JP2789853B2 - Image sensor reading circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reading circuit of an image sensor used for reading an image of a facsimile or an image scanner, and more particularly to a light receiving circuit composed of two diodes connected in series with opposite polarities. The present invention relates to a reading circuit of an image sensor having a plurality of elements arranged in a line.

[0002]

2. Description of the Related Art An image sensor used for reading an image of a facsimile or the like uses, for example, a light receiving element line having substantially the same length as the width of a document, and reads an image signal of one line on the document surface by electrical scanning in a line direction. At the same time, the original is moved by the original feeder (in the sub-scanning direction), and the electrical scanning is sequentially performed to read the entire original surface. In this type of image sensor, for example, as shown in FIG. 6, a photodiode PD and a blocking diode BD are connected in series so that they have opposite polarities to form one receiving element 61. A configuration has been proposed in which a plurality of 61 are arranged one-dimensionally in a line.

The reading of the image signal from the image sensor is performed as follows. That is, reflected light from the original surface (not shown) is irradiated to the already charged photodiode PD, and a photocurrent proportional to the irradiation amount of the light flows into the anode side of the photodiode PD,
The charge accumulated in the photodiode PD is 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 biased forward to reset the cathode voltage between the diodes to a substantially constant value (signal reading period). Accordingly, the same amount of charge as the positive charge of the cathode electrode flowing out as a photocurrent during the accumulation period is supplemented (charged) from the outside by the signal reading operation. The replenishment of the charge is transferred to the common signal line 6.
An image signal output can be obtained by detecting as a voltage by the integrator 64 via 3. The above operation is performed each time a drive pulse is sequentially applied to each light receiving element 61 from each terminal of the shift register SR.
Line image signals can be obtained in time series.

[0004]

However, the reading of the image signal from the image sensor has the following problems. FIG. 7A shows an image sensor in which a diode semiconductor is formed of amorphous silicon.
When a chart in which white, black, and white are arranged in the sub-scanning direction as shown in FIG. 7 is read, as shown in FIG. An output signal having a whiter level than the level (FIG. 7)
(B)) is output. In other words, there is a problem that a document image cannot be read accurately at a place where the black and white density on the document surface changes suddenly.

This phenomenon will be described with reference to FIGS. 8 and 9 for one pixel of the image sensor. 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 light is After recharging the discharged charge and resetting it to the initial state,
After the drive pulse returns to the “Low” level (GND), as shown by the hatched portion in FIG. 9B, the charging current (a forward current, the integration of which becomes an image signal) is 1% or less. When the driving pulse is “Hig”
h "flows in the opposite direction to the current. Since this overlaps for all pixels, the above phenomenon occurs. FIG. 9 (b)
The current waveform shown in the figure shows a case where the integration time is sufficiently long.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an image sensor comprising an array of a plurality of light-receiving elements formed of two diodes connected in series with opposite polarities is used to form an image. It is an object of the present invention to provide a reading circuit capable of obtaining an image signal faithful to a document when outputting a signal.

[0007]

In order to solve the above-mentioned problems of the prior art, a reading circuit of an image sensor according to the present invention comprises:
An image sensor in which a plurality of light receiving elements composed of two diodes connected in series with opposite polarities are arranged in a line, and driving means for sequentially applying a driving pulse to one end of each light receiving element. A reading circuit of an image sensor having a common signal line commonly connecting the other end of each of the light receiving elements and an integrator connected to the common connection line, wherein one end of the light receiving element is set to a GND level after the driving pulse is applied. A compensating current that differs only in polarity from the reverse current generated when the current is generated is generated by the additional circuit, and is fed back to the input of the integrator.

That is, a reading circuit according to a first aspect of the present invention includes a capacitor section for charging an electric charge proportional to the output of the integrator, a leak section for discharging the electric charge with a predetermined time constant, and an output of the leak section to the integrator. And a resistor for feedback to the input.

FIG. 5 shows the above configuration in an equivalent circuit. The 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. Capacitor 2
Is provided to store the output of the constant charge source 1 because the reverse current is the sum of the currents from all the pixels. Since the reverse current is attenuated by a certain time constant in the leak resistor 3, the capacitor 2
Are connected in parallel. The feedback resistor 4 is connected to the capacitor 2
The terminal voltage of the parallel circuit of the leakage resistor 3 is connected back to the input of the integrator.

The reading circuit according to a second aspect of the present invention includes a low-pass filter connected to an output of the integrator, and a feedback resistor for feeding an output of the low-pass filter back to an 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 first aspect of the invention, a compensation current having only a polarity different from the reverse current is generated by the capacitor portion and the leak portion, and the compensation current is fed back to the input side of the integrator through the feedback resistor. The direction current can be offset by the compensation current.

According to the second aspect of the present invention, the low-pass filter generates a compensation current that differs only in polarity from the reverse current, and feeds it back to the input side of the integrator via the feedback resistor. It can be offset by current.

[0013]

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

The one-dimensional image sensor 10 is formed 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 as one pixel in a line. Have been.
Each light receiving element 11 is, as shown in FIG.
On the lower electrode 102 (formed of, for example, a metal film) formed thereon, two semiconductor layers 103a and 103b made of amorphous silicon in which an n layer and an i layer are stacked are formed separately. , 103b, the upper electrodes 104a, 104b (formed of, for example, a transparent conductive film) are stacked on each other, so that two diodes can be arranged to face each other and connected in series with opposite polarities. . The upper electrodes 104a, 104b
Is a contact hole 106 formed in the insulating layer 105.
a, 106b are connected to the individual drive line 12 and the common signal line 13 made of aluminum, respectively.

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. The anode side of each photodiode PD is connected to one common signal line 13. The shift register SR is provided with a data terminal DATA and a clock terminal CK, and data pulses input from the data terminal DATA are shifted and sequentially output as drive pulses from each terminal of the shift register SR. Then, 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 communicated. It is taken out from the line 13 outside.

The incomplete integrator 20 operates as an integrator at a high frequency and as a current-voltage amplifier at a low frequency.
Complete integrator 22 having integration capacitor 21 in the negative feedback section
And 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 perfect integrator 23 and the input side of the perfect integrator 22. The complete 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. The output voltage divided by the resistors 25 and 26 is input to the non-inverting amplifier 23. The resistor 24 operates to leak the capacitor 21 at the output of the non-inverting amplifier 23.

The leak recovery circuit 30 includes a capacitor 3 having one end connected to the output of the non-inverting amplifier 23.
1, 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, and It comprises sample and hold circuits 37 and 38 connected to the other end. The constant current sources 33 and 34 are configured to output a current value substantially proportional to the output voltage value of the incomplete integrator 20. Also,
The reset switches 35 and 36 are connected to the reference power supply Vos.

In the leak reproducing circuit 30, one ends of the capacitors 31, 32 are driven by the output of the incomplete integrator 20, and the other ends of the capacitors 31, 32 are connected to the constant current sources 33, 34 compensates for the leakage due to the resistor 24, and an output obtained by integrating the sensor output current I appears here. After the completion of the integration, the reset switches 35 and 36 reset the potential to a constant reference potential, and restart the integration. The integration and reset operations described above are performed by the capacitor 31 (32), the constant current source 33 (34),
Since the two sets of reset switches 35 and 36 are used alternately and complementarily, the leak recovery circuit 30 can always be set to the integration mode.
The operating speed is not limited by the loss due to the switching time of 5, 36.

The compensation circuit 40 includes the sample-and-hold circuit 3
A multiplexer 43 for integrating the two signal output lines 41 and 42 via the signal lines 7 and 38 into one system;
Diodes 44, 45 having cathode sides connected to 1, 42
A transistor 46 having a base connected to the anodes of the diodes 44 and 45; a resistor 47 connected between the emitter of the transistor 46 and the reference power supply Vos; It comprises a capacitor 48 and a leak resistor 49 connected in parallel, and a feedback resistor 50 connected between the collector of the transistor 46 and the input of the incomplete integrator 20.

That is, a simple current source is constituted by using the reset potential by the reference power supply Vos and the output terminals of the sample and hold circuits 37 and 38. The current flowing on the emitter side of the transistor 46 is determined so as to be inversely proportional to the clock cycle. Two diodes 44 and 45 provided at the current source input (the base side of the transistor 46)
The potential lower than the signal output lines 41 and 42 is
Is determined, thereby enabling current correction of all pixels.

The charge accumulated in the capacitor 48 from the current source output (collector side of the transistor 46) is represented by a time constant τ (the capacitance of the capacitor 48 is C and the value of the resistor 49 is R
Τ = CR). The resistance 49 needs to be sufficiently smaller than the feedback resistance 50 so as not to cause an operation error. 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 described with reference to FIG. When the drive 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 from the anode side of the photodiode PD1 in the forward direction. Next, the anode side of the blocking diode BD1 becomes “Low”, and the anode side of the blocking diode BD2 of the light receiving element 11 becomes “High” by application of the drive pulse 72.
The current (reverse current) flows in the photodiode PD1 in the reverse direction, and the photodiode PD2
, A forward current (sensor output current) flows. Further, the same operation is performed for each light receiving element 11 by the driving pulses 73, 74. FIG. 3 shows a current waveform in a case where the clock cycle input to the shift register SR is lengthened and the integration time is sufficiently long for simplification of the description.

When the above-described image signal reading operation is performed for all pixels (four pixels for simplicity of description), the current flowing through the common signal line 13 is defined as the sum of the forward current (sensor output current) and the reverse current. Fig. 3
become that way. When these outputs are respectively integrated, they become forward current integration and reverse current integration. According to the conventional reading circuit, the forward / backward integration obtained by summing the forward current integration and the reverse current integration becomes an image signal output from the integrator, and the image on the document surface cannot be accurately converted into an electric signal. In the above embodiment, the charge proportional to the forward current integrated value is transferred to the capacitor 4 for each clock.
8 and leak this charge via a resistor 49 inserted in parallel so as to have the same time constant as the reverse current.
Then, the feedback resistor 5 is obtained from the terminal voltage of the capacitor 48.
The signal is fed back to the input of the incomplete integrator 10 via 0 as a compensation current. As shown in FIG. 3, the compensation current has a sign opposite to that of the reverse current and has the same magnitude. When the forward current, the reverse current, and the compensation current are totaled, the compensation current becomes a compensation current composite integral. The image signal output from the multiplexer 43 can be a signal faithful to the document surface.

Next, the value of the reverse current in the reading circuit of the above embodiment is obtained by calculation. Reverse current Ir
Is proportional to the magnitude of the forward current and is expressed by the following equation. Ir = −k · If · exp (−t / τ) Here, τ is a constant determined by amorphous silicon which is a semiconductor of the diode, k is a proportional constant, and If is a forward current. Assuming that the time when the drive pulse from the shift register SR rises is nΔt, the reverse current of the photodiode PD after t seconds is Ir = −k · If · exp (− (t + nΔt) / τ). , If (n) = If (n + 1) =... = If (m) = If, 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 · Δt / τ)) / (1-exp (-Δt / τ)) When m is sufficiently large with respect to 1 and τ is sufficiently large with respect to Δt, (1-exp (−m · Δt / τ)) = 1 (1−exp (−Δt / τ)) = Δt / τ, so that ΣIr (n) = k · If · τ / Δ
t · exp (−t / τ). Here, if the clock cycle (integration time) Δt is constant, k · If · τ is a constant, and ΔIr (n) = Constant · exp (−t / τ) (1)

The above equation (1) represents a CR having a time constant of τ.
Corresponds to the expression representing the output of the low-pass filter. As described in the above embodiment, in order to obtain the compensation current, the electric charge proportional to the output of the incomplete integrator 20 is discharged in the time constant τ while accumulating the electric charge in the capacitor 48 every clock. Is fed back to the input of the incomplete integrator 20 through the feedback resistor 50 as a compensation current. This compensation current is calculated by the clock (integration time)
Is fixed, the output is the same as the output of the low-pass filter, as can be seen from equation (1). Therefore, as shown in FIG. 4, if the output of the imperfect integrator 20 is passed through the low-pass filter 51 and then fed back to the input of the imperfect integrator 20 via the feedback resistor 50, the above-described embodiment will be described. Similar effects can be obtained. The other components in FIG. 4 are the same as those in FIG. 1, and are denoted by the same reference numerals and detailed description is omitted.

[0026]

According to the present invention, an additional circuit for generating a compensation current having only a different polarity with respect to a reverse current generated at the fall of a drive pulse is provided, and feedback is provided to the input side of the integrator via a feedback resistor. Therefore, the reverse current can be offset by the compensation current. As a result, the effect of the reverse current on the sensor output can be prevented, and an image signal faithful to the original image can be obtained.

[Brief description of the drawings]

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

FIG. 2 is an explanatory sectional view of a light receiving element.

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

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

FIG. 5 is a schematic equivalent circuit diagram showing a 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 a problem of a conventional example;
(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 the image sensor.

9A is a driving pulse waveform output from a shift register, and FIG. 9B is a current waveform flowing to a light receiving element.

[Explanation of symbols]

1 ... charge source, 2 ... capacitor, 3 ... leakage resistance,
4: feedback resistor, 10: one-dimensional image sensor, 11
... light receiving element, 12 ... individual drive line, 13 ... common signal line, 20 ... incomplete integrator, 30 ... leak reproduction circuit,
40 ... compensation circuit, 44 ... diode, 45 ... diode, 46 ... transistor, 47 ... resistor, 48 ...
Capacitor, 49: Leakage resistance, 50: Feedback resistance,
51 ... Low-pass filter

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H04N 1/024-1/036 H04N 1/40-1/409

Claims (2)

(57) [Claims] An image sensor comprising a plurality of light-receiving elements, each of which is composed of two diodes connected in series with opposite polarities, arranged in a line, and a drive pulse is sequentially applied to one end of each of the light-receiving elements. In a reading circuit of an image sensor, comprising: a driving unit for applying a voltage; a common signal line commonly connecting the other ends of the light receiving elements; and an integrator connected to the common connection line. An image sensor, comprising: a capacitor section for charging an electric charge; a leak section for discharging the electric charge at a predetermined time constant; and a feedback resistor for feeding an output of the leak section back to an input of the integrator. Reading circuit. 2. An image sensor in which a plurality of light receiving elements composed of two diodes connected in series with opposite polarities are arranged in a line, and a driving pulse is sequentially applied to one end of each light receiving element. In the reading circuit of the image sensor, the driving circuit for applying, a common signal line connecting the other end of each light receiving element in common, and an integrator connected to the common connection line, the read circuit is connected to the output side of the integrator. A read-out circuit for an image sensor, comprising: a low-pass filter, and a feedback resistor that feeds back an output of the low-pass filter to an input of the integrator.