JPH05167790A - Picture read method and its device - Google Patents

Abstract

PURPOSE:To improve read accuracy by a high frequency current called a capacitance kick caused when an initial drive pulse rises in a matrix drive type picture reader (image sensor). CONSTITUTION:An open drain type buffer is adopted for each buffer connecting among each output terminal of a shift register SR and each of blocks B0, B1...Bm. Then a pull-up resistor RL whose resistance is large is connected to an output terminal of a first buffer and a pull-up resistor RS whose resistance is equal to a conduction resistance RON of a final stage transistor(TR) FET in the inside of the buffer is connected to the output terminal of other buffer. Thus, only the leading time of a first drive pulse V0 is extended and the trailing time and the leading time of other drive pulses V0, V1...Vm are set equal with each other.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reading method and an apparatus therefor, and more particularly to a method and an apparatus for reading an image on an original such as a facsimile, an image scanner, a digital copying machine, an electronic blackboard in a time series. Regarding improvement.

[0002]

2. Description of the Related Art In recent years, a charge coupled device (CC) has been used in a document reading unit such as a facsimile.
An image reading apparatus generally called a contact image sensor is used instead of the image reading apparatus of the reduction optical system using D). This image reading device has a plurality of one-dimensionally formed photoelectric conversion elements made of a thin film semiconductor such as amorphous silicon a-Si on a glass substrate, and can read an image on a document at the same size. Photodiodes are usually used for these photoelectric conversion elements, but since the photocurrent generated in the photodiodes is extremely weak, a charge storage method in which the photocurrent is temporarily stored in the capacitance of the photodiode and then detected is adopted. ing. Further, since the number of photoelectric conversion elements is extremely large, a matrix driving method which requires a small number of switching elements is usually adopted.

In a conventional image reading apparatus, as shown in FIG. 6, for example, a photodiode PD as a photoelectric conversion element is used.
Are arranged (m + 1) × n, and blocking diodes BD are connected in series to these photodiodes PD with opposite polarities. The photodiode PD and the blocking diode BD are divided into (m + 1) blocks B _0, B _{1, ...} B _m for every n, and one block has n channels ch _1, ch. _2, ... ch _n are provided.
The anode terminals of these photodiodes PD are connected to a common current-voltage converter IV which is common among the blocks B _0, B _{1, ...} B _m and are connected to an integrator IN. Has been done. On the other hand, the anode terminal of the blocking diode BD is each block B _0, B _{1, ...} B _m.
Each of them is commonly connected, and further connected to the shift register SR via the buffer array BA as shown in FIG. The first block B ₀ is a dummy block that is not directly involved in image reading.

As shown in the time chart of FIG. 8, drive pulses V _0, V _{1, ...} V _m are sequentially transferred to the photodiode PD in units of blocks B _0, B _{1, ...} B _m by the shift register SR. When applied to, the current i corresponding to the optical signal accumulated in the capacitance C _PD of each photodiode PD flows,
This current i is converted and integrated by the current-voltage converter IV and the integrator IN, and the image signal is sequentially read out from each photodiode PD in units of blocks B _0, B _{1, ...} B _m .

By the way, these drive pulses V _0, V _{1, ...}
Since the rising and falling of V _m contains high frequency components, the high-frequency current through a capacitor C _PD of the capacitor C _BD photodiode PD of the blocking diode BD when falls and when these rises Flows.
This high frequency current is generally called a capacitance kick, and flows at a certain high frequency component when the impedances of these capacitors C _BD and C _PD become smaller than the forward resistance of the blocking diode BD and the reverse resistance of the photodiode PD. It is considered to be a surge current.

Further, this high frequency current is applied to the channel ch _1,
ch _2, ... ch channel ch _1, ch ₂ adjacent through the stray capacitance Cs parasitic to between _n, it flows also ... ch _n, there is a problem that adversely affects. Therefore, in order to prevent such adverse effects, the rising and falling timings of the drive pulse are made to coincide between the adjacent blocks B _0, B _{1, ...} B _m , and the high frequency currents cancel each other out. It is designed to fit. That is, each channel ch _1,
The line impedance of ch _2, ... ch _n is constant irrespective of the intensity of incident light, and the high-frequency current flowing when the driving pulses V _1, V _{2, ...} V _m rise and the driving pulses V _0, V _{1 , ...} V
There is no inflow or outflow to the current-voltage converter IV because it has the same magnitude as the high-frequency current flowing when _m-1 falls and flows in the opposite direction. Therefore, drive pulses V _0, V
The influence of high frequency components contained in _{1, ...} V _m does not appear.

[0007]

However, since there is no trailing edge of another driving pulse for canceling the high frequency current at the leading edge of the first driving pulse V ₀ ,
There is a problem that the adjacent block B ₁ is adversely affected. The reason for this is that, firstly, the high-frequency current flowing when the first drive pulse V ₀ rises flows into the adjacent block B ₁ via the stray capacitance Cs as described above, and the common potential of the blocking diode BD and the photodiode PD. It is considered that Vj is changed and the image signal from the adjacent block B ₁ is adversely affected. A second cause is considered that the first high-frequency current is extremely large and the integrator IN is saturated, and the influence remains immediately after the driving pulse V ₁ is applied to the next block B _1. To be Here, the current flowing in the photodiode PD is i, the negative feedback resistance in the current-voltage converter IV is R ₀ , the integration resistance in the integrator IN is R ₁ , the integration capacitor is C, and the integrator IN actually performs the integration operation. The output voltage Vout of the integrator IN is expressed by the following equation 1 where t is the time during which

[Equation 1]

Since the value of the effective current for actually charging the capacitance C _{PD of the} photodiode PD is on the order of nanoamperes, the value of R ₀ / (R ₁ · C) in the equation ₁ is set large. There is. Therefore, when a large current such as a high frequency current flows, the output voltage Vout is saturated immediately and the imaginary short circuit of the integrator IN is broken. As a result, while the drive pulse V ₀ is applied to the first block B ₀ , a certain constant voltage Vt is continuously applied to the inverting input side, and the drive pulse V ₁ is applied to the next block B _1. Immediately after that, the influence of the constant voltage Vt appears on the output voltage Vout.

Therefore, the inventors of the present invention have conducted intensive studies to reduce the influence of the high frequency current associated with the rise of the first driving pulse as much as possible, and as a result, the present invention has been achieved.

[0010]

The gist of the image reading method according to the present invention is to apply a drive pulse to a plurality of one-dimensionally arranged photoelectric conversion elements in order to obtain an image from the photoelectric conversion elements. In the image reading method for reading a signal, a driving pulse having a long rise time is applied to the first photoelectric conversion element of the photoelectric conversion elements.

On the other hand, the image reading apparatus according to the present invention is used directly for carrying out this image reading method, and its gist is that a plurality of photoelectric conversion elements arranged in one dimension and the photoelectric conversion elements are arranged. In the image reading apparatus including a drive circuit for sequentially applying a drive pulse to each other and a signal processing circuit for reading an image signal from the photoelectric conversion element, the drive pulse applied first among the drive pulses. Is to provide a high-frequency removing means for increasing the rise time.

Further, in such an image reading apparatus, a buffer is connected to each output terminal of the drive circuit, and at least the buffer associated with the photoelectric conversion element to which a drive pulse is applied is an open drain type. The high-frequency removing means is configured by connecting a pull-up resistor having a value sufficient to prolong the rise time of the drive pulse applied to the photoelectric conversion element to the output terminal of the buffer. It has been done.

Further, in such an image reading apparatus, all the buffers other than the buffer relating to the photoelectric conversion element to which the drive pulse is first applied are also open drain type, and the conduction of the final stage transistor in these buffers is conducted. A pull-up resistor having a value equal to that of the resistor is connected to each output terminal of the other buffer.

[0014]

According to such an image reading method, the image signal is read from the photoelectric conversion element by sequentially applying the drive pulse to the photoelectric conversion element, but the first photoelectric conversion element has a long rise time, That is, a drive pulse having a slow rising edge is applied. Since a high frequency component is not included in the rising edge of such a drive pulse, a high frequency current does not flow as in the conventional case, and an accurate image signal is read out from the adjacent photoelectric conversion element.

On the other hand, according to such an image reading apparatus, drive pulses are sequentially applied to the photoelectric conversion elements by the drive circuit, and the image signals are read from these photoelectric conversion elements by the signal processing circuit. The rising time of the generated drive pulse is lengthened by the high frequency removing means. That is, the rising edge of the drive pulse is blunted. Since a high frequency component is not included in the rising edge of such a drive pulse, a high frequency current does not flow as in the conventional case, and an accurate image signal is read out from the adjacent photoelectric conversion element.

A buffer is connected between the drive circuit and the photoelectric conversion element, and the drive pulse is sequentially applied to the photoelectric conversion element via these buffers. Of these buffers, the buffer to which the drive pulse is first applied is an open drain type (including an open collector type).
Since a pull-up resistor having a sufficiently large value is connected to its output terminal, the rise time of the drive pulse applied from this buffer becomes sufficiently long. That is, the time constant of the rising of the drive pulse is determined by this pull-up resistor, and therefore is sufficiently large. On the other hand, since the fall time of this drive pulse is determined by the conduction resistance of the final stage transistor inside this buffer, it remains short as usual.

Further, all the buffers other than the above-mentioned buffers are open drain type (including open collector type), and each output terminal of these buffers has a value equal to the conduction resistance of the final stage transistor inside the buffer. Since the pull-up resistor of is connected, the time constant of the trailing edge of the drive pulse is the same as the time constant of the leading edge of the next drive pulse. Therefore, the high-frequency current that flows when the drive pulse falls and the high-frequency current that flows when the next drive pulse rises have the same magnitude and flow in the opposite directions, so these cancel each other out. The effect of high frequency components contained in does not appear.

[0018]

Embodiments of the image reading method and apparatus according to the present invention will now be described in detail with reference to the drawings.

As shown in FIG. 1, the image reading apparatus according to the present invention includes (m + 1) × n photodiodes PD.
And blocking diodes BD connected in series to them in reverse polarity are arranged one-dimensionally, and (m + 1) blocks B _0, B _{1 with} n photodiodes PD and blocking diodes BD as one unit. _{, ...} B _m . In other words, one block is provided with _n channels ch _1, ch _2, ... Ch _n . The first block B ₀ is a dummy block that is not directly involved in image reading.

The anode terminals of these photodiodes PD are connected to a current-voltage converter IV which is common among the blocks B _0, B _{1, ...} B _m at the same relative position, and It is connected to the integrator IN. Here, the current-voltage converter IV and the integrator IN constitute a signal processing circuit for reading an image signal from the photodiode PD.

On the other hand, the anode terminals of the blocking diode BD are commonly connected to each block B _0, B _{1, ...} B _m , and as shown in FIG.
It is connected to the shift register SR via A ^* . These buffer arrays BA ^* are C-MOS type gate ICs with eight built-in open drain type buffers.
Then, blocks B _1, B _{2, ...} B other than the first block B ₀
An output terminal connected to _m is connected to a pull-up resistor R _S having a value equal to the conduction resistance R _ON of the final stage field effect transistor FET inside the buffer. The output terminal connected to the first block B ₀ has a pull-up resistor R of a value larger than these pull-up resistors R _S.
L is connected. Here, the photodiode PD
The drive circuit for sequentially applying the drive pulses V _0, V _{1, ...} V _m to the drive circuit is composed of a shift register SR.
Also, the buffer connected to the first block B ₀ is an open drain type, and the pull-up resistor R _L is connected to this output terminal.
Is connected to form a high-frequency removing unit that lengthens the rising time of the drive pulse V ₀ applied first.

The photodiode PD is a photoelectric conversion element in which thin film semiconductors such as amorphous silicon a-Si are laminated in a pin structure or the like, and the blocking diode BD is between blocks B _0, B _{1, ...} B _m . It is a switching element for preventing crosstalk. Usually, the photodiode PD and the blocking diode BD are formed at the same time and have the same structure.

Next, the operation of this image reading apparatus will be described with reference to the time chart of FIG.

The data input pulse Din input to the shift register SR is sequentially shifted in the shift register SR in accordance with the clock pulse CLK, and drive pulses V _0, V _{1, ... From} the respective output terminals of the shift register SR. _. V _m
Are sequentially output as these drive pulses V _0, V
_{1, ...} V _m are sequentially applied to each photodiode PD via the buffer array BA ^* in units of blocks B _0, B _{1, ...} B _m .

The rising time constants of these drive pulses V _0, V _{1, ...} V _m are the pull-up resistors R _L and R _S connected to each output terminal, the capacitance C _{BD of} the blocking diode _BD , and the photo diode. The rise time of the first drive pulse V ₀ is longer than the rise times of the other drive pulses V _1, V _{2, ...} V _m because it is determined by the combined capacitance of the capacitance C _PD of the diode PD. This is because R _L > R _S. The rising times of the other drive pulses V _1, V _{2, ...} V _m remain short as usual.

On the other hand, these drive pulses V _0, V _{1, ...} V _m
The falling time constant of R is independent of the pull-up resistors R _L and R _S , the conduction resistance R _ON of the final stage field effect transistor FET inside each buffer, the capacitance C _BD of the blocking diode _BD and the photodiode PD. Capacity C _PD
And the combined capacitance of the drive pulses V _0, V _{1, ...} V _m are the same,
It remains short as usual. Moreover, the pull-up resistor R _S connected to the output terminals of buffers other than the first buffer
Is equal to the conduction resistance R _ON of the field effect transistor FET at the final stage, these drive pulses V _0, V _{1, ...} V
_The fall time of _m-1 is the next drive pulse V _1, V
_It is always the same as the rise time of _{2, ...} V _m .

Therefore, although there is no fall of another drive pulse for canceling the high frequency current at the rise of the first drive pulse V ₀ , the rise time is long,
Since the rising edge does not include the high frequency component, the high frequency current does not adversely affect the adjacent block B ₁ or the like, such as flowing into the adjacent block B ₁ through the stray capacitance as in the conventional case. Further, since the high frequency current does not flow in this way, the integrator IN is not saturated.
On the other hand, since the fall time of the drive pulse V _0, V _{1, ...} V _m-1 is short and the fall includes a high frequency component, a high frequency current flows, but the next drive pulse V
It is canceled by the high frequency current flowing when _1, V _{2, ...} V _m rises.

In other words, this is equivalent to the addition of the low-pass filter only when the first drive pulse V ₀ rises, and this low-pass filter allows the first drive pulse V ₀ to be added.
Only the rising edge of ₀ is blunted and high frequency components are removed. Therefore, the value of the pull-up resistor R _L that constitutes this low-pass filter is _RL and C _BD , C _PD.
The time constant of the combined capacitance may be set to a value that matches the time constant that can remove the frequency component of the capacitance kick described above.

In this way, drive pulses V _0, V are sequentially applied to each photodiode PD in units of blocks B _0, B _{1, ...} B _{m.
When 1, ...} V _m is applied, a current i corresponding to the optical signal accumulated in the capacitance C _PD of each photodiode PD flows, and this current i is fed to the current-voltage converter IV and the integrator IN. The image signal is read out from each photodiode PD after being converted and integrated by.

In this embodiment, since the first buffer is an open drain type and the output terminal thereof is connected to the pull-up resistor R _L having a relatively large value, a high frequency current is generated when the first drive pulse V ₀ rises. It does n’t flow,
The influence on adjacent channels ch _1, ch _2, ... ch _n can be significantly reduced. This can significantly improve the reading accuracy. Further, with such a simple configuration, only the rising time of the drive pulse V ₀ applied first can be lengthened, which is extremely low cost and practical. On the other hand, for example, a method of increasing the rising time constant of the drive pulse V ₀ by using a CR circuit may be considered, but this method also increases the falling time constant. In view of this, it is obvious that the present embodiment realizes to make only the rising edge of the drive pulse V ₀ dull with an extremely simple structure.

Further, all the buffers other than the first buffer are open drain type, and the output terminals of these other buffers have a value equal to the conduction resistance R _ON of the field effect transistor FET at the final stage inside the buffer. Since the up resistance R _S is connected, the high-frequency currents that flow when these drive pulses V _0, V _{1, ...} V _m rise and fall, cancel each other out, and the adverse effects of such high-frequency currents occur. Can also be reduced.

The embodiment of the image reading method according to the present invention and the apparatus used directly for carrying out the method have been described above in detail. However, the present invention is not limited to the above-described embodiment and can be carried out in other modes. I will get it.

For example, as shown in FIG. 4, a normal totem pole type buffer may be used for the buffers other than the first buffer. That is, at least the first of these buffers may be an open drain type. In this case, if the final-stage field effect transistor FET inside the first buffer and the other final-stage field effect transistor FET inside the buffer have the same characteristics, the fall time of the first drive pulse V ₀ and The rising time of the next drive pulse V ₁ becomes the same, and the high frequency currents flowing at this time cancel each other out.

Further, in the above-mentioned embodiment, the open-drain type buffer in which the final-stage transistor is the field effect transistor FET is used, but the final-stage transistor may be an open-collector type buffer in which it is a normal transistor. That is, in the present invention, the open drain type is
It is a concept that includes the open collector type. Therefore, as described above, at least the first buffer may be the open collector type, and the other buffers may be the totem pole type as shown in FIG. In this case, the use of a gate IC or the like designed such that the collector resistance R _C is equal to the conduction resistance of the final stage transistor Tr makes it possible to cancel the high-frequency currents flowing at the fall and rise of the drive pulse. preferable.

Further, in the above-mentioned embodiment, the first buffer is an open drain type, and the high frequency removing means is constituted by connecting the pull-up resistor R _L to the output terminal of this buffer. Any drive pulse can be used as long as it can increase only the rising time of the drive pulse.

Up to now, the number of photodiodes PD per block is represented by n, and the number of effective blocks B _0, B _{1, ...} B _m other than the dummy block B ₀ is represented by m. For example, in the case of an A4 size image reading apparatus having a resolution of 8 dots / mm, normally n is 32 and m is 5
Set to 4. Further, it may be a type in which n is 1, that is, a type in which an image signal is individually read from each photodiode by sequentially applying a drive pulse to each photodiode. Although it is desirable to provide the dummy block B _0, the effect of reducing the high-frequency current flowing when the first drive pulse V ₁ rises can be sufficiently exhibited even if the dummy block B ₀ is not provided.

Further, the photodiode PD and the blocking diode BD are connected at their anode terminals, and the cathode terminal side of the photodiode PD is connected to the current-voltage converter I.
It may be connected to V and the cathode terminal side of the blocking diode BD may be connected to the buffer array BA ^* . Furthermore, the positions of the photodiode PD and the blocking diode BD may be exchanged, the buffer array BA ^* may be connected to the photodiode PD side, and the current-voltage converter IV may be connected to the blocking diode BD side.

In addition, the present invention can be applied not only to the blocking diode BD but also to a type selectively driven by a TFT or the like, and can be applied not only to the contact type but also to the perfect contact type image reading apparatus. Further, although the present invention is mainly applied to a charge storage type in which a high frequency current is apt to occur, a CdS-CdS photoelectric conversion element is used.
The present invention can be applied to a photoconductive type using e and the like, and can be carried out in various modes with various improvements, modifications and variations based on the knowledge of those skilled in the art without departing from the spirit of the invention.

[0039]

According to the image reading method of the present invention, since the drive pulse having a long rise time is applied to the first photoelectric conversion element of the photoelectric conversion elements, a high frequency current flows when the first drive pulse rises. Never,
The influence on the adjacent photoelectric conversion elements can be significantly reduced. This can significantly improve the reading accuracy.

On the other hand, since the image reading apparatus according to the present invention is provided with the high frequency removing means for lengthening the rising time of the drive pulse applied first among the drive pulses, the high frequency current is generated when the first drive pulse rises. It does not flow, and the influence on adjacent photoelectric conversion elements can be greatly reduced. This can significantly improve the reading accuracy.

Further, a buffer is connected to each output terminal of the drive circuit, and at least the buffer related to the photoelectric conversion element to which the drive pulse is applied is of the open drain type, and the buffer is connected to each output terminal. The pull-up resistor having a value sufficient to prolong the rise time of the drive pulse applied to the element is connected to the output terminal of this buffer to form the high-frequency removing means, so that the configuration is extremely simple. Therefore, it is possible to prolong only the rise time of the drive pulse applied first, which is low cost and practical.

Further, all the buffers other than the buffer related to the photoelectric conversion element to which the drive pulse is first applied are also open drain type and have pull-ups having a value equal to the conduction resistance of the final stage transistor inside these buffers. Since the up resistance is connected to each output terminal of these other buffers, the high-frequency current flowing when the drive pulse falls and the high-frequency current flowing when the drive pulse rise are canceled by each other, and the high-frequency component of these drive pulses It has excellent effects such that the influence can be reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of an image reading apparatus according to the present invention.

FIG. 2 is a circuit diagram showing a portion of a drive circuit in the image reading apparatus shown in FIG.

FIG. 3 is a time chart for explaining the operation of the image reading apparatus shown in FIGS. 1 and 2.

FIG. 4 is a circuit diagram for explaining another embodiment of the image reading apparatus according to the present invention.

FIG. 5 is a circuit diagram for explaining still another embodiment of the image reading apparatus according to the present invention.

FIG. 6 is a circuit diagram showing an example of a conventional image reading apparatus.

7 is a circuit diagram showing a part of a drive circuit in the conventional image reading apparatus shown in FIG.

FIG. 8 is a time chart for explaining the operation of the conventional image reading apparatus shown in FIGS. 6 and 7.

[Explanation of symbols]

PD; Photodiode (photoelectric conversion element) V _0, V _{1, ...} V _m ; Drive pulse SR; Shift register (drive circuit) IV; Current-voltage converter (signal processing circuit) IN; Integrator (signal processing circuit) ) BA ^* ; Open drain type buffer array R _L , R _S ; Pull-up resistance FET, Tr; Final stage transistor R _ON ; Conduction resistance

Claims (4)

[Claims] 1. An image reading method for reading an image signal from a plurality of one-dimensionally arranged photoelectric conversion elements by sequentially applying drive pulses to the photoelectric conversion elements, wherein the first photoelectric conversion element among the photoelectric conversion elements is used. An image reading method characterized in that a drive pulse having a long rise time is applied to the element. 2. A plurality of one-dimensionally arranged photoelectric conversion elements, a drive circuit for sequentially applying drive pulses to the photoelectric conversion elements, and a signal processing circuit for reading an image signal from the photoelectric conversion elements. An image reading apparatus comprising: a high-frequency removing unit that lengthens a rising time of a drive pulse applied first among the drive pulses. 3. A buffer is connected to each output terminal of the drive circuit, and at least the buffer related to the photoelectric conversion element to which the drive pulse is applied is an open drain type, and the buffer is connected to each output terminal of the drive circuit. The high-frequency removing unit is configured by connecting a pull-up resistor having a value sufficient to prolong the rise time of the drive pulse applied to the conversion element to the output terminal of the buffer. The image reading device according to claim 2. 4. All the buffers other than the buffer related to the photoelectric conversion element to which the drive pulse is first applied are also open drain type and have a value equal to the conduction resistance of the final stage transistor inside these buffers. The image reading device according to claim 3, wherein a pull-up resistor is connected to each output terminal of the other buffer.