009]     Also, the number of the reset circuits is set equal to or less than half the number of the photoelectric conversion circuits, and the number of the connection circuits is set less than the number of the photoelectric conversion circuits. Further, part of the connection circuits is turned ON to realize a function of outputting an average value of the outputs of the adjacent photoelectric conversion circuits through the amplifier circuit. Further, all or part of the connection circuits is turned ON and all or part of the reset circuits is turned ON to realize a function of resetting all of the photoelectric conversion circuits. According to another aspect of the present invention, there is provided a signal reading circuit, including: plural holding circuits; a shift register composed of plural flip-flops; and a common signal line, for sequentially reading plural signals held in the plural holding circuits into the common signal line based on a signal from the shift register, in which clock signals input to the flip-flops and part of inverted signals of the clock signals are both set to Hi to realize a function of skipping to read part of the signals.  
         [0010]     With the photoelectric converter of the present invention, by providing the necessary minimum number of inter-light-receiving-element circuits and of reset circuits, outputs of the adjacent light receiving elements can be averaged and output at a low resolution mode.  
         [0011]     As described above, the photoelectric converter having a resolution switching function with a simple construction can be provided. In addition, it is possible to provide the inexpensive contact image sensor by mounting the plural photoelectric converters linearly. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     In the accompanying drawings:  
         [0013]      FIG. 1  is a circuit block diagram of a close image sensor of the present invention;  
         [0014]      FIG. 2  is a circuit diagram showing signal reading circuits and light receiving elements for four bits, of an image sensor according to a first embodiment of the present invention;  
         [0015]      FIG. 3  is a timing chart of an operation of the image sensor at a high resolution mode according to the first embodiment of the present invention;  
         [0016]      FIG. 4  is a timing chart of an operation of the image sensor at an intermediate resolution mode according to the first embodiment of the present invention;  
         [0017]      FIG. 5  is a timing chart of an operation of the image sensor at a low resolution mode according to the first embodiment of the present invention;  
         [0018]      FIG. 6  is a circuit diagram showing signal reading circuits and light receiving elements for four bits, of an image sensor according to a second embodiment of the present invention;  
         [0019]      FIG. 7  is a timing chart of an operation of the image sensor at a high resolution mode according to the second embodiment of the present invention;  
         [0020]      FIG. 8  is a timing chart of an operation of the image sensor at a low resolution mode according to the second embodiment of the present invention;  
         [0021]      FIG. 9  is a circuit block diagram of a conventional image sensor;  
         [0022]      FIG. 10  is a circuit block diagram of a shift register and light receiving elements of the conventional image sensor;  
         [0023]      FIG. 11  is an equivalent circuit of the light receiving element of the conventional image sensor; and  
         [0024]      FIG. 12  is an operation timing chart of the conventional image sensor. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]      FIG. 1  is a circuit block diagram of a close image sensor of the present invention. The close image sensor of the present invention has plural photoelectric converters  21  mounted therein. According to this embodiment mode,  11  photoelectric converters  21  are arranged and mounted in a single line to constitute the close image sensor, for example.  
         [0026]     The photoelectric converter  21  includes plural light receiving elements  20 , a logic circuit  22 , a signal processing circuit  13 , a common signal line  11 , and a control signal line  23 . Though not shown, a constant current source is connected to the common signal line  11 . Connected to all the photoelectric converters  21  are a clock φCLK for driving the photoelectric converter, a start pulse φFS, a resolution switching signal φMODE, and a signal output PSIG. A start signal φSI of the contact image sensor is connected to the first photoelectric converter  21 .  
         [0027]     The logic circuit  22  receives input signals φSI, φCLK, φMODE, and φFS, produces from the signals a control signal for controlling the light receiving elements  20  and a read-start signal for the next chip, and reads and outputs them to the control signal line  23  and a read-start signal line  24 , respectively. The signal processing circuit  13  amplifies a signal output from the light receiving elements  20  to the common signal line  11  and outputs the amplified signal as the SIG signal. Such a construction is adapted that plural kinds of resolutions are obtained according to the state of φMODE.  
         [0028]     Hereinafter, embodiments of the present invention will be described by showing specific circuits.  
       First Embodiment  
       [0029]     In a first embodiment, a description will be given to a contact image sensor having a function of switching three kinds of resolutions: a high resolution mode of 1,200 DPI, an intermediate resolution mode of 600 DPI, and a low resolution mode of 300 DPI. Such a construction is adopted that the high resolution mode, the intermediate resolution mode, and the low resolution mode are respectively obtained when φMODE is at a high level, is at a low level, and is switched from the high level to the low level.  
         [0030]      FIG. 2  is a circuit diagram showing signal reading circuits and light receiving elements for four bits, of the image sensor according to the first embodiment. This includes four light receiving elements  101 ,  102 ,  103 , and  104  as photoelectric conversion circuits, amplifiers  2  as amplifier circuits such as source follower amplifiers, signal voltage holding capacities  3 , reference voltage holding capacities  4 , signal voltage reading switches  5 , reference voltage reading switches  6 , source follower amplifiers  7  for reading, read-selection switches  10 , signal voltage reading switches  8 , reference voltage reading switches  9 , a common signal line  11 , flip-flops  19 , three inter-light-receiving-element switches  15 ,  16 , and  17  as connection circuits, two reset switches  14  and  18  as reset circuits, etc. The common signal line  11  is connected to a constant current source  12  and the signal processing circuit  13  outside this block for the four bits.  
         [0031]     The logic circuit  22  of  FIG. 1  produces control signals input to this block, namely, φR 1 , φR 2 , φS 1 , φS 2 , φRIN, φSIN, K 1 , K 2 , φDIN, φC 1 , φC 1 X, φC 2 , φC 2 X, φC 3 , and φC 3 X. VRT denotes a reset voltage of the light receiving element and is produced in the photoelectric converter  21  of  FIG. 1 . GND is usually 0 V.  
         [0032]     An operation of the contact image sensor of the present invention will be described below based on a timing chart for each resolution.  
         [0033]      FIG. 3  is a timing chart of an operation of the contact image sensor at the high resolution mode according to the present invention. This applies to a case when all bit signals are read.  
         [0034]     First, a signal batch reading operation will be described. This operation is for reading signal voltages and reference voltages of the light receiving elements into the respective holding capacities, which is simultaneously performed for all light receiving elements of all photoelectric converters.  
         [0035]     Each pulse, φR 1 , φR 2 , φS 1 , φS 2 , φRIN, and φSIN, is produced by being generated at a predetermined timing from an input start pulse φFS, which is simultaneously performed for all photoelectric converters.  
         [0036]     With φSIN, optical signals generated according to lights respectively entering the light receiving elements  101  to  104  are respectively forwarded to the signal voltage holding capacities  3  through the amplifiers  2  as signal voltages. Next, with φR 1 , φR 2 , and φS 1 , the reset switches  14  and  18  and the inter-light-receiving-element switches  15  and  17  are turned ON, and outputs from all the light receiving elements  101  to  104  are reset to VRT. Next, when φS 1  becomes Lo and the inter-light-receiving-element switches  15  and  17  are turned OFF, output terminals of the light receiving elements  102  and  103  are reset to a level on which off-noises of the inter-light-receiving-element switches  15  and  17  are respectively superimposed. Next, when φR 1  and φR 2  become Lo and the reset switches  14  and  18  are turned off, output terminals of the light receiving elements  101  and  104  are reset to a level on which off-noises of the inter-light-receiving-element switches  14  and  18  are respectively superimposed. When the inter-light-receiving-element switches and the reset switches respectively have the same sizes, the off-noises are substantially equalized. Thus, reset levels of all the light receiving elements are substantially equalized. Note that the inter-light-receiving-element switch  16  is always kept ON with φ 2 . After that, accumulation of charge generated according to light entering each light receiving element starts.  
         [0037]     Next, with φRIN, reset levels of all the light receiving elements  101  to  104  are forwarded as the reference voltages to the reference voltage holding capacities  4  through the amplifiers  2 , respectively.  
         [0038]     Next, a signal reading operation will be described. This signal reading operation is for sequentially reading voltages of holding capacities into the common signal line, that is, after reading of the first photoelectric converter ends, reading of the second photoelectric converter starts. φDIN determines the start of reading, which is produced in the logic circuit with the read-start signal φSI and an input signal of the read-start signal line  24 . The number of bits to be read varies depending on resolutions, and therefore the timing for outputting φDIN is changed according to φMODE.  
         [0039]     Further, the clock signals supplied to the shift register, that is, φC 1 , φC 1 X, φC 2 , φC 2 X, φC 3 , and φC 3 X, are also produced in the logic circuit  22 .  
         [0040]     When φDIN is input to the flip-flop  19  for the first bit, with φSCH 1  and φOS 1 , the read-selection switch  10  and the signal voltage reading switch  8  are turned ON. Then, the signal voltage for the first bit held in the signal voltage holding capacity  3  is read into the common signal line  11  through the source follower amplifier  7 .  
         [0041]     Next, with φOS 1  and φOR 1 , when the signal voltage reading switch  8  is turned OFF and thereafter the reference voltage reading switch  9  is turned ON, the reference voltage for the first bit held in the reference voltage holding capacity  4  is read into the common signal line  11  through the source follower amplifier  7 .  
         [0042]     Next, with φSCH 1  and φSCH 2 , the first bit read-selection switch  10  is turned OFF, the second-bit read-selection switch  10  is turned ON, and reading of the signal voltage for the second bit starts. After that, similarly, reading of the signal voltage and the reference voltage for the second bit starts.  
         [0043]     Offsets of the amplifiers  2  and  7  for each bit are superimposed on the signal voltage and the reference voltage for each bit. A fixed pattern noise between the bits can be eliminated by extracting a difference between the signal voltage and the reference voltage by a clamp circuit in the signal processing circuit  13 , etc.  
         [0044]      FIG. 4  is a timing chart of an operation of the contact image sensor at the intermediate resolution mode according to the present invention. This applies to a case when odd-numbered bit signals are read.  
         [0045]     First, a signal batch reading operation will be described.  
         [0046]     Since φS 1  is always kept Hi, the inter-light-receiving-element switches  15  and  17  are kept ON. An output of the light receiving element  101  becomes equal to that of the light receiving element  102  and the output value is the average of the two outputs. The same holds true for outputs of the light receiving elements  103  and  104 .  
         [0047]     With φSIN, an optical signal of each average value is forwarded to the signal voltage holding capacity  3  through the amplifier  2  as the signal voltage. Next, φR 1  and φR 2  become Hi, the reset switches  14  and  18  are turned ON, and the outputs of all the light receiving elements  101  to  104  are reset to VRT. Next, when φR 1  and φR 2  become Lo and the reset switches  14  and  18  are turned OFF, output terminals of the light receiving elements  101  and  102  and output terminals of the light receiving elements  103  and  104  are reset to a level on which off-noises of the reset switches  14  and  18  are respectively superimposed. When the inter-light-receiving-element switches and the reset switches respectively have the same sizes, the off-noises are substantially equalized. Thus, reset levels of all the light receiving elements are substantially equalized. After that, accumulation of charge generated according to light entering each light receiving element starts.  
         [0048]     Next, with φRIN, reset levels of all the light receiving elements  101  to  104  are respectively forwarded to the reference voltage holding capacities  4  through the amplifiers  2  as reference voltages.  
         [0049]     Next, a signal reading operation will be described.  
         [0050]     Among the clock signals (φC 1 , φC 1 X, φC 2 , φC 2 X, φC 3 , and φC 3 X) supplied to the shift register, φC 2  and φC 2 X are always kept Hi, data skips the flip-flops for the second and fourth bits and are not read thereinto. At this time, by setting K 1  to Hi, φSCH 2  and φSCH 4  become Lo, and short-circuit of signals to be read into the common signal line is avoided.  
         [0051]     When φDIN is input to the flip-flop  19  for the first bit, with φSCH 1  and φOS 1 , the read-selection switch  10  and the signal voltage reading switch  8  are turned ON and the signal voltage for the first bit held in the signal voltage holding capacity  3  is read into the common signal line  11  through the source follower amplifier  7 .  
         [0052]     Next, with φOS 1  and φOR 1 , when the signal voltage reading switch  8  is turned OFF and thereafter the reference voltage reading switch  9  is turned ON, the reference voltage for the first bit held in the reference voltage holding capacity  4  is read into the common signal line  11  through the source follower amplifier  7 .  
         [0053]     Next, with φSCH 1  and φSCH 3 , the first-bit read-selection switch  10  is turned OFF, the third-bit read-selection switch  10  is turned ON, and reading of the signal voltage for the third bit starts. After that, similarly, reading of the signal voltage and the reference voltage for the third bit starts.  
         [0054]      FIG. 5  is a timing chart of an operation of the contact image sensor according to the present invention at the low resolution mode (300 DPI). This is the case of reading (4N-3)-th bit signal (N is a natural number).  
         [0055]     First, a signal batch reading operation will be described.  
         [0056]     Since φS 1  and φS 2  are always kept Hi, the inter-light-receiving-element switches  15 ,  16 , and  17  are kept ON. Outputs of the light receiving elements  101  to  104  are equalized and the output value is the average of the outputs of the four light receiving elements.  
         [0057]     With φSIN, an average value of optical signals generated according to lights respectively entering the light receiving elements  101  to  104  is forwarded to the signal voltage holding capacity  3  through the amplifier  2  as the signal voltage.  
         [0058]     Next, φR 1  becomes Hi, the reset switch  14  is turned ON, and the outputs of all the light receiving elements  101  to  104  are reset to VRT. Next, when φR 1  becomes Lo and the reset switch  14  is turned OFF, output terminals of the light receiving elements  101  to  104  are reset to a level on which an off-noise of the reset switch  14  is superimposed. When the inter-light-receiving-element switches and the reset switches respectively have the same sizes, the off-noises are substantially equalized. Thus, reset levels of all the light receiving elements are substantially equalized. After that, accumulation of charge generated according to light entering each light receiving element starts.  
         [0059]     Next, with φRIN, reset levels of all the light receiving elements  101  to  104  are respectively forwarded to the reference voltage holding capacities  4  through the amplifiers  2  as reference voltages.  
         [0060]     Next, a signal reading operation will be described.  
         [0061]     Among the clock signals (φC 1 , φC 1 X, φC 2 , φC 2 X, φC 3 , and φC 3 X) supplied to the shift register, φC 2 , φC 2 X, φC 3 , and φC 3 X are always kept Hi, data skips the flip-flops for the second, third, and fourth bits and are not read thereinto. At this time, by setting K 1  and K 2  to Hi, φSCH 2 , φSCH 3 , and φSCH 4  become Lo, and short-circuit of signals to be read into the common signal line is avoided.  
         [0062]     When φDIN is input to the flip-flop  19  for the first bit, with φSCH 1  and φOS 1 , the read-selection switch  10  and the signal voltage reading switch  8  are turned ON and the signal voltage for the first bit held in the signal voltage holding capacity  3  is read into the common signal line  11  through the source follower amplifier  7 .  
         [0063]     Next, with φOS 1  and φOR 1 , when the signal voltage reading switch  8  is turned OFF and thereafter the reference voltage reading switch  9  is turned ON, the reference voltage for the first bit held in the reference voltage holding capacity  4  is read into the common signal line  11  through the source follower amplifier  7 .  
         [0064]     Next, with φSCH 1  and φSCH 5 , the first-bit read-selection switch  10  is turned OFF, the fifth-bit read-selection switch  10  is turned ON, and reading of the signal voltage for the fifth bit starts. After that, similarly, reading of the signal voltage and the reference voltage for the fifth bit starts.  
       Second Embodiment  
       [0065]     In a second embodiment, a description will be given to a contact image sensor having a function of switching two kinds of resolutions: 1,200 DPI and 600 DPI. Such a construction is adopted that the high resolution mode and the low resolution mode are respectively obtained when MODE is at a high level and is at a low level.  
         [0066]      FIG. 6  is a circuit diagram showing signal reading circuits and light receiving elements for four bits, of the image sensor according to the second embodiment. This includes four light receiving elements  101 ,  102 ,  103 , and  104  as photoelectric conversion circuits, amplifiers  2  as amplifier circuits such as source follower amplifiers, signal voltage holding capacities  3 , reference voltage holding capacities  4 , signal voltage reading switches  5 , reference voltage reading switches  6 , source follower amplifiers  7  for reading, read-selection switches  10 , signal voltage reading switches  8 , reference voltage reading switches  9 , a common signal line  11 , flip-flops  19 , two inter-light-receiving-element switches  15  and  16  as connection circuits, two reset switches  14  and  18  as reset circuits, etc. The common signal line  11  is connected to a constant current source  12  and a signal processing circuit  13  outside this block for the four bits.  
         [0067]     The logic circuit  22  of  FIG. 1  produces control signals input to this block, namely, φR 1 , φS 1 , φRIN, φSIN, K 1 , φDIN, φC 1 , φC 1 X, φC 2 , and φC 2 X. VRT denotes a reset voltage of the light receiving element and is produced in the photoelectric converters  21  and  21 ′ of  FIG. 1 . GND is usually 0 V.  
         [0068]     An operation of the contact image sensor of the present invention will be described below based on a timing chart for each resolution.  
         [0069]      FIG. 7  is a timing chart of an operation of the contact image sensor at the high resolution mode (1,200 DPI) according to the present invention. This applies to a case when all bit signals are read.  
         [0070]     First, a signal batch reading operation will be described. This operation is for reading signal voltages and reference voltages of the light receiving elements into the respective holding capacities, which is simultaneously performed for all light receiving elements of all photoelectric converters.  
         [0071]     Each pulse of φR 1 , φS 1 , φRIN, and φSIN is produced by being generated at a predetermined timing from an input start pulse φFS, which is simultaneously performed for all photoelectric converters.  
         [0072]     With φSIN, optical signals generated according to lights respectively entering the light receiving elements  101  to  104  are respectively forwarded to the signal voltage holding capacities  3  through the amplifiers  2  as signal voltages. Next, with φR 1  and φS 1 , the reset switches  14  and  18  and the inter-light-receiving-element switches  15  and  16  are turned ON, and outputs from all the light receiving elements  101  to  104  are reset to VRT. Next, when φS 1  becomes Lo and the inter-light-receiving-element switches  15  and  16  are turned OFF, output terminals of the light receiving elements  102  and  104  are reset to a level on which off-noises of the inter-light-receiving-element switches  15  and  16  are respectively superimposed. Next, when φR 1  becomes Lo and the reset switches  14  and  18  are turned off, output terminals of the light receiving elements  101  and  103  are reset to a level on which off-noises of the inter-light-receiving-element switches  14  and  18  are respectively superimposed. When the inter-light-receiving-element switches and the reset switches respectively have the same sizes, the off-noises are substantially equalized. Thus, reset levels of all the light receiving elements are substantially equalized. After that, accumulation of charge generated according to light entering each light receiving element starts.  
         [0073]     Next, with φRIN, reset levels of all the light receiving elements  101  to  104  are forwarded as the reference voltages to the reference voltage holding capacities  4  through the amplifiers  2 , respectively.  
         [0074]     Next, a signal reading operation will be described. This signal reading operation is for sequentially reading voltages of holding capacities into the common signal line, that is, after reading of the first photoelectric converter ends, reading of the second photoelectric converter starts. φDIN determines the start of reading, which is produced in the logic circuit with an input signal of the read-start signal line  24 . The number of bits to be read varies depending on resolutions, and therefore the timing for outputting φDIN is changed according to φMODE.  
         [0075]     Further, the clock signals supplied to the shift register, that is, φC 1 , φC 1 X, φC 2 , and φC 2 X are also produced in the logic circuit  22 .  
         [0076]     When φDIN is input to the flip-flop  19  for the first bit, with φSCH 1  and φOS 1 , the read-selection switch  10  and the signal voltage reading switch  8  are turned ON. Then, the signal voltage for the first bit held in the signal voltage holding capacity  3  is read into the common signal line  11  through the source follower amplifier  7 .  
         [0077]     Next, with φOS 1  and φOR 1 , when the signal voltage reading switch  8  is turned OFF and thereafter the reference voltage reading switch  9  is turned ON, the reference voltage for the first bit held in the reference voltage holding capacity  4  is read into the common signal line  11  through the source follower amplifier  7 .  
         [0078]     Next, with φSCH 1  and φSCH 2 , the first bit read-selection switch  10  is turned OFF, the second-bit read-selection switch  10  is turned ON, and reading of the signal voltage for the second bit starts. After that, similarly, reading of the signal voltage and the reference voltage for the second bit starts.  
         [0079]     Offsets of the amplifiers  2  and  7  for each bit are superimposed on the signal voltage and the reference voltage for each bit. A fixed pattern noise between the bits can be eliminated by extracting the difference between the signal voltage and the reference voltage by a clamp circuit in the signal processing circuit  13 , etc.  
         [0080]      FIG. 8  is a timing chart of an operation of the contact image sensor according to the present invention at the low resolution mode (600 DPI). This is the case of reading odd-numbered bit signals.  
         [0081]     First, a signal batch reading operation will be described.  
         [0082]     Since φS 1  is always kept Hi, the inter-light-receiving-element switches  15  and  16  are kept ON. An output of the light receiving element  101  becomes equal to that of the light receiving element  102  and the output value is the average of the two outputs. The same holds true for outputs of the light receiving elements  103  and  104 .  
         [0083]     With φSIN, an optical signal of each average value is forwarded to the signal voltage holding capacity  3  through the amplifier  2  as the signal voltage. Next, φR 1  becomes Hi, the reset switches  14  and  18  are turned ON, and the outputs of all the light receiving elements  101  to  104  are reset to VRT. Next, when φR 1  becomes Lo and the reset switches  14  and  18  are turned OFF, output terminals of the light receiving elements  101  and  102  and output terminals of the light receiving elements  103  and  104  are reset to a level on which off-noises of the reset switches  14  and  18  are respectively superimposed. When the inter-light-receiving-element switches and the reset switches respectively have the same sizes, the off-noises are substantially equalized. Thus, reset levels of all the light receiving elements are substantially equalized. After that, accumulation of charge generated according to light entering each light receiving element starts.  
         [0084]     Next, with φRIN, reset levels of all the light receiving elements  101  to  104  are respectively forwarded to the reference voltage holding capacities  4  through the amplifiers  2  as reference voltages.  
         [0085]     Next, a signal reading operation will be described.  
         [0086]     Among the clock signals (φC 1 , φC 1 X, φC 2 , and φC 2 X) supplied to the shift register, and φC 2  and φC 2 X are always kept Hi, data skips the flip-flops for the second and fourth bits and are not read thereinto. At this time, by setting K 1  to Hi, φSCH 2  and φSCH 4  become Lo, and short-circuit of signals to be read into the common signal line is avoided.  
         [0087]     When φDIN is input to the flip-flop  19  for the first bit, with φSCH 1  and φOS 1 , the read-selection switch  10  and the signal voltage reading switch  8  are turned ON and the signal voltage for the first bit held in the signal voltage holding capacity  3  is read into the common signal line  11  through the source follower amplifier  7 .  
         [0088]     Next, with φOS 1  and φOR 1 , when the signal voltage reading switch  8  is turned OFF and thereafter the reference voltage reading switch  9  is turned ON, the reference voltage for the first bit held in the reference voltage holding capacity  4  is read into the common signal line  11  through the source follower amplifier  7 .  
         [0089]     Next, with φSCH 1  and φSCH 3 , the first-bit read-selection switch  10  is turned OFF, the third-bit read-selection switch  10  is turned ON, and reading of the signal voltage for the third bit starts. After that, similarly, reading of the signal voltage and the reference voltage for the third bit starts.