JP2000011150A - Output circuit for image reading device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an output circuit for image reading device operable only with a single positive power source to reduce power consumption and cost by improving the deterioration of S/N due to the secureness of fact reading and depth of focus. SOLUTION: In this output circuit in which an image sensor output is used as the input voltage of a DC level shift circuit or a first impedance converter, an analog switch 7 and a capacitor 8 are connected in series between the output terminal and reference potential of the DC level shift circuit or the first impedance converter, the analog switch 7 is constituted so as to be turned ON/OFF based upon a sensor driving signal, and a voltage generated at the capacitor 8 is set as the operating point of an amplifier through a second impedance converter 10. Thus, the output circuit of the image reading device operable only with a single positive power source is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an output circuit of an image reading device in a facsimile, a personal computer, or the like.

[0002]

2. Description of the Related Art In recent years, among image reading apparatuses, contact image sensors used for facsimile and personal computers have been increasingly used for handy scanners and color scanners, and have a large depth of focus and high-speed reading. Things are sought.

[0003] When the depth of focus becomes deeper, the aperture angle of the lens must be narrowed, and the aperture angle which has been conventionally 20 degrees is reduced to 8.7 degrees. As a result, even when a double-row lens is used, the light amount ratio becomes 20% or less, and the amount of exposure to the sensor element is drastically reduced. In addition, the reading time is conventionally 10 ms / l to 5 ms / l, but the reading time is 2.5 ms / l.
The demand of ms / l or less is becoming mainstream, and the charge accumulation type image sensor further reduces the amount of light.

In such a case, a measure is taken to improve the S / N ratio by increasing the luminance of the illumination light source. However, a mobile handy scanner that is basically driven by a battery cannot easily increase the light source current. Therefore, good S / N
It is desired to realize an image reading device having a ratio satisfying the above conditions. Conventionally, both positive and negative power supplies have been supplied to the image reader in order to reduce power consumption and reduce the cost of products such as facsimile machines. There are issues that cannot be addressed.

Hereinafter, a conventional technique will be described with reference to the drawings. FIG. 3 is a circuit diagram of an output circuit of a conventional image reading apparatus when driven by dual power supplies. This output circuit operates an amplifier circuit with positive and negative power supplies.

This output circuit is composed of an image sensor element 1
An analog switch 2 for switching at a sensor driving frequency, a capacitor 3 for converting a photoelectric conversion current of the image sensor element 1 into a voltage, a non-inverting amplifier 9 by an operational amplifier, and a degree of amplification of the non-inverting amplifier 9 The feedback resistors 11 and 12 are provided.

The operation of the output circuit of the conventional image reading apparatus configured as described above will be described. The output of the image sensor element 1 is sequentially output for each pixel in the drive block indicated by the voltage at the point A. The image sensor element 1 outputs a photocurrent during a Low period of the driving block,
The capacitor 3 is charged, and the electric charge charged during the High period of the drive block is discharged by closing the analog switch 2 so that the photocurrent of the next pixel is not affected.

The signal output is shown at point B in FIG. The non-inverting amplifier 9 operates on both positive and negative power supplies with 0v as a reference potential, and the feedback resistors 11 and 12 determine the degree of amplification.

FIG. 4 is a circuit diagram of an output circuit of the conventional image reading apparatus when a single power supply is driven.

This output circuit includes voltage dividing resistors 4 and 5 for determining the operating point of the non-inverting amplifier 9 and PN
An impedance converter (PNP transistor for emitter follower, hereinafter referred to as a PNP transistor) 6 composed of a PNP transistor with a grounded P collector, an impedance converter 10 composed of an operational amplifier, and a voltage dividing resistor 13 for determining an operating point of the non-inverting amplifier 9. , 14 and a smoothing capacitor 15.

The operation of the output circuit driven by a single positive power supply of the conventional image reading apparatus configured as described above will be described.

The output from the image sensor element 1 is the same as the output at the point B shown in FIG. Here, since the voltage from the image sensor element 1 is an output voltage based on 0 V, in order to operate the operational amplifier and the differential amplifier with a single power supply and obtain an appropriate amplification degree, a constant DC voltage is required. Bias needs to be applied. Therefore, PNP
An appropriate operating point is designed based on the ratio of V _{BE of the} transistor 6 to the voltage _dividing resistors 4 and 5.

The voltage dividing resistors 4 and 5 are R ₄ and R ₅ and PN
Assuming that the base-emitter voltage of the P transistor 6 is V _BE , the input voltage Vin of the non-inverting amplifier 9 is represented by the following equation.

[0014]

(Equation 1)

Here, V _B represents a base voltage of the PNP transistor 6, that is, an image sensor output signal.

On the other hand, the voltage dividing circuit for setting the operating point of the non-inverting amplifier 9 composed of the voltage dividing resistors 13 and 14 is as described in the above (Equation 1).
Is set to be equal to the DC bias component, and the output impedance is lowered by the impedance converter 10 and connected to the feedback resistor 12.

Thus, if the output of the voltage divider composed of the voltage dividing resistors 13 and 14 is E, E is represented by the following equation.

[0018]

(Equation 2)

In order to make the DC bias term of the above (Equation 2) equal to that of the above (Equation 1), the following equation must be satisfied.

[0020]

(Equation 3)

According to the above configuration, the output of the non-inverting amplifier 9 is

[0022]

(Equation 4)

The DC bias is subtracted by (Vin-E), and only the signal is amplified. The operating point is determined by the final term E.

In the above-described conventional technique, the DC bias determined at the time of single power supply operation is the
At the same time, the voltage dividing ratio determined by the _BE and the voltage dividing resistors 4 and 5 must be set at the same time, and the voltage dividing ratio determined by the voltage dividing resistors 13 and 14 must be set equal. In this case, the offset voltage becomes an offset voltage, and the offset voltage is amplified by the amplification degree determined by the feedback resistors 11 and 12, thereby causing saturation of the output of the image sensor, deterioration of linearity, and the like. In order to prevent this, the voltage dividing resistor 14 or 13 is changed to a variable resistor to perform offset voltage removal adjustment, but this is disadvantageous in terms of cost.

Further, in the case of a contact type image sensor, the length is increased in order to form an image at a ratio of 1: 1 with respect to the width of the read original. As a result, the resistance value of the ground pattern between the image sensor chips varies, and a potential difference occurs in the output signal of each pixel. In the case of the conventional technique, since the operation reference voltage E of the non-inverting amplifier 9 is a DC voltage obtained by dividing V _CC by resistance, the potential difference of the output signal of each pixel is directly multiplied by the amplification factor, and the potential difference is also amplified. Would.

[0026]

As described above, in the prior art, the variable resistor is used to absorb the offset voltage caused by the variation of the voltage dividing resistor which determines the operating point in the single power supply operation output circuit. Therefore, it is necessary to take measures such as adjustment with a heater and narrowing the range of variation of the voltage dividing resistor, which is disadvantageous in cost. Further, the potential difference of the output signal of each pixel caused by the variation of the resistance value of the earth pattern of the printed wiring board is amplified by the non-inverting amplifier, and it is difficult to improve the S / N ratio.

According to the present invention, an inexpensive circuit configuration and a variable resistor are used to reduce the swell of an image sensor dark output signal due to the generation of an offset voltage during the operation of a single power supply and the variation in the resistance value of the ground pattern of a printed wiring board. It is an object of the present invention to provide an output circuit of an image reading apparatus which does not require any countermeasures such as adjustment by an image reader and narrowing of an allowable range of resistance value variation.

[0028]

In order to achieve the above object, an output circuit of an image reading apparatus according to the present invention comprises a first capacitor and a first electric switch connected in parallel to an output terminal of a photoelectric conversion element. An output circuit of the image reading device, wherein an output signal terminal of the photoelectric conversion element is connected to a DC level shift circuit or a first impedance converter, and is used as an input voltage of the DC level shift circuit or the first impedance converter. Wherein a second electrical switch element that switches with a sensor drive signal and a second capacitor are connected in series,
The second electric switch element and the second capacitor are connected between the output terminal of the DC level shift circuit or the first impedance converter and a reference potential, and the voltage across the second capacitor is changed to the second voltage. In addition to the impedance converter, the operating voltage of the amplifier is configured.

Thus, it is possible to provide an output circuit of an image reading apparatus capable of improving the generation of an offset voltage due to a variation in resistance value and the potential difference between output signals of respective pixels due to a variation in ground pattern resistance of a printed wiring board.

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the first aspect of the present invention, a first capacitor and a first electrical element are connected in parallel to an output terminal of a photoelectric conversion element for converting light reflected from an original into an electric signal. A switch is connected, an output signal terminal of the photoelectric conversion element is connected to a DC level shift circuit or a first impedance converter, and an output of the photoelectric conversion element is connected to an input voltage of the DC level shift circuit or the first impedance converter. An output circuit of an image reading device, wherein a second electric switch element and a second capacitor that are switched by a sensor drive signal are connected in series, and the second electric switch element and the second capacitor are connected to each other. Connected between the output terminal of the DC level shift circuit or the first impedance converter and a reference potential, and applying a voltage across the second capacitor to the second impedance converter; An output circuit of the image reading apparatus and the operating voltage of the width circuit, ON / OF in the sensor drive signal
The switching element that performs F charges the capacitor with a DC bias applied to the photoelectric conversion element signal output,
By adding the voltage across the capacitor to the impedance converter and setting it as the operating voltage of the non-inverting amplifier, the variation in the DC bias can always be the voltage across the capacitor. As a result, the DC bias and the operating voltage of the non-inverting amplifier become equal, and it becomes possible to multiply only the signal component by the amplification degree, and to reduce the S / N ratio deterioration due to the amplification of the offset voltage and the potential difference of the output signal of each pixel. It has the effect of being able to improve.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. (Embodiment 1) FIG. 1 is a circuit diagram of an output circuit of an image reading apparatus according to Embodiment 1 of the present invention, and FIG. 2 is an output voltage waveform diagram at points A to E in the circuit diagram of FIG. And FIG. 5 are voltage waveform diagrams from point A to point E in FIG. 1 for explaining improvement of the potential difference of the signal output of each pixel.

In FIG. 1, 1 is an image sensor element, 2 is an analog switch for switching at a sensor driving frequency, 3 is a capacitor for converting the photoelectric conversion current of the image sensor element 1 into a voltage, and 4 and 5 are DC biases. 6 is a PNP transistor used as an emitter follower, 9 is a non-inverting amplifier, 10
Are impedance converters using operational amplifiers, 11, 1
Reference numeral 2 denotes a feedback resistor for determining the degree of amplification of the non-inverting amplifier 9, which is configured in the same manner as in the conventional example.

The first embodiment is characterized in that an analog switch 7 for switching at a sensor drive frequency and a capacitor 8 for charging a DC voltage at point C are provided. The one-dot chain line portion in FIG. 1 indicates the above-mentioned characteristic portion.

The image sensor element 1 sequentially outputs the photoelectric conversion current of each pixel in accordance with the drive clock seen at the voltage at point A in FIG. 2, and in the first embodiment, in the low period of the drive clock. The signal output of the image sensor element 1 is charged in the capacitor 3, the analog switch 2 is turned on during the High period of the clock, and the photoelectric conversion charge stored in the capacitor 3 is discharged to prevent the next pixel from being affected.

The signal waveform obtained as described above is shown as a voltage waveform at point B in FIG. Here, ideally, the output of each pixel is
Since the analog switch 2 is grounded during the High period of the clock, the potential of the voltage output during the High period of the clock is stable at 0V. A DC voltage is applied to the signal voltage generated at the point B by an emitter follower including the PNP transistor 6 and the voltage dividing resistors 4 and 5 so as to fall within the range of the input voltage width of the non-inverting amplifier 9. The setting of the DC bias is represented by the above equation (1). In the case of an inexpensive operational amplifier, the input voltage range is V _CC = 5 vV _EE = 0 v
In the case of, there is a need to fall within the range of _{V CC -1.5v>Vin> V EE} + 1.5v. The point C voltage in FIG. 2 indicates a sensor signal output voltage waveform to which an appropriate DC bias has been applied.

Next, in synchronization with the analog switch 2, the analog switch 7 and the capacitor 8, which are switched by a clock, are an image sensor signal output to which a direct current bias generated at the point C is applied, and the voltage of the signal portion of the CLK high period is output. The capacitor 8 is charged by turning the analog switch 7 ON. The High period of the clock is
As described above, it is ideally grounded, and a DC bias component applied by the emitter follower of the PNP transistor 6 is generated. Therefore, by setting the capacity of the capacitor 8 to a small capacity and setting a time constant that allows sufficient charging in one half cycle of one clock, the voltage across the capacitor 8 is the same as the DC bias at the point C. Generate voltage.

Therefore, the relationship between the DC bias at the point C and the voltage E at the point D required in the above (Formula 3) can be automatically established, and the occurrence of an offset voltage can be prevented.

Therefore, as described above,

[0039]

(Equation 5)

SIG. OUT can be obtained. The above shows the case where the image sensor output is ideally obtained on the basis of 0 V. However, in the case of a contact type image sensor, the width of the read original and the length of the image sensor have a 1: 1 relationship, Become As a result, a potential difference occurs in the output voltage of each pixel due to a variation in the resistance value of the ground pattern of the printed wiring board between the image sensor chips. The actual waveform of the point B voltage in FIG. 1 is shown in the point B voltage waveform of FIG. A dotted line portion of the voltage waveform at point B indicates a bright output (white original reading output), and a solid line indicates a dark output.

Since the potential difference of the output voltage of the image sensor element 1 as described above affects the gradation resolution of a facsimile or the like, it is desirable that the potential difference be as small as possible. As described in the related art, when the voltage at the point D is fixed at the DC voltage, when the above-described potential difference occurs, Vin expressed by (Equation 4) becomes (Vin ± △ Vin), and ± As for the ΔVin component, the amplification degree of the non-inverting amplifier directly affects, and the potential difference serving as the noise component is amplified in the same manner as the signal component, and the S / N
The ratio cannot be improved.

However, according to the first embodiment, the B voltage changes following the fluctuation of the C point voltage for each pixel.
(Equation 4) is

[0043]

(Equation 6)

Therefore, the amplification degree of the non-inverting amplifier 9 does not affect the potential difference of each pixel, which is the noise component, but only the change of the DC component.

If the peak-to-peak potential difference of each pixel is 10 mVpp and the bright output is 100 mV,
Assuming that the amplification degree is 10 times in the conventional technology, a bright signal output of 1000 mV is obtained for a noise of 100 mVpp.
N = 20 db, but according to the first embodiment, 10 m
A bright signal output of 1000 mV with respect to Vpp noise is obtained, and S / N = 40 db can be achieved.

The first embodiment shows an example in which the output of the image sensor is output on the basis of 0 V by the capacitor 3 and the analog switch 2. However, even when the output from the image sensor is output based on an arbitrary reference voltage. It is easy to see that a similar effect is obtained.

[0047]

As described above, according to the present invention, the saturation of the signal output and the deterioration of the linearity due to the generation of the offset voltage can be prevented, and the output of each pixel due to the variation of the resistance value of the earth pattern of the printed wiring board can be prevented. It is possible to realize an output circuit of an image reading apparatus with an improved potential difference and a good S / N ratio.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an output circuit of an image reading device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing output voltage waveforms at points A to E in the circuit diagram of FIG.

FIG. 3 is a circuit diagram of an output circuit when the conventional image reading apparatus is driven by dual power supplies.

FIG. 4 is a circuit diagram of an output circuit when a conventional image reading apparatus is driven by a single power supply.

FIG. 5 is a diagram showing voltage waveforms at points A to E in FIG. 1 for describing improvement of a potential difference in signal output of each pixel.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Image sensor element 2 Analog switch 3 Capacitor 4 Divider resistor 5 Divider resistor 6 PNP transistor 7 Analog switch 8 Capacitor 9 Non-inverting amplifier 10 Impedance converter 11 Feedback resistor 12 Feedback resistor

Claims (1)

[Claims] A first capacitor and a first electrical switch are connected in parallel to an output terminal of a photoelectric conversion element for converting light reflected from a document into an electric signal, and an output signal terminal of the photoelectric conversion element is connected to the first capacitor. An output circuit of an image reading device connected to a DC level shift circuit or a first impedance converter and using the output of the photoelectric conversion element as an input voltage of the DC level shift circuit or the first impedance converter, wherein A second electrical switch element and a second capacitor that are switched by a signal are connected in series, and the second electrical switch element and the second capacitor are connected to the DC level shift circuit or the first impedance converter. It is connected between an output terminal and a reference potential, and a voltage across the second capacitor is applied to a second impedance converter to obtain an operating voltage of the amplifier circuit. The output circuit of the image reading apparatus.