JPH0142186B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical reading device, and more particularly, to a device for converting a document such as a calligraphy or drawing into a scanned electrical signal in a facsimile or character reading device.

As this type of optical reading device, a large number of light receiving elements (photoelectric conversion elements) are arranged in a straight line, and each element is selected and scanned by an external driving voltage. A method is known in which the signal is added as a time-series signal to a single or small number of signal readout processing circuits (for example, an integrator or a sampling circuit).

In devices that implement these methods, the lines from each light-receiving element to the signal readout circuit are multilayered and densely wired, so there is parasitic capacitance, and as a result, the charge accumulated in the parasitic capacitance is transferred from the light-receiving element to the signal readout circuit. There is a problem in that it affects the signal charges read out, making it impossible to obtain accurate scanning output signals. In response to this problem, the present inventors have devised an invention to eliminate the influence of the parasitic capacitance, which will be explained later with reference to FIG. 2 (Japanese Patent Application No. 129258/1983, "Light-receiving element" Umaji et al.). Although the above invention is effective, it has been found that when the parasitic capacitances generated in each light-receiving element are different, the unevenness of the parasitic capacitances becomes a cause and appears as noise in the scanning output signal.

Therefore, an object of the present invention is to solve the problem even if there is unevenness in the parasitic capacitances that exist for multiple light-receiving elements.
It is an object of the present invention to realize an optical reading device in which the electric signal read out from the light receiving element is not affected by the parasitic capacitance.

In order to achieve the above object, the present invention includes a plurality of light receiving elements that hold charges corresponding to the amount of light, a drive circuit that selects and switches the operating state of the light receiving elements, and a light receiving element selected by the drive circuit. An optical reading device comprising a signal readout circuit for reading out an electric signal corresponding to the amount of light from the drive circuit, the signal readout circuit having an integrating means, integrating the electric signal and reading it out as an amount of charge, and reading out an electric signal corresponding to the amount of light from the drive circuit. and at least a portion of the signal readout circuit is provided with means for discharging the charge accumulated in the parasitic capacitance of the selected light receiving element immediately before the signal readout circuit reads out the electrical signal corresponding to the amount of light. It is characterized by being configured.

According to the present invention, since the charge accumulated in the parasitic capacitance of each optical element is discharged immediately before the read mode, even if the value of the parasitic capacitance varies, there is no problem.

The light receiving element in the present invention may be a combination of a separation diode and a photodiode as described below, or a combination of a photodiode and a capacitor, or a separation diode and a photoconductive film.

Hereinafter, the present invention will be explained in detail using the drawings.

First, in order to facilitate understanding of the invention, the general configuration of an optical reading device and the previously invented optical reading device will be explained.

FIG. 1 shows a perspective view of a line sensor portion of an optical reading device to which the present invention is applied, in which a large number of light receiving elements are arranged. In the figure, 1 is a contact reading license sensor, 2 is a light receiving element, 3 is a lens array, 4 is a light source such as an LED, 5 is a document, 6 is a document traveling direction (sub-scanning direction), and 7 is a main scanning direction.

FIG. 2 is a circuit diagram of an optical reading device previously invented by the inventors of the present invention, and FIG. 3 is a time chart for explaining the operation of the circuit shown in FIG. In the same figure, 2
denotes a plurality of light-receiving element sections, and each light-receiving element has a separation diode 11 and a photodiode 13 connected in series. a1 and a2 are light-receiving element drive circuits for selecting the above-mentioned light-receiving elements and switching each element to modes such as idle reading, charge accumulation, readout, etc. In order to simplify the circuit for selecting and scanning each light-receiving element, a
1 is a switch group 9 for dividing the plurality of light-receiving elements into a plurality of groups and switching a line commonly connected to the diodes 11 of each group (referred to as a row wiring) to ground and bias power supply 10; It consists of a circuit 8 (hereinafter referred to as a row scanning circuit) that generates a row scanning pulse for driving the above-mentioned switch group, and a2 is a line connecting the photodiodes 13 at the same position of each block (referred to as a column wiring) to a power source 17. It consists of a switch group 15 for switching between the input signal and the input of the signal readout circuit, and a circuit (hereinafter referred to as a column scanning circuit) that generates a signal for driving the switch group 15.

The signal readout circuit b includes a bias charge cancel reset switch 19, a bias charge cancel capacitor 20, and a bias charge cancel voltage source 2.
1. Operational amplifier 22 for signal charge integration and capacitor 2
3, a reset switch 24, a switch 25 constituting a sample and hold circuit, a capacitor 26, and a buffer amplifier 27. Note that 32 is a wiring resistance.

This optical reading device has the above-mentioned switches 9, 15,
By the operations 19 and 24, the idle reading, signal charge accumulation, and readout modes are switched.

Next, the read operation will be explained using the time chart of FIG. 3.

First, during time _t1 to _t2 , the switch 19 is closed by the reset signal RST, and the potential _V29 of the signal line is set to the voltage of the voltage source 21 _-VB . At the same time, close switch 24 to reset (discharge) the integrator capacity. From time _t2 to _t4 , the column switch 15 (X _{o -1} in this case) is connected to the signal line side, and at the same time the previously closed set switches 19 and 24 are opened. As a result, the separation diode 11 is turned on, and the signal charge accumulated in the capacitor 13 and the diode 11 is transmitted to the integrator capacitor 23, and the positive charge accumulated in the column line parasitic capacitance 14 is transferred to the capacitor 20 and the capacitor 18. cancels the negative charge accumulated in the As a result, almost only the signal charge, such as V ₃₀ , is integrated and appears at the integrator output terminal. Switch this to switch 25 (signal S/
H) and held by the amplifier 27 and capacitor 26 to obtain the output _V31 . The sampling period is performed from time t ₃ to t ₄ when the integral value is stable.

In the above circuit, the influence of charge due to parasitic capacitance is removed as follows.

A reverse bias voltage V _B (17 in the figure) is applied to column wirings that are not being read. This voltage charges the column wiring parasitic capacitance C _Li (total of capacitors 14 in the figure), and this bias charge Q _Bi Q _Bi = C _Li・V _B ...(1) switches the column switch X _i to the signal line side. and is integrated by the integrator through the signal line. On the other hand, the signal charge Q _S obtained by the light receiving element 13 is Q _S =I _ph ×τ _s (2). Here, I _ph is a photocurrent source for one pixel, and τ _s is an accumulation time. I _ph has a value of 3 pA/x, the saturation illuminance of the element is approximately 100x, and τ _s is 5 mS. As a result, the value of the charge Q _S becomes approximately 1.5 pC. On the other hand, since the capacitance C _L is approximately 100PF and V _B is 5V, the bias charge Q _B is 500pF. Therefore, integrator 2
2 becomes saturated at Q _Bi , making it impossible to read the signal Q _S. Also, in order to read the signal, it is necessary to turn on the isolation diode, but if the potential of the column wiring is raised to V _B by this parasitic capacitance C _Li , a higher voltage is needed to turn it on. must be applied to the anode (row wiring) of the diode. Therefore, in the reading device shown in Figure 2 above, the capacity
C _L1 and C _L2 (18 and 20 in the figure) are charged in advance to a voltage -V _B ' with the opposite sign to the reverse bias voltage V _B , and the bias charge Q _Bi is charged at the same time as the column switch is switched to the signal line. It is something to cancel. The residual charge Q _R resulting from this is Q _R = Q _Bi − (C _L1 + C _L2 ) V _B ′ = C _Li ·V _B − (C _L1 + C _L2 ) V _B ′ (3). By appropriately selecting the capacitance value C _L2 and voltage V _B ′, the residual charge Q _R can be reduced.
The signal charge Q _S can be read without the integrator becoming saturated. Also, the isolation diode can be turned on without applying a high voltage to the row wiring.

By the way, the bias charge Q _Bi is proportional to the column wiring parasitic capacitance C _Li (i=1 to n). Since this value varies from column to column, the residual charge Q _R also varies from column to column, and this appears in the output signal. signal charge
Since the value of the charge Q _Bi is large compared to Q _S , the signal variation caused by this greatly deteriorates the signal-to-noise (S/N) ratio of the output of the optical reading device.

FIG. 4 is a circuit diagram of an embodiment of an optical reading device according to the present invention. The difference from the circuit of FIG. 2 is that a switch 34 is added to ground the anode of the separation diode in synchronization with the pixel readout cycle, and the bias charge chiman cell circuit 19, 20, 21
Instead, a switch 33 is provided for grounding the integrator input end during the reset period.

The operation of the embodiment shown in FIG. 4 will be described below in conjunction with the timing chart shown in FIG. Note that this type of optical reading device performs a blank reading in which a constant charge is sequentially applied to the photodiodes of a large number of light receiving elements arranged in a line, and also inputs the charge added in the blank reading for a certain period of time τ _s after the blank reading. Charge accumulation that discharges in response to an optical signal and signal readout that reads out the discharged charge after the charge accumulation is performed, and the present invention relates to the part that eliminates the influence of parasitic capacitance during signal readout. Therefore, the time chart in FIG. 5 shows only the part related to the operation at the time of signal reading. In other words, among a large number of light receiving elements, Y _j - ₁
The operation of the portion in which signals are sequentially read from the X _o - ₁ and X _o -th elements of the block Y and the X ₁ , X ₂ , X ₃ , and X ₄ of the Y _j block is shown.

First, we will discuss the signal readout operation of the X _{o -1} light receiving element of the Y _j block. When the row scanning pulse Y _j - ₁ of the row scanning circuit a1 is at a high level, the example scanning pulse X _o - ₁ of the column scanning circuit a2 becomes a high level, so that the signal readout mode (time t ₁ to t ₃ ).
In this mode, at time _t1 to _t2 , the light receiving element
X _{o -1} is connected to the signal readout circuit, but switch 34 is closed to the ground side, and switches 33 and 24 are closed. (In FIG. 5, the signal Y _CLR indicates the drive signal for the switch 34, and is switched to the ground side at high level and to the power supply 10 side at low level.
RST indicates a drive signal for the switches 24 and 33, and a high level closes the switch, and a low level opens the switch. ) Therefore, the voltage Y' _j - ₁ applied to the cathode of the separation diode of the light-receiving element becomes 0, and the separation diode is cut off. Therefore, the bias charge Q _Bi accumulated in the parasitic capacitance of X _o − ₁ all flows to the ground. After all of the charge Q _Bi has been discharged (indicated by Q _B in FIG. 5), switch 34 is switched to the power supply 10 side at time t ₂ to _{t 3} and switches 24 and 33 are opened. _T (voltage of power supply 10) is applied, the isolation diode is forward biased, and Q _S ⁺ is applied to the capacitor 2 of the integrator.
It is accumulated in 3. This charge is Q _S ⁺ =I _ph・τ _s +C _a・C _d・V _T (C _a +C _d )...(4).

Here, C _a and C _d are each photodiode 13
and the capacitance of the isolation diode 11. The first term in equation (4) is a signal component corresponding to equation (2), and the second term is an offset component corresponding to the voltage applied to the row wiring. If we ground the voltage on the row wire again at time t ₃ , we get
The integrator integrates the charge with the opposite sign Q _S ^- = -C _a・C _d・V ^- _T / (C _a + C _d ) ...(5), and the sum with the previous integrated value is Q _S Q _S = Q _S ⁺ +Q _S ^- = I _ph・τ _s ...(6) appears in the output. The value of integrator capacitor 23 is
In the case of C _F , the following voltage appears at the output V ₃₀ : V ₃₀ = I _ph ·τ _s /C _F (7). This is sampled and held at time _t3 to obtain an output _V31 .

The above has described the operation of the X _o - ₁ light receiving element of the Y _j - ₁ block. At time t ₄ , the column scanning pulse X _o
becomes a high level, and the above-described operation from time t ₁ to _{t 4} is performed on the X _o light receiving element of the Y _{j -1} block. Further, when the signal reading operation of the X _o light receiving element of the Y _{j -1} block is completed, only the row scanning pulse Y _j becomes a high level, and the aforementioned signal reading operation is performed for the X ₁ of the Y _j block. Similarly below,
Sequential scanning is performed.

As described above, in the optical reading device according to the present invention,
Time t ₁ immediately before readout without destroying the signal charge Q _S
The bias charge Q _Bi is completely discharged in advance from ~ _t2 , and only the signal component is read out from time _t2 to _t4 . As a result, variations in the output signal due to variations in the column wiring parasitic capacitance C _Li are not observed. Furthermore, the fifth
If sample and hold is performed at time _t3 as shown in the figure, the variation in signal offset due to variation in C _a and C _d will be eliminated, and only the signal component can be read as shown in (7).

Although an operational amplifier is used as the integrator as shown in the figure, it is preferable to configure this part as shown in FIG. 6, especially in an apparatus that requires high speed and low noise. Here, 35 is a coupling capacitor, 36 is a low-noise junction field effect transistor, 37 is a load resistor connected to its drain, and 38 is a high amplification factor G.
39 is a feedback resistor R _F , 40 is a coupling capacitor, and 41 is a resistor for determining the bias point of the transistors, which are arranged symmetrically with respect to each transistor as shown in the figure. 42 is an AC bypass capacitor, and 43 has the same value as the resistor 44 connected to the emitter of the PNP transistor 45.
46 is an npn transistor complementary to 45, 47 is an integration container, 48 is its reset (discharge) switch, and 49 and 50 are MOS type field effect transistors forming a buffer (source follower) with high input impedance. . 51,5
2 are positive and negative power supplies, respectively.

The operation of the circuit shown in FIG. 6 will be briefly explained below.
Now, if the reading time of one pixel is τ _V , then the signal charge is
Q _S is read as an equivalent (average) current I _S =Q _S /τ _V (8). If the gain G of the amplifier 38 is sufficiently high, this can be read at its output terminal as V ₅₃ = _-RF ·I _S = _RF ·Q _S /τ _V (9). Next, in accordance with this voltage, the balance between the transistors 45 and 46 is lost, and the following current is charged to the capacitor C _F by the transistor 46.

I=V ₅₃ /R= _RF ·Q _S /τ _V /R (10) As a result, V ₅₄ increases by the following value.

V ₅₄ =I・τ _V /C _F =R _F・Q _S /R・C _F ...(11) This voltage is passed through a 1:1 buffer (consisting of 49 and 50) to the S/H circuit. tell. Since the low-noise junction field effect transistor 36 is used at the first input stage, the noise is low, and the high-speed bipolar transistors 45 and 46 are used as the integrators, resulting in high speed. Furthermore, by making the circuit symmetrical with respect to the transistors 45 and 46 and using complementary elements, stability against temperature changes is also high.

As described above, according to the present invention, even if the parasitic capacitances parasitic to a large number of light receiving elements are uneven, it is possible to achieve the effect that the electrical signals read from the light receiving elements are not affected by the parasitic capacitance. can.

[Brief explanation of drawings]

Figure 1 is a perspective view of the close reading licensor;
Figure 3 is a circuit diagram and timing chart of an optical reading device previously invented by the present inventor, and Figures 4 and 5 are a circuit diagram and timing chart of an embodiment of an optical reading device according to the present invention, respectively.・Chart, FIG. 6 is a circuit diagram of one embodiment of an integrator circuit used in the present invention. 11... Separation diode, 12... Photocurrent of photoconductive film, 13... Capacity of photoconductive film, 14... Parasitic capacitance of column wiring, 17... Separation diode reverse bias power supply, 21... Bias Power supply for charge canceling, 33... switch for discharging bias charge, 34... switch for grounding row wiring.

Claims (1)

[Scope of Claims] 1. A plurality of light-receiving elements that hold charges corresponding to the amount of light, a drive circuit that selects and switches the operating state of the light-receiving elements, and receives the light amount from the light-receiving element selected by the drive circuit. An optical reading device comprising a signal readout circuit for reading out an electric signal corresponding to the drive circuit and the signal readout circuit, the signal readout circuit having an integrating means to integrate the electric signal and read it out as an amount of charge, and for reading out an electric signal corresponding to the drive circuit and the signal readout circuit. At least a portion of the circuit is configured to include means for discharging the charge accumulated in the parasitic capacitance of the selected light receiving element immediately before the signal readout circuit reads out the electrical signal corresponding to the amount of light. An optical reading device characterized by: 2 In the device described in item 1, the light receiving element includes a photodiode and a capacitive element, a photoconductive film and a separation diode, or a photodiode connected in series;
An optical reader consisting of one of the isolation diodes with the opposite rectifying direction. 3. In the device according to item 1, the plurality of light receiving elements are divided into a plurality of groups, and the drive circuit switches between the row wiring and the ground or the first bias power supply corresponding to all the elements in each group. a row scanning circuit consisting of a first switch group and a first switch drive circuit that drives the first switch group; and a column wiring that connects elements at relatively the same position in each group; A column scanning circuit includes a second switch group for switching between a second bias power source or the input of the signal readout circuit, and a second switch drive circuit for driving the second switch group, and An optical reading device characterized in that the means comprises means for connecting row wiring and column wiring corresponding to a selected light receiving element to ground. 4 In the device described in item 3, the signal readout circuit has the column wiring connected to the inverting input terminal, the non-inverting input terminal connected to ground, and the inverting input terminal and the output terminal connected to each other via the second switch group. 1. An optical reading device comprising an operational amplifier having a capacitor with a reset switch connected therebetween, and a sample-and-hold circuit that samples and holds the output of the operational amplifier.