JPH0961478A - Electric charge detection circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a highly sensitive electric charge detection circuit which is not effected by an output capacitance and a wiring capacitance of a switching element by using capacitances, the switching element, differential amplifiers and resistances in a specified configuration. SOLUTION: A signal electric charge is stored into a capacitance C1 while a switching element S1 is OFF. This electric charge Q flows to an electric charge detection circuit by turning ON the switching element S1 and a voltage of Vout2=(Q/C3)×(R1/R2) is generated at an output terminal of a differential amplifier 2. That is the voltage of Vout2 is proportional to the electric charge Q. The value of the resistance R1 is preferably larger than that of the resistance R2, hence allowing raising of the sensitivity of a circuit output. The electric charge Q is inputted into a current-voltage conversion circuit constituted of the differential amplifier 2, a capacitance C3 and a resistance R3 through a current-voltage conversion circuit constituted of a differential amplifier 1, the capacitances C1 and C2 and the resistances R1 and R2. Thereby effect of a wire capacitance C4 can be eliminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charge detection circuit for detecting signal charges.

[0002]

2. Description of the Related Art Optical sensors are widely used for measuring light intensity, as position sensors in robots and various automation systems, and for reading image information in information communication and information processing. In particular, with the recent advances in image information processing technology and capabilities, there is a strong demand for advances in image sensors as high-performance image information input devices.
Facsimile machines, word processors, electronic file systems, etc. are typical devices that require an image input device.

As such an input device, a device for taking out two-dimensional information such as a video camera and an image scanner for scanning and reading an image by using a line sensor can be considered, but usually a sufficient resolving power (number of pixels) is used. Image scanner using a line sensor is used to obtain a). As a line sensor, a charge coupled device (CCD) using crystalline silicon is typical, but there is a limit to the size of the device, and a reduction optical system is used to read an image of a large area. It is necessary to arrange a large number of high precision. On the other hand, a sensor having a photoconductive surface made of cadmium sulfide or amorphous silicon can have a relatively large area, and a unit-size contact type line sensor that also uses a rod lens array has been put into practical use.

In such an image reading apparatus, a method is generally used in which the output current of each pixel is accumulated in a capacitor for a fixed time and the accumulated charge is detected by a charge detection circuit. FIG. 3 shows an example of a charge detection circuit used conventionally. While the switching element S1 is in the OFF state, the signal charge Q accumulated in C1 flows into the detection circuit by turning S1 in the ON state, and is output to the output terminal of the differential amplifier 3 (usually an operational amplifier is used). The voltage V expressed by the following equation (1)
outl occurs. After that, the voltage at the output terminal decreases at the time constant t1 represented by the equation (2).

[0005]

## EQU1 ## Vout1 = Q / C3 (1) t1 = R3 × C3 (2)

[0006]

However, since the capacitance (C4) exists between the negative terminal of the differential amplifier and the constant potential due to the output capacitance and wiring capacitance of the switching element (S1), the output waveform rises slowly. However, there are problems such as a decrease in sensitivity to signal charges and oscillation of the output waveform.

[0007]

DISCLOSURE OF THE INVENTION As a result of intensive studies on the above-mentioned problems, the inventors of the present invention have eliminated the influence of the capacitance C4 to increase the sensitivity to signal charges by changing the S / The present invention has been completed by finding that the N ratio can be improved.

That is, the gist of the present invention is to connect the first capacitance for accumulating signal charges to the negative terminal of the first differential amplifier via the first switching element, The output terminal and the negative terminal of the first differential amplifier are connected by a first resistor, and the output terminal of the first differential amplifier and the positive terminal of the first differential amplifier are connected by a second resistor, The positive terminal of the first differential amplifier and the negative terminal of the second differential amplifier are connected to keep the positive terminal of the second differential amplifier at a substantially constant potential, and the negative terminal of the second differential amplifier is connected. The second capacitance is connected between the terminal and the constant potential by connecting the output terminal of the second differential amplifier and the negative terminal of the second differential amplifier by the third capacitance. Use the third resistor or the second switching element to connect the output terminal and the negative terminal of the second differential amplifier. Consists in the charge detecting circuit, characterized in that connection was.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION In the charge detection circuit of the present invention, the current flowing when the first switching element is turned on is supplied to the first differential amplifier, the fourth capacitance, the first resistance, and the second differential amplifier. A current-voltage conversion circuit composed of a second differential amplifier, a third capacitor, a third resistor or a second switching element via a current-current conversion circuit composed of a resistor and a second capacitor. Since it is input to, it is possible to eliminate the influence of the fourth capacitance on the current-voltage conversion circuit.

[0010]

FIG. 1 shows an embodiment of the charge detection circuit of the present invention. C4 is the output capacitance of the switching element S1. The signal charge is accumulated in the capacitor C1 while the switching element S1 is in the OFF state. This charge Q is a switching element S
When 1 is turned on, it flows to the charge detection circuit,
The voltage Vout2 is generated at the output terminal of the second differential amplifier 2.

[0011]

## EQU00002 ## Vout2 = (Q / C3) .times. (R1 / R2) (3)

Vout2 is proportional to Q. R1 to R
A value larger than 2 is preferable, which makes it possible to increase the sensitivity of the circuit output to Q. After this, the voltage at the output terminal of the differential amplifier 2 has a time constant t.
Decrease by 2.

[0013]

## EQU00003 ## t2 = R3.times.C3 (4)

Here, it is desirable that the values of R1, C4, R2 and C2 satisfy the following equation.

[0015]

(R2 × C2) × 0.8 ≦ R1 × C4 ≦ (R2 × C2) × 1.2 (5)

Further, as shown in FIG. 2, a switching element S2 may be used instead of R3 in FIG. In this case, the voltage at the output terminal of the differential amplifier 2 is maintained at Vout2 while S2 is in the OFF state, and is reset by turning S2 into the ON state. In the present invention, a circuit for adjusting the offset may be added to each of the first and second differential amplifiers 1 and 2.

[0017]

With the above-described structure of the present invention, it is possible to provide a highly sensitive charge detection circuit which is not affected by the output capacitance of the switching element or the wiring capacitance.

[Brief description of drawings]

FIG. 1 is a charge detection circuit according to the present invention.

FIG. 2 is a charge detection circuit according to the present invention.

FIG. 3 is an example of a charge detection circuit used conventionally.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 1st differential amplifier 2 2nd differential amplifier 3 Differential amplifier in a prior art example S1, S2 switching element C1-C4 capacitance R1-R3 resistance

Front page continuation (72) Inventor Kiyoshi Matsuda 1000 Kamoshida-cho, Aoba-ku, Yokohama-shi, Kanagawa Mitsubishi Chemical Corporation Yokohama Research Institute

Claims (3)

[Claims] 1. A first capacitance for accumulating signal charges is connected to a negative terminal of a first differential amplifier via a first switching element, and an output terminal of the first differential amplifier and a first terminal of the first differential amplifier are connected. The negative terminal of the differential amplifier is connected with the first resistor, the output terminal of the first differential amplifier and the positive terminal of the first differential amplifier are connected with the second resistor, and the first differential amplifier The positive terminal of the second differential amplifier is connected to the negative terminal of the second differential amplifier, and the positive terminal of the second differential amplifier is maintained at a substantially constant potential. Is connected with the second capacitor, and the output terminal of the second differential amplifier and the negative terminal of the second differential amplifier are connected with the third capacitor,
A charge detection circuit characterized in that the output terminal of the second differential amplifier and the negative terminal of the second differential amplifier are connected by a third resistor or a second switching element. 2. The product of the fourth resistance and the first resistance existing between the negative terminal of the first differential amplifier and the constant potential /
The charge detection circuit according to claim 1, wherein (the product of the second resistance and the second capacitance) is between 0.8 and 1.2. 3. The charge detection circuit according to claim 1, wherein the first resistor has a resistance value larger than that of the second resistor.