JP3895689B2 - Image sensor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a one-dimensional image sensor that receives reflected light from a light-irradiated document and converts it into an electrical signal, and is applied to an image reading apparatus such as a FAX or a scanner.
[0002]
[Prior art]
FIG. 9 shows a circuit diagram of a contact type one-dimensional image sensor IC used in a conventional FAX reader. As shown in FIG. 9, the output of the phototransistor PTR is read to the common signal line LSIG by sequentially turning on the switches S1 to Sn.
[0003]
Such a contact-type phototransistor one-dimensional image sensor is described in Japanese Patent Application Laid-Open No. 61-124171.
[0004]
[Problems to be solved by the invention]
However, in such a one-dimensional image sensor, since the carriers remaining at the base of the phototransistor PTR are removed through the emitter, the base potential cannot be sufficiently reset to the initial state, and there is a problem that the afterimage is large.
To obtain an effective light output, turn off the light source and turn it dark before reading the document, read the dark output, record the value, turn the light source on and light the document The light output was read, the difference from the dark output recorded previously was obtained, and the effective light output was obtained. However, this method has a problem in that it has to have a memory for recording the output in the dark, resulting in an increase in cost. In addition, the previous dark output is not always at all, but is often incorporated into the final product such as a facsimile machine and set only once at the factory shipment stage, and the dark output has temperature characteristics. However, the dark output recorded at the time of shipment from the factory is different from the dark output when the document is actually read, so that the effective light output cannot be obtained correctly and the reproduced image quality is inferior.
[0005]
Therefore, in order to solve the conventional problems, it is an object of the present invention to supply an image sensor with a small afterimage at low cost and to obtain an effective bright output with high accuracy.
[0006]
[Means for Solving the Problems]
In order to solve the above problems, the present invention has an image sensor configured as follows.
(1) In an image sensor configured by linearly arranging and mounting a plurality of linear image sensor ICs, the output of a plurality of photodiodes is converted into an exposure amount via a first amplifier connected to each photodiode. The image signal that changes accordingly and the initial signal when the photodiode is returned to the initial state are held for each light receiving element, and then the image signal and the initial signal of each light receiving element are sequentially read out by a switch. Thus, the image sensor IC is input to the second amplifier corresponding to each light receiving element, and the output signal is read as a signal of the unit light receiving element.
(2) When the image signal is held, the read switch is turned off, and when the initial signal is held, the read switch is turned on and an initial signal is given to the input gate of the second amplifier.
(3) When the initial signal is held, the readout switch is turned on and the second amplifier is also in the operating state, and the initial signal is applied to the input gate of the second amplifier.
(4) The second amplifier is in an operating state when reading a signal from the light receiving element corresponding to the second amplifier, and is configured to suppress current consumption in most of the other periods.
(5) When reading the signal to the outside, an initial signal was output for each light receiving element, and then an image signal was output, and this operation was sequentially performed for all the light receiving elements.
(6) The first amplifier is in an operating state when holding a signal from the light receiving element corresponding to the first amplifier, and operates to suppress current consumption in most of the other periods.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, a photodiode is used as a light receiving element of a unit light receiving circuit, and an amplifier is connected to the photodiode in the block, and a terminal voltage of the photodiode that changes in accordance with the amount of received light is output through the amplifier. In addition, a MOS switch for resetting each photodiode to the initial state is also provided in the unit block, and the terminal voltages before and after the resetting of the light receiving element are respectively held through an amplifier, and then sequentially read out. The signals of the light receiving elements in the blocks were sequentially output to the outside.
[0008]
【Example】
Hereinafter, the present invention will be described with reference to the drawings.
FIG. 1 is a first embodiment of a unit light receiving circuit constituting an image sensor of the present invention.
The unit light receiving circuit 1 is configured as follows. A photodiode 2 serving as a light receiving element, a first amplifier 6 for amplifying a signal of the photodiode 2, a first holding means 8 for holding an output signal of the first amplifier 6, and a first holding means. A first readout switch 10 for reading out the image signal charge, a reset switch 4 for returning the photodiode 2 to the initial state, and a second holding means 11 for holding the initial signal when the photodiode 2 is returned to the initial state. And a second readout switch 13 and a first readout switch 10 for reading out the initial signal charges held in the second holding means 11, and a second amplifier 16 for amplifying a signal output from the second readout switch 13. The N region of the first electrode of the photodiode 2 is connected to the power supply potential VDD, and the P region of the second electrode of the photodiode 2 Connecting a drain of the first electrode of the MOS transistor 4 as a reset switch to return to the initial state, and to the input terminal of the first amplifier 6 for amplifying a signal of the photodiode diode.
[0009]
A reference voltage VREF5 serving as an initial potential of the photodiode is applied to the source of the second electrode of the MOS transistor 4 serving as a reset switch for returning the photodiode to the initial state. The output terminal of the first amplifier 6 is connected to the input terminal of the first holding means 8 and the input terminal of the second holding means 11 so that the signals held in the respective holding means can be read out. The first read switch 10 is connected to the second holding means 11, the second read switch 13 is connected to the second holding means 11, the output terminal of each read switch is connected to the input terminal of the second amplifier 16, and the second amplifier 6 is connected. The unit light receiving circuit 1 is configured so as to read the output signal. A plurality of blocks having the same configuration as that of the unit light receiving circuit 1 are arranged side by side in one linear image sensor. Here, it is assumed that an N-bit block is included in the linear image sensor for convenience.
[0010]
FIG. 2 is a timing chart showing the operation of the unit light receiving circuit 1 shown in FIG.
Although the clock φCLK is not shown in FIG. 1, various drive signals are input in synchronization with the clock φCLK serving as a reference signal for operation. The operation will be described from the operation in which the potential of the photodiode 2 changes in response to the reflected light from the document and holds that potential as an image signal. The potential of the photodiode 2 changed by receiving the reflected light from the document is amplified through the first amplifier. The amplified image signal is held in the first holding means 8 when the pulse φSH1 is applied to the first holding means 8.
[0011]
Here, the holding means is composed of one MOS switch and one capacitor. After holding the image signal in the first holding means, in order to return the photodiode 2 to the initial state, the pulse φRST is applied to bring the reset switch 4 into the conductive state, and the photodiode 2 is driven at the potential of the reference voltage 5. Return to the initial state. Thereafter, in order to hold the initial potential of the photodiode 2 at the time of returning to the initial state in the second holding means 11 as an initial signal via the first amplifier, the pulse φSH2 is applied.
[0012]
Up to this point, the first holding means 8 and the second holding means 11 hold the image signal and the initial signal, respectively. After that, the operation of reading the signal held at a desired timing to the outside is continued. In order to read the image signal held in the first holding means 8, a pulse φRO 1 is applied so as to make the first reading gate 10 conductive, the image signal is read from the first holding means 8, and is passed through the second amplifier 16. And outputs a signal φSIG. Thereafter, in order to read the initial signal held in the second holding means 11 with a delay of half a clock, the pulse φRO2 is applied so as to make the second reading gate 13 conductive, and the initial signal is read from the first holding means 8. The signal is amplified through the second amplifier 16 and a signal φSIG is output. When the original is black and there is almost no reflected light, the image signal is almost the same as the initial signal, and the initial signal can be regarded as a dark signal. Therefore, an initial signal that can be regarded as an image signal and a dark signal is output for each pixel.
[0013]
The signal can be read out even in the operation according to the timing chart of FIG. 2, but when the initial signal is read out, the input capacitance of the second amplifier 16 and the floating capacitance of the node 14 are slightly affected by the residual charge of the immediately preceding image signal. The signal is slightly larger than the original initial signal. Since the slight amount depends on the reflected light amount of the original, the read initial signal also varies depending on the reflected light amount of the original. An operation method that suppresses this influence is shown in FIG.
[0014]
FIG. 3 is another timing diagram for operating the first embodiment of the unit light receiving circuit constituting the image sensor of the present invention.
The difference between FIG. 2 and FIG. 3 is how to conduct the first read gate 10 and the second read gate 13. In FIG. 3, when the initial signal is held in the second holding means 11, the second read gate 13 is also conducted in synchronization with the input capacitance of the second amplifier 16 and the pulse φSH2 so as to give the initial signal to the node 14 as well. The pulse φRO2 is also applied so that the holding operation is performed. After that, when reading the held signal at a desired timing, the pulse φRO2 is applied so that the second read gate 13 is first conducted, and the initial signal held in the second holding means 11 is changed to the second signal. First, the signal φSIG is read out to the outside through the amplifier 16. Thereafter, the second read gate 13 is turned off, and at the same time, the pulse φRO1 is applied so that the first read gate 10 is turned on, the image signal held in the first holding means 8 is read, and the second amplifier 16 is turned on. And output as a signal φSIG. With this operation, a constant initial signal is always output without depending on the amount of reflected light from the document.
[0015]
In the present invention of FIG. 1, two amplifiers are added to one light receiving element as compared with the conventional example of FIG. 9, and inferior to the conventional example from the viewpoint of a generally required low current consumption. Yes. A circuit configured to suppress this is shown below.
FIG. 4 shows a second embodiment of a unit light receiving circuit constituting the image sensor of the present invention. A MOS switch 17 is connected to selectively operate the first amplifier 6 and is controlled by a pulse φA1. Similarly, the MOS switch 18 is connected so as to selectively operate the second amplifier 16 and is controlled by the pulse φA2.
[0016]
FIG. 5 is a timing diagram for operating the second embodiment of the unit light receiving circuit constituting the image sensor of the present invention.
The operation of the circuit shown in FIG. 4 will be described with reference to the timing chart 5. The first amplifier 6 operates so that the node 3 is not affected by the first amplifier 6 when the image signal or the initial signal of the photodiode 2 is held in the holding unit and when the photodiode 2 is returned to the initial state. Thus, the pulse φA1 is applied. As for the second amplifier 16, as described with reference to FIG. 3, when the initial signal of the photodiode 2 is held in the second holding unit 11, the second read gate is also turned on, and the node 14 and the second amplifier 16. Since the initial signal is also applied to the input gate, the pulse φA2 is applied when the initial signal is held so that the second amplifier 16 is also in an operating state so as to be in the same state as when the signal is read. Further, when the image signal and the initial signal are read from the first holding unit 8 and the second holding unit 11, the pulse φA2 is applied so that the second amplifier 16 operates. By operating in this way, current consumption was reduced.
[0017]
FIG. 6 is a block diagram showing an embodiment of a linear image sensor IC using a unit light receiving circuit constituting the image sensor of the present invention. A plurality of unit light receiving circuits 1 are arranged in a straight line, and a linear image sensor IC 100 including at least a scanning circuit 20 is obtained. In FIG. 6, for convenience, n unit light receiving circuits 1 are arranged, various pulses are applied from the scanning circuit 20 to the plurality of unit light receiving circuits, and image signals and initial signals obtained by the plurality of unit light receiving circuits are displayed. It was configured to read sequentially. An input signal 21 necessary for the scanning operation is input to the scanning circuit 20, and a scanning output signal 22 necessary for the operation when a plurality of linear image sensor ICs 100 are arranged in a straight line is output.
[0018]
FIG. 7 shows a timing chart showing a part of the operation of the block diagram of FIG. The respective image signals and initial signals in all the unit light receiving circuits are held by the pulse φSH1 (all) and the pulse φSH2 (all) having the same timing, and the timing for resetting the respective photodiodes is also made simultaneously by the pulse φRST (all). The Further, in order to perform these operations normally, the first amplifiers are also operated by applying the pulse φA1 (all), and the respective second amplifiers are simultaneously operated by the pulse φA2 (i).
[0019]
When reading out the held signal to the outside, a signal is given from the scanning circuit so that the initial signal and the image signal are sequentially read out in order of arrangement from the unit light receiving circuits arranged linearly. For convenience, a case where signals from the i-th and (i + 1) -th unit light receiving circuits are read will be described. A pulse φSW (i) is applied to read out the signal held in the i-th unit light receiving circuit to the outside, and in the first half of the pulse, the pulse of the i-th unit light receiving circuit is synchronized with the pulse φRO2 (i). Read the initial signal. Thereafter, in synchronization with the pulse φRO1 (i) in the second half of the pulse φSW (i), the image signal of the i-th unit light receiving circuit is output. When the image signal from the i-th unit light-receiving element is thus output to the outside in this way, the pulse φSW (i + 1) is used to read out the signal held in the next (i + 1) -th unit light-receiving circuit. Apply. In the first half of the pulse, the initial signal of the (i + 1) -th unit light receiving circuit is read in synchronization with the pulse φRO2 (i + 1). Thereafter, the image signal of the (i + 1) -th unit light receiving circuit is output in synchronization with the pulse φRO1 (i + 1) in the second half of the pulse φSW (i + 1). When these series of operations are performed from the first unit light receiving circuit to the nth unit light receiving circuit, an image signal as a linear image sensor for n bits is obtained.
[0020]
FIG. 8 shows an embodiment of the image sensor of the present invention. A plurality of linear image sensor ICs 100 shown in FIG. 6 are arranged in a straight line to form an image sensor 200 that reads an original at an equal magnification. For convenience, the scanning input signal 21 is input to the linear image sensor IC disposed at the left end, and the scanning output signal 22 is input as the input signal 21 to the second linear image sensor 100 disposed from the left. . Similarly, a plurality of linear image sensors are connected, and signal output terminals of the respective linear image sensors are connected so that an image signal can be read as φSIG.
[0021]
In this way, by using a photodiode as the light receiving element, by applying an initial potential to the second electrode whose potential changes according to the amount of received light after the signal is read, through the reset switch, the initial state is restored. The deterioration of image quality such as afterimage due to residual charge, which was observed in an image sensor using a phototransistor of a conventional example as a light receiving element, was greatly improved, and good image quality was obtained.
[0022]
【The invention's effect】
As described above, the present invention provides an image sensor with little afterimage, a reference voltage that is a potential when a photodiode equivalent to a dark output voltage is always initialized during operation, Since the output voltage is output for each light receiving element, an image sensor that can calculate the effective light output accurately with a simple external circuit and almost no external memory or with a small external memory is supplied. I was able to do it. High-speed operation is also possible.
[Brief description of the drawings]
FIG. 1 is a unit light receiving circuit diagram of a first embodiment constituting an image sensor of the present invention.
FIG. 2 is a timing diagram of a unit light receiving circuit of a first embodiment constituting the image sensor of the present invention.
FIG. 3 is another timing diagram of the unit light receiving circuit of the first embodiment constituting the image sensor of the present invention.
FIG. 4 is a timing diagram of a unit light receiving circuit of a second embodiment constituting the image sensor of the present invention.
FIG. 5 is a timing chart of a unit light receiving circuit of a second embodiment constituting the image sensor of the present invention.
FIG. 6 is a block diagram of a linear image sensor IC constituting the image sensor of the present invention.
FIG. 7 is a timing chart of an embodiment of a linear image sensor IC constituting the image sensor of the present invention.
FIG. 8 is a block diagram of the image sensor of the present invention.
FIG. 9 is a circuit diagram of a conventional image sensor.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Unit light reception circuit 2 Photodiode 4 Reset switch 6 1st amplifier 8 1st holding means 10 1st read-out switch 11 2nd holding means 13 2nd read-out switch 16 2nd amplifier

Claims (2)

The unit light receiving circuit includes a photodiode serving as a light receiving element, a first amplifier that amplifies a signal of the photodiode, a first switch that controls power supply to the first amplifier, and an output of the first amplifier. First holding means for holding an image signal, a first read switch for reading the image signal held in the first holding means, a reset switch for returning the photodiode to an initial state, and an output of the first amplifier A second holding means for holding an initial signal, a second readout switch for reading the initial signal held in the second holding means, and signals output from the first readout switch and the second readout switch. A second amplifier for amplifying and a second switch for controlling power supply to the second amplifier, and a plurality of the unit light receiving circuits are linearly formed. The first holding means, the second holding means, and the reset switch are arranged so that the initial signal of each unit light receiving circuit and the image signal corresponding to the amount of received light are sequentially read out to the outside in the arrangement order of the unit light receiving circuits. a linear image sensor IC that Sukunakutomo includes a scanning circuit which scans the first read switch and the second read switch and said second amplifier and said first switch and the second switch and, all the Tan'i The image signal in the light receiving circuit is held in each of the first holding means with a pulse φSH1 (all) of the same timing, and then each reset switch is further turned on with the same pulse φRST (all), and each photodiode is connected. Simultaneously reset the initial signal of each photodiode to the same timing. In order to read out the held signals from the i-th and (i + 1) -th unit light receiving circuits after being held in the second holding means of each of the unit light receiving circuits with the pulse φSH2 (all), first, the pulse φSW (I) is applied to the switch that outputs the signal of the second amplifier of the i-th unit light receiving circuit, and is turned on, and the second readout switch of the i-th unit light-receiving circuit is turned on in the first half of the pulse φSW (i). The initial signal of the i-th unit light receiving circuit is read out in synchronization with the pulse φRO2 (i) to be performed, and then the pulse φRO1 for conducting the first read switch of the i-th unit light receiving circuit in the latter half of the pulse φSW (i) In synchronization with (i), the switch outputs an image signal of the i-th unit light-receiving circuit, and subsequently outputs a signal of the second amplifier of the next (i + 1) -th unit light-receiving circuit. The pulse φSW (i + 1) is applied to be conducted, and in the first half of the pulse φSW (i + 1), in synchronization with the pulse φRO2 (i + 1) for conducting the second readout switch of the (i + 1) th unit light receiving circuit (i + 1) The initial signal of the 1st unit light receiving circuit is read, and then in the latter half of the pulse φSW (i + 1), in synchronization with the pulse φRO1 (i + 1) that makes the first read switch of the (i + 1) th unit light receiving circuit conductive (i + 1). An image sensor IC that outputs an image signal of the first unit light receiving circuit. An image sensor comprising the image sensor IC according to claim 1 arranged and mounted in a straight line.

Images