JPH04355563A - Read circuit for image sensor - Google Patents

Abstract

PURPOSE:To obtain a picture signal with fidelity to an original picture by providing an additional circuit generating a compensation current whose polarity differs from a reverse current and feeding back the current to an input of an integration device via a feedback resistor to prevent the effect of the reverse current in a sensor output. CONSTITUTION:A charge in proportion to a forward current integral value is being stored to a capacitor 48 for each clock and the charge is leaked via a resistor 49 inserted in parallel to have a same time constant as that for a reverse current. Then the charge is fed back as a compensation current to the input of an incomplete integration device 20 via a feedback resistor 50 from a terminal of the capacitor 48. The compensation current is the same current as the reverse current but its polarity is reverse, and a picture signal outputted from a multiplexer 43 is formed to be a signal with fidelity to an original picture by summing the forward current, the reverse current and the compensation current resulting in obtaining a compensation current synthesis integral value.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to a reading circuit for an image sensor used for reading images of facsimile machines and image scanners, and in particular, the present invention relates to a reading circuit for an image sensor used for reading images of facsimile machines and image scanners. The present invention relates to a reading circuit for an image sensor in which a plurality of elements are arranged in a line.

0002

2. Description of the Related Art Image sensors used for reading images in facsimile machines, for example, use a light-receiving element line of approximately the same length as the document width, and read an image signal of one line on the document surface by electrical scanning in the line direction. At the same time, the document is moved by a document feeding device (in the sub-scanning direction), and the electrical scanning is sequentially performed to read the entire surface of the document. For example, as shown in FIG. 6, this type of image sensor includes a photodiode PD and a blocking diode B.
It has been proposed that the light receiving elements 61 are connected in series so as to have opposite polarities to each other to form one light receiving element 61, and a plurality of light receiving elements 61 are arranged one-dimensionally in a line.

[0003] Reading of the image signal of the image sensor is carried out as follows. That is, the already charged photodiode PD is irradiated with reflected light from the document surface (not shown), and a photocurrent proportional to the amount of the irradiated light flows into the anode side of the photodiode PD.
The charges accumulated in the photodiode PD are discharged (accumulation period). Subsequently, a drive pulse is applied to the anode side of the blocking diode BD via the individual drive line 62 by the shift register SR, and the blocking diode B
D is forward biased to reset the cathode voltage across the diode to a substantially constant value (signal read period). Therefore, the same amount of charge as the positive charge of the cathode electrode flowing out as a photocurrent during the accumulation period is replenished (charged) from the outside by the signal reading operation. The supplementary amount of this charge is applied to the common signal line 6.
By detecting the voltage as a voltage with an integrator 64 via 3, an image signal output can be obtained. The above operation is performed for each light receiving element 61 every time a driving pulse is sequentially applied from each terminal of the shift register SR, and an image signal of one line on the document can be obtained in time series.

0004

However, reading the image signals of the image sensor has the following problems. An image sensor in which the diode semiconductor is made of amorphous silicon, as shown in FIG.
When reading a chart in which white, black, and white are lined up in the sub-scanning direction as shown in FIG. Output signal with a level even whiter than the level (Figure 7(b))
is output. In other words, there is a problem in that the original image cannot be read accurately in areas where the black and white density on the original surface changes suddenly.

To explain this phenomenon for one pixel of an image sensor with reference to FIGS. 8 and 9, the driving pulse shown in FIG. 9(a) from the shift register SR is applied to the anode side of the blocking diode BD, and the driving pulse is applied to the anode side of the blocking diode BD. After recharging the discharged charge and resetting to the initial state,
After the drive pulse returns to the "Low" level (GND), as shown by the hatched area in FIG. The drive pulse of the minute current (reverse current) is “High”
The current flows in the opposite direction to the current when . The above phenomenon occurs because these overlap for all pixels. Note that the current waveform in FIG. 9(b) shows the case where the integration time is set to be sufficiently long.

The present invention has been made in view of the above-mentioned circumstances, and it is possible to obtain an image from an image sensor having a plurality of light-receiving elements arranged in a line, each consisting of two diodes connected in series with opposite polarities. It is an object of the present invention to provide a reading circuit that can obtain an image signal faithful to the original when outputting a signal.

[0007]

[Means for Solving the Problems] In order to solve the problems of the above-mentioned conventional example, an image sensor reading circuit according to the present invention has the following features:
An image sensor comprising a plurality of light receiving elements arranged in a line, each consisting of two diodes connected in series with opposite polarities, and a driving means for sequentially applying a driving pulse to one end of each of the light receiving elements. , in a reading circuit for an image sensor having a common signal line that connects the other end of each of the light receiving elements in common, and an integrator connected to the common connection line, one end of the light receiving element is brought to GND level after the driving pulse is applied; An additional circuit generates a compensation current that differs only in polarity from the reverse current that occurs when the current is generated, and the compensation current is fed back to the input of the integrator.

That is, the reading circuit of claim 1 includes a capacitor section that charges a charge proportional to the integrator output, a leak section that discharges the charge at a predetermined time constant, and an output of the leak section that is connected to the integrator. It is equipped with a resistor for feeding back to the input.

FIG. 5 shows an equivalent circuit of the above configuration. A constant charge source 1 (constant current source if the clock is constant) is provided to generate a reverse current proportional to the sensor output (forward current) of the image sensor. The capacitor 2 is provided to store the output of the constant charge source 1 since the reverse current is the sum of currents from all pixels. Since the reverse current attenuates with a certain time constant, the leak resistance 3 is connected in parallel to the capacitor 2 in order to match this time constant. Feedback resistor 4 is connected to feed back the terminal voltage of the parallel circuit of capacitor 2 and leakage resistor 3 to the input of the integrator.

The reading circuit according to the second aspect of the present invention includes a low-pass filter connected to the output side of the integrator, and a feedback resistor for feeding back the output of the low-pass filter to the input of the integrator. The low-pass filter has the same time constant as the time constant when the reverse current attenuates.

[0011]

According to the invention as claimed in claim 1, the capacitor section and the leakage section generate a compensation current that differs only in polarity from the reverse direction current, and the compensation current is fed back to the input side of the integrator via the feedback resistor. Directional current can be offset by compensation current.

According to the second aspect of the invention, a compensation current having a polarity different from that of the reverse current is generated by the low-pass filter, and is fed back to the input side of the integrator via the feedback resistor, so that the reverse current is compensated. It can be canceled out with electric current.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the image sensor reading circuit of the present invention will be described with reference to the drawings. FIG. 1 is an overall configuration diagram of an image sensor reading circuit, which includes a one-dimensional image sensor 10, an incomplete integrator 20, and a leakage regeneration circuit 30.
and a compensation circuit 40.

The one-dimensional image sensor 10 is constructed by connecting a photodiode PD and a blocking diode BD in series with opposite polarities to form a light receiving element 11, and arranging a plurality of these in a line as one pixel. has been done. Each light receiving element 11 is connected to an insulating substrate 101 as shown in FIG.
Two semiconductor layers 103a and 103b made of amorphous silicon in which an n layer and an i layer are laminated are formed separately on the lower electrode 102 (formed, for example, of a metal film) formed thereon, and this semiconductor layer 103a , 103b, respectively, by stacking upper electrodes 104a, 104b (for example, made of a transparent conductive film), two diodes can be arranged facing each other and connected in series with opposite polarities. . The upper electrodes 104a and 104b are connected to contact holes 106a formed in the insulating layer 105.
, 106b to the individual drive lines 12 and common signal line 13 made of aluminum.

The anode side of the blocking diode BD constituting each light receiving element 11 is connected to each terminal of the shift register SR via each individual drive line 12. Further, the anode side of each photodiode PD is connected to one common signal line 13. In the shift register SR,
A data terminal DATA and a clock terminal CK are provided, and a data pulse input from the data terminal DATA is shifted and sequentially output as a drive pulse from each terminal of the shift register SR. This drive pulse is applied to each light receiving element via the individual drive line 12, and when this drive pulse becomes "High", the sensor output current I corresponding to the photocurrent generated in the light receiving element 11 is It is taken out from line 13.

The imperfect integrator 20 operates as an integrator at high frequencies and as a current-voltage amplifier at low frequencies.
Perfect integrator 22 having an integrating capacitor 21 in the negative feedback section
, a non-inverting amplifier 23 for voltage amplifying the output of the perfect integrator 22, and a resistor 24 connected between the output side of the non-inverting amplifier 23 and the input side of the perfect integrator 22. The perfect integrator 22 has a non-inverting input and an inverting input, the non-inverting input is grounded, and the common connection line 13 of the one-dimensional image sensor 10 is connected to the inverting input side. Further, the configuration is such that the output voltage divided by the resistors 25 and 26 is input to the non-inverting amplifier 23. The resistor 24 is connected to the capacitor 21 at the output of the non-inverting amplifier 23.
works to leak.

The leak regeneration circuit 30 includes capacitors 31 each having one end connected to the output of the non-inverting amplifier 23.
, 32; constant current sources 33, 34 connected to the other ends of the capacitors 31, 32; reset switches 35, 36 connected to the other ends of the capacitors 31, 32; It is composed of sample and hold circuits 37 and 38 connected to the end sides. Constant current source 3
3 and 34 are configured to output a current value approximately proportional to the output voltage value of the imperfect integrator 20. Further, the reset switches 35 and 36 are connected to the reference power source Vos.

In this leak regeneration circuit 30, one end of the capacitors 31, 32 is driven by the output of the incomplete integrator 20, and the other end of the capacitors 31, 32 is connected to a constant current source 33, 34 compensates for the leakage caused by the resistor 24, and an output obtained by integrating the sensor output current I appears here. After the integration is completed, the voltage is reset to a constant reference potential by the reset switches 35 and 36, and the integration is started again. The above-mentioned integration and reset operations are performed alternately and complementarily by two sets of circuits: the capacitor 31 (32), the constant current source 33 (34), and the reset switch 35 (36). Since it can always be set to integral mode, the reset switch 35,
36 switching time losses do not limit operating speed.

The compensation circuit 40 includes the sample and hold circuit 3
A multiplexer 43 that integrates two signal output lines 41 and 42 into one system via 7 and 38, and each signal output line 41
, 42, a transistor 46 whose base side is connected to the anode side of the diodes 44, 45, and a transistor 46 connected between the emitter side of the transistor 46 and the reference power source Vos. a resistor 47; a capacitor 48 and a leak resistor 49 connected in parallel to the collector side of the transistor 46; and a feedback resistor 50 connected between the collector side of the transistor 46 and the input side of the imperfect integrator 20. It consists of

That is, a simple current source is constructed by using the reset potential from the reference power source Vos and the output terminals of the sample and hold circuits 37 and 38. The current flowing to the emitter side of the transistor 46 is determined to be inversely proportional to the clock cycle. The two diodes 44 and 45 provided at the current source input (on the base side of the transistor 46) have a lower potential than the signal output lines 41 and 42.
The base potential of each pixel is determined, thereby making it possible to correct the current of all pixels.

Further, the charge accumulated in the capacitor 48 from the current source output (collector side of the transistor 46) is determined by the time constant τ (the capacitance of the capacitor 48 is C, the value of the resistor 49 is R).
In this case, discharge occurs at τ=CR). The resistor 49 needs to be sufficiently smaller than the feedback resistor 50 so as not to cause calculation errors. Moreover, the capacitance C of the capacitor 48 is
The CR is determined so as to match the time constant of the reverse current generated at the fall of the drive pulse applied to the light receiving element 11.

Next, the operation of the circuit having the above configuration will be explained with reference to FIG. When the driving pulse 71 is applied from the terminal of the shift register SR and the anode side of the blocking diode BD1 of the light receiving element 11 becomes "High," a current (sensor output current) flows in the forward direction from the anode side of the photodiode PD1. Next, the anode side of the blocking diode BD1 becomes "Low" and the blocking diode B of the light receiving element 11 becomes "Low".
The anode side of D2 becomes “H” by applying the driving pulse 72.
"high", and a current (reverse direction current) flows in the photodiode PD1 in the reverse direction, and a forward current (sensor output current) flows in the photodiode PD2. Further, similar operations are performed for each light receiving element 11 by driving pulses 73, 74, . . . . In order to simplify the explanation, FIG. 3 shows the current waveform when the clock cycle input to the shift register SR is made long and the integration time is made long enough.

When the above image signal reading operation is performed for all pixels (four pixels are used to simplify the explanation), the current flowing through the common signal line 13 is calculated as the sum of the forward current (sensor output current) and the reverse current. Figure 3 shows the total and
become that way. Integrating these outputs respectively results in forward current integration and reverse current integration. According to the conventional reading circuit, the image signal outputted from the integrator is a forward and reverse integral combination of the forward current integral and the reverse current integral, and the image on the document surface cannot be accurately converted into an electrical signal. In the above embodiment, a charge proportional to the forward current integral value is transferred to the capacitor 4 every clock.
8, and this charge is leaked through a resistor 49 inserted in parallel so as to have the same time constant as the reverse current. Then, from the terminal voltage of this capacitor 48, the feedback resistor 5
0 to the input of the imperfect integrator 10 as a compensation current. As shown in FIG. 3, this compensation current has the opposite sign and the same magnitude as the reverse current, and when the forward current, reverse current, and compensation current are summed, it becomes a compensation current composite integral, The image signal output from the multiplexer 43 can be a signal that is faithful to the original surface.

Next, the value of the reverse current in the reading circuit of the above embodiment is calculated. Reverse current Ir
is proportional to the magnitude of the forward current and is expressed by the following formula. Ir=-k.If.exp(-t/.tau.) Here, .tau. is a constant determined by amorphous silicon, which is the semiconductor of the diode, k is a proportionality constant, and If is a forward current. The time required for the drive pulse from the shift register SR to rise is n
Assuming Δt, the reverse current of the photodiode PD after t seconds is Ir=-k・If・exp(-(t+nΔt)/τ) If these are added for 0 to m pixels, If(n)=If(n+1)= ……=If(m)=If
As, ΣIr(n)=−k・If・exp(−
t/τ) −k・If
・exp(-(t+Δt)/τ)
−k・If・exp(−(t+2Δt)/
τ) …………
−k・If・exp(−(
t+mΔt)/τ) =k
・If・exp(−t/τ)・(1−exp(−m・Δ
t/τ))/(1-exp(-Δt/τ)) If m is sufficiently large for 1 and τ is sufficiently large for Δt,
(1-exp(-m・Δt/τ))=1 (1-e
Since it can be approximated as xp(-Δt/τ))=Δt/τ,
ΣIr(n)=k・If・τ/Δt・exp(−t/
τ). Here, if the clock period (integration time) Δt is constant, k・If・τ becomes a constant, and ΣIr(n)=Constant・exp(−t/
τ) ......Equation (1) is obtained.

Equation (1) above represents CR with a time constant of τ.
This corresponds to the expression representing the output of a low-pass filter. As explained in the above embodiment, in order to obtain a compensation current, a charge proportional to the output of the imperfect integrator 20 is accumulated in the capacitor 48 every clock, and is discharged with a time constant τ. The terminal voltage is fed back through the feedback resistor 50 to the input of the imperfect integrator 20 as a compensation current. This compensation current is the clock (integration time)
If is fixed, as can be seen from equation (1), it will be the same as the output of the low-pass filter. Therefore, as shown in FIG. 4, if the output of the imperfect integrator 20 is configured to pass through the low-pass filter 51 and then be fed back to the input of the imperfect integrator 20 via the feedback resistor 50, the above embodiment can be achieved. A similar effect can be obtained. The other configurations in FIG. 4 are the same as those in FIG. 1, are given the same reference numerals, and detailed explanations are omitted.

[0026]

[Effects of the Invention] According to the present invention, an additional circuit is provided to generate a compensation current with a different polarity for the reverse current generated at the falling edge of the drive pulse, and the compensation current is fed back to the input side of the integrator via the feedback resistor. Therefore, the reverse current can be offset by the compensation current. As a result, it is possible to prevent the influence of reverse current on the sensor output and obtain an image signal that is faithful to the original image.

[Brief explanation of the drawing]

FIG. 1 is an equivalent circuit diagram of a reading circuit of an image sensor showing an embodiment of the present invention.

FIG. 2 is a cross-sectional explanatory diagram of a light receiving element.

FIG. 3 is a waveform diagram for explaining the operation of the reading circuit.

FIG. 4 is an equivalent circuit diagram of a reading circuit of an image sensor showing another embodiment.

FIG. 5 is a schematic equivalent circuit diagram showing the configuration of the present invention.

FIG. 6 is an equivalent circuit diagram of a reading circuit of a conventional image sensor.

FIG. 7 is a diagram for explaining problems in the conventional example, in which (a) is a chart, (b) is an output waveform corresponding to the chart, and (c) is a reproduced image corresponding to the output waveform.

FIG. 8 is an equivalent circuit diagram corresponding to one pixel of an image sensor.

FIG. 9(a) shows a driving pulse waveform output from a shift register, and FIG. 9(b) shows a current waveform flowing through a light receiving element.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1... Charge source, 2... Capacitor, 3... Leak resistance, 4... Feedback resistor, 10... One-dimensional image sensor, 11... Light receiving element, 12... Individual drive line, 1
3...Common signal line, 20...Incomplete integrator, 30...
leak regeneration circuit, 40...compensation circuit, 44...diode, 45...diode, 46...transistor,
47...Resistor, 48...Capacitor, 49...Leak resistance, 50...Feedback resistor, 51...Low pass filter

Claims (2)

[Claims] 1. An image sensor comprising a plurality of light-receiving elements arranged in a line, each consisting of two diodes connected in series with opposite polarities, and a driving pulse sequentially applied to one end of each of the light-receiving elements. In a reading circuit for an image sensor, the image sensor has a driving means for applying a voltage, a common signal line that commonly connects the other end of each of the light receiving elements, and an integrator that is connected to the common connection line. An image sensor comprising: a capacitor section that charges electric charge; a leak section that discharges the electric charge at a predetermined time constant; and a feedback resistor that feeds back the output of the leak section to the input of the integrator. reading circuit. 2. An image sensor comprising a plurality of light-receiving elements arranged in a line, each consisting of two diodes connected in series with opposite polarities, and a driving pulse sequentially applied to one end of each of the light-receiving elements. In a reading circuit for an image sensor, the image sensor has a driving means for applying a voltage, a common signal line that commonly connects the other end of each of the light receiving elements, and an integrator connected to the common connection line, which is connected to the output side of the integrator. What is claimed is: 1. A reading circuit for an image sensor, comprising: a low-pass filter; and a feedback resistor for feeding back an output of the low-pass filter to an input of the integrator.