JPS62122372A - Original reader - Google Patents

Abstract

PURPOSE:To obtain a reading signal removing noise of a fixed pattern by turning the second switching element on to read the signal, turning the first switching element on and off, thereafter reading the signal again, subtracting this to obtain an output signal and successively repeating operations on respective picture elements. CONSTITUTION:When a MOS transistor TR11 is turned on by a digital signal phiS1, a signal charge stored in a parasitic capacity C1 is read from an output terminal 11 as a voltage by a buffer amplifier BA1. At this time, not only a noise component but also a noise component on which an offset voltage due to a residual charge of a BA1 and an A points is overlapped is included. Then, a TR21 is turned on by a digital signal phiR1 to make the potential of the A point zero, and thereafter, the TR21 is turned off, the voltage of the A point is read by the signal phiS1 by making the offset voltage by the residual charge to obtain the noise component. From photoelectric transducers LE2-LEn, the signal component and the noise component including the noise are successively obtained. Thereafter, the signal component out1 including the noise is delayed by a changeover switch 15 to subtract a noise component out2 by a differential amplifier 13 and an output signal 3 is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a document reading device used in facsimiles and the like, and particularly to a contact type document reading device.

[1T view of the invention] In general, a contact type document reading device is disclosed in Japanese Patent Application Laid-Open No. 59-1407.
As disclosed in Japanese Patent Application No. 66, it is composed of a plurality of photoelectric conversion elements (photoelectric conversion element array) and a circuit for switching and scanning the elements. The second photoelectric conversion probe array has the same length as the original, and reads the original through one-to-one imaging using an optical system such as an optical fiber array or lens array. This makes it possible to shorten the length and significantly downsize the document reading device.

FIG. 1 is a plan view of a conventional contact type document reading device. In FIG. 1, a photoelectric conversion element LEA.

■, Ez, ...E, form electrodes on the insulating substrate 1,
A photoconductive thin film 2 is formed over one end of this electrode, and a transparent conductive film 3 is further provided on top of this to form a laminated structure. Further, each of the photoelectric conversion elements is directly read by a switching circuit 4 constituted by a semiconductor integrated circuit.

FIG. 2 is a circuit diagram of the document reading device shown in FIG. 1.

In FIG. 2, photoelectric conversion elements LEt, LEz, and LEg.

..., LEn are equivalently represented by photodiodes. These photoelectric conversion elements LEI, LE2. LEst...
.

LEn is each buffer amplifier B A t @B
The inputs of A1 gRAs..., BA are read directly, and the output of each buffer amplifier is connected to the shift register SR.
MOS) - transistor T whose on/off is controlled by
R engineeringt, T Rtz, T Rtat..., TR
1- via common signal v&1.0 and read amplifier 13
, are connected to the output terminal 11. Also, each photoelectric conversion element LEz.

LE2. LEδ, ..., LEn are shift registers SR
MOS transistor T Rx t v controlled by
TR2i TRaa, -, selectively grounded via TRzn. Furthermore, each photoelectric conversion element has an input capacitance and a wiring capacitance of the photoelectric conversion element and a MOS transistor.

and the parasitic capacitance CI formed by the drain capacitance,
It has CI Ca,..., Cn. Moreover, the photoelectric conversion element LEtw LE2. LEA, "', LEn are biased by power supply 12.

Here, the operation of the above-mentioned document reading device will be explained. First, when an original image is formed on the light weight conversion element LEz, a photocurrent corresponding to the light intensity flows to the photodiode,
Accordingly, a signal charge is accumulated in the parasitic capacitance C1, and the buffer amplifier BAz outputs a signal voltage corresponding to the light intensity. Next, by turning on the MOS transistor T Rs t for an appropriate time under the control of the shift register SR, a signal voltage corresponding to the optical signal of the photoelectric conversion element is transmitted via the common signal line 10 and the read amplifier 13. Output terminal 1
It is read from 1. Thereafter, the MOS transistor TRLI controlled by the shift register SR is turned on for an appropriate period of time to discharge the charge accumulated in the parasitic capacitance of the photoelectric conversion element and bring it into an initial state. Thereafter, the original image is sequentially read out by the photoelectric conversion elements LEz, LEa, . . . , LE in the same manner.

By the way, in the above-mentioned document reading device, since the offset voltage of each buffer amplifier varies, there is a fixed bump noise in which the output signal varies even if each photoelectric conversion element has incident light of the same light intensity, and the signal pair of the output signal varies. Noise ratio (S/
There was a problem of deteriorating the N ratio.

In addition, in order to discharge the charge accumulated in the parasitic capacitance of the photoelectric conversion element, a discharge MOS)-transistor T Rztt
When TRzzt..., TRzn is turned on and then turned off, a part of the charge in the channel region of the MOS transistor remains as a residual charge of the parasitic capacitance of the photoelectric conversion element, causing an offset voltage. Ta.

Further, the parasitic capacitance of the photoelectric conversion element is not uniform.

It has distribution and variation. For example, as shown in FIG. 3, in actual devices, wiring conductors often extend to the sides of the switching circuit 4 configured by a semiconductor 9 and an integrated circuit. They are formed on a substrate with different sizes. Therefore, differences occur in the parasitic capacitance of each wiring conductor, and the parasitic capacitances C, Cz, Ca, . Further, the residual charge itself has variations depending on each photoelectric conversion element, and as a result, the offset voltage generated by the residual charge has variations and distribution depending on each photoelectric conversion element, resulting in fixed pattern noise.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a document reading device that obtains a read signal from which the above-mentioned offset voltage and fixed pattern noise, which is a variation thereof, are removed.

[Summary of the invention]

SUMMARY OF THE INVENTION Therefore, the present invention provides a circuit configuration method that applies the concept of so-called double sampling, which is known for fixed imaging devices, to a document reading device using a buffer amplifier, and a driving method that fully brings out the effects of double sampling. It is.

Conventional double sampling involves reading out the electric charge accumulated in the optical FL conversion element, reading it out by opening and closing the transistor twice, and converting the signal component containing the fixed pattern noise read the first time into the fixed pattern read out the second time. The idea was to obtain signal components by subtracting pattern noise. Note that since the conventional method was to read out charges, the charges accumulated in the photoelectric conversion element were discharged at the same time during the first readout.

In the present invention, a signal voltage including an offset voltage and fixed pattern noise is read out at the first time, and then the discharge transistor is turned on to discharge the charge accumulated in the photoelectric conversion element, and then the discharge transistor is turned off to discharge the charge accumulated in the photoelectric conversion element. When a portion of the charge in the channel region of the transistor remains as a residual charge, a second readout is performed and a signal component is obtained by subtracting this readout signal from the first readout signal. By the driving method of performing the second reading after turning on/off the discharge transistor, it is possible to eliminate not only the offset voltage of the buffer amplifier but also the offset voltage fixed pattern noise due to the residual charge of the discharge transistor.

[Embodiments of the invention]

A first embodiment of the present invention will be described below with reference to FIG.

Note that the same reference numerals are used for parts that perform the same functions as in FIG.

The output of each buffer amplifier is a MOS transistor T R11, T R11, whose on/off is controlled by the signal φS, φSt, ..., φSn of the shift register SR1.
...Connected to the common borrowed line 1o via the HT R1n. Further, the common signal line 10 is connected to a changeover switch 15, and one output of the changeover switch 15 is connected to the delay element 1.
4 to one input of the differential amplifier 13, and the other output of the changeover switch 15 is directly connected to the other input of the differential amplifier 13. Further, the output of the differential amplifier 13 is connected to the output terminal 11. Each photoelectric conversion element also receives a control signal φR19φR of another shift register SRx.
2,.

..., MO3I whose on/off is controlled by φRn
-Ran resistor TRx! , TRtaxt-, and TRzn.

Next, the operation of the document reading device according to this embodiment will be explained.

First, the MoS transistor TRzz is turned on for an appropriate period of time, and a bias voltage -vB is applied to the photoelectric conversion element LEI by the power supply 12. At this time, the potential at point A becomes zero, and the primary MoS transistor TR21 is turned off. At this time, a part of the charge in the channel region of the transistor TRzt remains as a residual charge in the parasitic capacitance C1 of the photoelectric conversion element LEz, and the potential at point A becomes an offset voltage due to the residual charge slightly deviated from zero. Next, when the original image is formed on the photoelectric conversion element LEz, a photocurrent corresponding to the light intensity flows, and the potential at point A changes. FIG. 5 shows the subsequent operation when the sequence continues. When the MoS transistor T Rs 1 is turned on for an appropriate time by the digital signal φ51, the signal charge accumulated in the parasitic capacitance C1 is read out from the output terminal 11 as a voltage by the buffer amplifier BAR. At this time, the read signal is a signal component Sl corresponding to the light intensity.
In addition, it includes a noise component Nl in which the offset voltage of the buffer amplifier BAz and the offset voltage due to the residual charge at the point A overlap (S1+N1). Next, the MoS transistor TRzz is turned on by the digital signal φR1,
The charge accumulated in the parasitic capacitance CI of the photoelectric conversion element LEz is discharged, and the potential at point A is made zero. Then transistor T
Turn off R21 and make the potential at point A an offset voltage due to residual charge. Therefore, the signal is read out using the digital signal φS1, and the noise component N is the overlap of the offset voltage of the buffer amplifier BA1 and the offset voltage due to the residual charge at point A.
Get 1.

In the same manner, the photoelectric conversion element L E 1t L E
x w..., LE, a signal component containing noise (S+
N) and noise component N are obtained sequentially. This is out in Figure 5 (
This is a signal that is set to "out".

Thereafter, the signal component S + N (outl) containing noise is delayed by the changeover switch 15, and this delayed signal (
A differential amplifier 13 subtracts the noise component N (out2) from the output signal (outl') to obtain an output signal (out3).

It has been explained above that an output signal from which noise has been removed can be obtained.

Note that the switch 17 connected to the common signal line 10 in FIG. 4 will be explained below.

This switch is effective in improving operating speed. This switch selectively connects the common signal line 1o to the ground potential. By turning on this switch while the common signal line 10 is not used for signal reading, the output potential of the common signal line 10 returns to zero. Therefore, when the operating speed of the buffer amplifier is slow, especially in the case of a buffer amplifier whose operating speed is fast when the output rises but slow when the output falls, instead of lowering the potential of the common signal line 10 by the buffer amplifier, switch 17
By lowering the power consumption, it is possible to improve the operating speed of the document reading device or reduce the power consumption of the buffer amplifier.

Although not shown in the drawings, the second embodiment described later also includes the above-mentioned switch, operates this switch in advance of noise reading, and returns the potential of the common signal line 10 to zero, thereby improving the operating speed. Of course you can.

FIG. 6 shows a second embodiment of the present invention. Compared to the first embodiment, the rotary converter of this embodiment has (1) a different configuration of the noise removal circuit, and (2) a driving method. has been changed to enable high-speed reading.

In this embodiment, the common signal line 10 is divided into two parts, two of which are connected to one input terminal of the differential amplifier through the delay element 14, and the other branch of the common signal line 10 is connected to one input terminal of the differential amplifier. Connected to the other input terminal. The output of the differential amplifier is connected to the output terminal 11 through the output sampler 16.

In the first example, in which the rqAS waveform of this example is shown in No. 71!21, a signal component (S+N) containing noise of a certain photoelectric conversion element is read out, the photoelectric conversion element is subsequently discharged, and further the signal component for discharging is read out. After turning off the MOS transistor, the noise component (N) of the photoelectric conversion element was read out. In this driving method, while discharging the photoelectric conversion element, the common signal 1s
10 has no signal.

There was time to waste. Therefore, in this embodiment, the speed is increased by operating each part in parallel so that there is no wasted time on the common signal line.

First, the operation will be explained focusing on the photoelectric conversion element LEz. First, a signal containing noise (Sz+Nz) is read out to the common signal line 10 using the control signal φs2.

Next, the charge accumulated in the parasitic capacitance Cz of the photoelectric conversion element LEz is discharged using the control signal φR2. After that, a second pulse of the control signal φsx causes a noise component N2, which is a combination of the offset voltage of the buffer amplifier and the offset voltage due to the residual charge after the photoelectric conversion element discharges, to the common signal line 10.
Read out.

In the above operation, during the time when the second photoelectric conversion element LEa is being discharged, the common signal line 10 is used to read out the noise component Nl of the first photoelectric conversion element LEs, and the noise component Nl of the third photoelectric conversion element LEs is read out. Signal component containing noise of LEa (Ss+N
By reading out a), there is no wasted time on the common signal line 10.

The operation of the final stage noise removal circuit is as follows.

The output (out) of the common signal line 10 is delayed and becomes the company signal (out'). The delay time is defined as a signal containing noise (S x + N
(lack) are set to match. A differential amplifier 13 takes the difference between the two signals to obtain an intermediate waveform (out2).

Output sampler 16 extracts only meaningful signal parts,
An output signal (out3) from which fixed pattern noise has been removed is obtained.

FIG. 8 is a diagram showing a third embodiment of the present invention.

In this embodiment, the second common signal line 17 and the MOS transistor TRAl are different from the first embodiment.

TRaz, ..., TRan and shift register SR
We are adding more males. The output of each buffer amplifier is the signal φNtt φN4 . ...,φN
MOS transistor T whose on/off is controlled by n
It is connected to the second common signal line 17 via Raz, TRaz, -, and TRan. The second common signal line 17 is connected to one input terminal of the differential amplifier 13. 1st
The common signal line '10 is connected to the other input terminal of the differential amplifier 13 through the delay element 14. The other configurations are the same as in the first embodiment.

Ninth, an example of the drive waveform of this embodiment is shown. The operation will be explained focusing on the photoelectric conversion element LEz. First, the control signal φs1
A signal containing noise (Sz
+Nt). Next, the charge accumulated in the parasitic capacitance Ci of the photoelectric conversion element LEz is discharged using the control signal φR1. After that, the control signal φNl causes the second common signal line 1 to
7, a noise component N1 in which the offset voltage of the buffer amplifier and the offset voltage due to the residual charge after the discharge of the photoelectric conversion element overlap is read out. The output signal of the first common signal line 10 (
S1+Ns) is delayed to align the time with the output signal N1 of the second common signal line 17, and the difference is taken by the differential amplifier 13 to obtain the signal component S1. Furthermore, high-speed operation is possible by operating each part in parallel as described below. That is, while the noise component N1 of the first photoelectric conversion element (2) and El is read out through the second common signal 17, the second photoelectric conversion element (2) and Ez are discharged to the initial voltage, and the third photoelectric conversion element (2) and Ez are discharged to the initial voltage. The signal component (S
a + N a ) is read out to the first common signal 11A10.

As explained in detail above, the discharging transistor is turned on to discharge the charge of the photoelectric conversion element, the dissipation main transistor is turned off to generate an offset voltage due to the residual charge, and then the signal is read out. , the noise component in which the offset voltage of the buffer amplifier and the offset voltage due to the residual charge overlap can be read out.

Note that if the variations in the offset voltage of the buffer amplifier are large and the variations in the offset voltage due to the residual charge after the photoelectric conversion element is discharged are small and do not pose a problem, the signal may be read out with the discharge transistor turned on. In this case, only the variation in the offset voltage of the operational amplifier is obtained as a noise component, and a certain fixed pattern noise suppression effect can be obtained.

〔Effect of the invention〕

As described in detail above, according to the present invention, it is possible to obtain a readout signal from which fixed pattern noise has been removed, thereby improving the S/N ratio of the readout signal.

[Brief explanation of drawings]

Fig. 1 is a plan view of a conventional contact type original reading device, Fig. 2 is a circuit diagram of a conventional contact type original reading device, Fig. 3 is a plan view showing the arrangement of conventional wiring conductors, and Fig. 4 is a plan view of a conventional contact type original reading device. A circuit diagram showing the first embodiment, FIG. 5 is a diagram showing operating waveforms of the first embodiment, FIG. 6 is a circuit diagram showing the second embodiment, and FIG. 7 is a diagram showing the operation waveforms of the first embodiment. FIG. 8 is a circuit diagram showing the third embodiment, and FIG. 9 is a diagram showing the operating waveforms of the third embodiment. 1... Insulating substrate, 2... Photoconductive thin film, 3...
Transparent conductive thin film, 4... switching circuit, 10.1
7... Common signal line, 11... Output terminal, 12...
Bias power supply, 13... Differential amplifier, 14... Delay element, 15... ¥J! ;Fig. can 29g3 ρut3S・S2S3 Fig. 7 fss ヂR3 Fig. 8 ￥: J q To the figure 3 ψR3

Claims (1)

[Claims] A document reading device comprising a photoelectric conversion element array formed of individual electrodes, a common electrode, and at least one type of photoconductive film sandwiched between the electrodes, wherein the electric charge accumulated in each of the photoelectric conversion elements is discharged. the first connected to each element
a switching element, a plurality of buffer amplifiers that take out voltages corresponding to charges accumulated in each photoelectric conversion element, and a second switching element that reads out the outputs of the buffer amplifiers corresponding to each buffer amplifier. In the reading device, the second switching element is turned on for each element to read out the signal, and after the first switching element is turned on and off, the signal is read out again, and the second readout signal is read out from the readout signal. A document reading device characterized in that subtraction and output signals are repeated for each pixel in sequence.