JPH05199361A - Image sensor - Google Patents

Abstract

PURPOSE:To improve the linearity between a light reflected from an original and a power supply by extracting all charges stored in a photodiode so as to contribute to the picture signal. CONSTITUTION:A charge stored in a photodiode is extracted as a current and it is subject to I/V conversion by an I/V converter 12 and an integration device 16 integrates a voltage signal for a time width. Then a picture signal corresponding to the total quantity of the charge stored in the photodiode is obtained through integration in this way. Furthermore, the circuit constant of the I/V converter 12 and the integration device 16 is set within a prescribed range so that the relation among the entire gain, the time constant of the I/V converter 12 and the circuit constant of the integration device 16 is made proper.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image sensor used as an image reading device such as a small facsimile and a handy scanner.

[0002]

2. Description of the Related Art In general, an image reading element of an image sensor is composed of a photodiode and a blocking diode whose anodes are connected back to back or whose cathodes are connected front to front. The photodiode photoelectrically converts the light reflected by the document to store the charge corresponding to the image information, and the blocking diode serves as a switching element for taking out the charge stored in the photodiode.

FIG. 3 shows an example of a circuit for processing an image signal obtained from the image reading element as described above. However, this circuit is for an image reading element consisting of a set of a photodiode and a blocking diode, and in practice, such a circuit is provided as many as a plurality of image reading elements existing in the same block. ..

In FIG. 3, an image signal obtained as a current from the photodiode is a current −
It is supplied to a voltage converter (hereinafter referred to as I / V converter) 52. Then, after being converted into a voltage signal here, it is amplified to a predetermined level in an amplifier circuit 54 and supplied to a sample hold circuit (hereinafter referred to as S / H circuit) 56. The S / H circuit 56 samples the voltage level in accordance with the timing at which the signal input to the S / H circuit reaches the peak value, and holds the voltage level for a certain period. Then, the held voltage level is taken out as an image signal, and the necessary processing thereafter is performed.

[0005]

It takes some time from the application of the drive pulse to the blocking diode to the extraction of the electric charge accumulated in the photodiode as a current. Generally ON characteristic (forward bias characteristic)
The better the blocking diode, the shorter this time,
On the contrary, a blocking diode having a poor on characteristic (forward bias characteristic) has a longer time. Therefore, the signal converted into a voltage by the I / V converter also changes over a certain time width as shown in FIG. 2 according to the ON characteristics of the blocking diode. Conventionally, the peak value of the voltage that changes in this way was captured as an image signal, but it is not the peak voltage value that corresponds to the charges accumulated in the photodiode, but the total amount of charges discharged from the photodiode. For example, the voltage signal indicated by a in FIG. 2 is a case where the blocking diode has a good ON characteristic, and the voltage signal of a rises quickly and the peak voltage value is large. On the other hand, the voltage signal indicated by b in FIG. 2 is the case where the ON characteristics of the blocking diode are bad, and the rising of the voltage signal of b is slow and the peak voltage value is small.
However, the integrated value of the voltage signal is constant in both a and b, which corresponds to the total amount of charges accumulated in the photodiode. Therefore, in the signal processing circuit of the conventional image sensor shown in FIG. 3, the voltage signal shown in FIG. 2 becomes more gradual when the accumulated charges are taken out as a current due to the ON characteristic of the blocking diode, for example. , The correlation between the amount of light reflected from the original and the peak value of the voltage after current-voltage conversion is reduced.

Further, since it is difficult to manufacture a large number of blocking diodes having a constant forward characteristic, the time when the current drawn from the photodiode reaches the peak value is not always constant. Therefore, it cannot be said that all of the voltage values sampled by the S / H circuit correspond to the peak voltage, and some deviation from the time when the peak is reached is inevitable.

Generally, the linearity of the sensor output with respect to the amount of input light is represented by γ. When the sensor output is completely proportional to the amount of input light, γ becomes 1. However, like the conventional image sensor, if the voltage peak after current-voltage conversion is taken out as the sensor output, for the reasons described above,
This γ is considerably smaller than 1, and is about 0.8, for example. This is 1 ± required by general users as the value of γ.
Compared to 0.1, it is much smaller and therefore the quality of the image obtained is reduced.

In general, many photodiodes and blocking diodes used for one image sensor are manufactured at the same time in the same process. Therefore, if the characteristics vary as described above, they must be discarded. However, there is a problem that the yield is reduced and therefore the overall cost is increased.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an image sensor capable of improving the yield and reducing the cost by improving the linearity of the sensor output with respect to the amount of input light. It is what

[0010]

An image sensor according to a first aspect of the present invention for achieving the above object divides a plurality of image reading elements arranged in a main scanning direction into blocks each having a predetermined number. Image information for one line is obtained by scanning all the blocks in such a manner that the image reading elements are connected in a matrix in units of blocks and the image reading elements in the same block are simultaneously driven to output parallel signals. In the image sensor for reading, a current-voltage conversion unit that operates for each block and converts each image signal obtained as a current signal from each image reading element into a voltage signal, and operates for each block, Integrating means for integrating the image signal, which is a voltage signal, for a certain period of time is provided.

When the feedback resistance forming the current-voltage converting means is Rf, the input resistance forming the integrating means is Rin, and the feedback capacitance is Cf, the range of Rf / Rin is 5 to 1000, Cf range from 10 to 1000
It is preferably pF.

An image sensor according to a second aspect of the present invention for attaining the above object comprises a voltage amplifying means between the current-voltage converting means and the integrating means of the image sensor according to the first aspect of the invention. The input resistance constituting the voltage amplification means is R
2. When the feedback resistance is R3, (Rf · R3) /
It is characterized in that the range of (Rin · R2) is 5 to 1000 and the value of Rf is 1 MΩ or less.

[0013]

According to the first aspect of the present invention, by integrating the voltage signal from the I / V converting means by the integrating means over a fixed period, a signal equivalently integrating the voltage change during that period can be obtained. Therefore, whether the charge is taken out sharply or gently from the image reading device, the integrated signal has a very high correlation with the amount of charge accumulated in the image reading device, that is, the amount of light reflected from the document. You will have sex.

The range of Rf / Rin is 5 to 100.
By setting 0 and setting the range of Cf to 10 to 1000 pF, the relationship between the gain of the entire circuit and the time constant of the I / V conversion means and the integration time of the integration means can be optimized.

According to the second aspect of the present invention, by providing the voltage amplifying means between the I / V converting means and the integrating means, the overall gain, especially the gain up to the integrating means can be increased. .. At this time, (Rf · R3) / (R
If the range of inR2) is 5 to 1000 and the range of Rf is 1 MΩ or less, the relationship between the gain of the entire circuit and the time constant of the I / V conversion means and the integration time of the integration means is optimized. be able to.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An image sensor according to an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram of a signal processing circuit of an image sensor according to an embodiment of the present invention. In a typical image sensor in which one block has 32 channels and a total of 54 blocks, such a circuit is provided in the number of channels of one block, that is, 32, and is commonly used in each block.

In the circuit of FIG. 1, the I / V converter 12,
The amplifier 14 is the same as that of FIG. 3, but in this embodiment, an integrator 16 is used instead of the sample hold (S / H) circuit used in FIG.

When the S / H circuit is used as in the prior art, for example, when the output voltage of the I / V converter changes as shown by the curve a in FIG. 2, the peak voltage at time t1 is maintained. Although the image output is used, when the integrator 6 is used as shown in FIG. 1, the curve a is integrated over a period longer than the time width of the curve. Therefore, due to variations in the characteristics of the blocking diode, I / V
Even if the output of the converter 14 shows a change as shown by the curve b in FIG. 2, if the integration time is sufficiently longer than the time width of this curve, it is accumulated in the photodiode by integration. An image signal corresponding to the total amount of electric charge can be obtained.

In FIG. 1, the total gain of the I / V converter 12, the amplifier 14, and the integrator 16 is (Rf · R).
3) / (Rin.R2.Cf) Here, the amplifier 14 can be omitted. When the amplifier 14 is omitted, R3 / R2 becomes 1, so that the overall gain becomes Rf./(Rin.Cf).

First, consider the case where the amplifier 14 is not provided. When considering only increasing the gain of the circuit, it is preferable to increase the value of Rf and decrease the value of Cf as much as possible. However, if Rf is large, I /
The time constant of the V converter also becomes large, and the time required to take out the charge accumulated in the photodiode as a current becomes long. In other words, the time width of the curve shown in FIG. 2 widens. Therefore, if Rf is too large, it becomes impossible to completely discharge all the charges accumulated in the photodiode within the integration period of the integrator 16.

On the other hand, the overall gain can be increased by reducing Cf. However, since the integrator 16 includes the reset switch SW, it is inevitable that noise charges are generated by the on / off operation of the reset switch SW. If this noise charge level is QNO, this is QNO / C by the integrator 16.
It is amplified f times and output. Therefore, when the value of Cf is reduced, the noise level appearing in the output is increased and the S / N ratio of the output image signal is reduced.

From the above, if the respective circuit constants are independently determined, the above inconvenience will occur. In the present embodiment, considering the above points, the range of Rf / Rin is set to 5 to 1000, and the range of Cf is set to 10 to 10.
It is set to 00 pF. With such a range, the inconvenience as described above can be avoided.

Next, consider the case where the amplifier 14 is provided as shown in FIG. By providing the amplifier 14, the signal level can be increased before reaching the integrator 16,
It is not necessary to consider increasing the gain in the integrator 16 so much. Therefore, to increase the overall gain, Cf
It is not necessary to reduce the value of, and it is mainly the role of the amplifier 14 that makes the gain of the entire circuit a predetermined gain. Therefore, the value of Cf can be made sufficiently large so that the noise generated in the integrator 16 can be suppressed small, and therefore the S / N ratio of the image output can be improved. Similarly, in this case, since it is not necessary to increase the gain in the I / V converter 12 so much, the R of the I / V converter 12 is increased.
I / V converter 12 in order not to make f so large
The time constant of can be kept sufficiently small.

Considering the above points, as shown in FIG.
When 4 is provided, (Rf ・ R3) / (Rin ・ R2)
Is set to 5 to 1000, and Rf is set to 1 MΩ or less. As actual concrete circuit constants, in this embodiment, Rf = 500 kΩ, Rin = 50 kΩ, Cf = 50 p
F, R2 = 50 kΩ and R3 = 500 kΩ.

In this way, the integration time of the integrator 16 is about 16 microseconds. On the other hand, the operation clock frequency of the image reading element required by the user is about 500 kHz for a high-grade image sensor, that is, 2 microseconds per data. Therefore, when one block operating simultaneously has 32 channels, the total time is 64 microseconds, which has a sufficient margin as compared with the integration time of 16 microseconds, and there is no problem in circuit operation. As a result of actually performing an experiment with setting such a circuit constant, the value of γ is about 0.
When the illuminance is about 20 lux, the S / N ratio is 30 dB or more, and the MTF (Modulation Trans
fer function) has improved to over 95%.

[0026]

As described above, according to the present invention, even if there is some variation in the ON characteristics of the blocking diode, the signal extracted as an image output is high with respect to the charge accumulated in the photodiode. It is possible to provide an image sensor that can obtain linearity and thus can obtain a high-quality image, improve yield, and reduce overall cost.

[Brief description of drawings]

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

FIG. 2 is a graph showing how a voltage of an image signal output from an I / V converter changes.

FIG. 3 is a circuit diagram of a signal processing circuit of a conventional image sensor.

[Explanation of symbols]

 10 input terminal 12 current-voltage converter (I / V converter) 14 amplifier 16 integrator Cf capacitor SW reset switch Rf, Rin, R2, R3 resistance

Claims (3)

[Claims] 1. A plurality of image reading elements arranged in the main scanning direction are divided into a predetermined number of blocks, and the image reading elements are connected in a matrix in units of the blocks, and the image reading elements in the same block are connected. In the image sensor that reads the image information of one line by scanning all blocks for the operation of simultaneously driving and outputting parallel signals, the image sensor is operated for each block, and a current signal is output from each of the image reading elements. Current-voltage conversion means for converting each of the obtained image signals into a voltage signal, and integration means that operates for each of the blocks and integrates the voltage signal image signal for a certain period are provided. Image sensor. 2. A feedback resistance forming the current-voltage converting means is Rf, an input resistance forming the integrating means is Rin,
The image sensor according to claim 1, wherein when the feedback capacitance is Cf, the range of Rf / Rin is 5 to 1000 and the range of Cf is 10 to 1000 pF. 3. The current of the image sensor according to claim 2,
When an amplifying means is provided between the voltage converting means and the integrating means and the input resistance and the feedback resistance forming the amplifying means are R2 and R3, respectively, the range of (Rf.R3) / (Rin.R2) is 5
To 1000 and the value of Rf is 1 MΩ or less.