JPH09232891A - Electric charge detection circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electric charge detection circuit with high sensitivity. SOLUTION: First capacity C1 storing signal electric charge is connected with the plus terminal of a first differential amplifier 1 and the minus terminal of a second differential amplifier 2 via first switching element S1, the output terminal of the first differential amplifier 1 and the plus terminal of the first differential amplifier are connected by a first resistor R1, the output terminal of the first differential amplifier 1 and the minus terminal of the first differential amplifier 1 by a second resistor R2, and the minus terminal of the first differential amplifier 1 and constant potential are connected by the second capacitor C2. The plus terminal of the second differential amplifier 2 is kept at a constant potential, the output terminal of the second differential amplifier 2 and the minus terminal of the second differential amplifier 2 are connected by the third capacitor C3, and the output terminal of the second differential amplifier 2 and the minus terminal of the second differential amplifier 2 are connected by a third resistor R3 or a second switching element.

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 filing 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 lines with high accuracy. 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. 5 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 the S1 in the ON state, and the following expression is applied to the output terminal of the differential amplifier 3 (usually an operational amplifier is used). The voltage Vout represented by (1)
Occurs. After that, the voltage at the output terminal decreases at the time constant t1 represented by the equation (2).

[0005]

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

[0006]

However, due to the output capacitance and wiring capacitance of the switching element (S1), a large capacitance (C
4) is present, there are problems that the rising of the output waveform is delayed, the sensitivity to signal charges is lowered, the output waveform oscillates, and the like.

[0007]

DISCLOSURE OF THE INVENTION As a result of intensive studies on the above problems, the inventors of the present invention have determined that the charge detection circuit has a specific configuration to eliminate the influence of the capacitance C4 and increase the sensitivity to signal charges. The present invention has been completed by finding that the N ratio can be improved.

That is, according to the first aspect of the present invention, the first capacitance for accumulating the signal charge is connected to the plus terminal of the first differential amplifier and the second differential amplifier via the first switching element. Connected to the negative terminal of the first differential amplifier and the positive terminal of the first differential amplifier with a first resistor, and the output terminal of the first differential amplifier and the first difference The negative terminal of the dynamic amplifier is connected with a second resistor, the negative terminal of the first differential amplifier is connected with a constant potential with a second capacitor, and the positive terminal of the second differential amplifier is kept at a constant potential. Keep
The output terminal of the second differential amplifier and the negative terminal of the second differential amplifier are connected by the third capacitor, and the output terminal of the second differential amplifier and the negative terminal of the second differential amplifier are connected to the third terminal. Is a charge detection circuit characterized by being connected by the resistor or the second switching element.

According to a second aspect of the present invention, the first capacitance for accumulating signal charges is connected to the positive terminal of the first differential amplifier and the second differential amplifier via the first switching element. Connected to the negative terminal, the output terminal of the first differential amplifier and the positive terminal of the first differential amplifier are connected by the second capacitor, and the output terminal of the first differential amplifier and the first differential The negative terminal of the amplifier is connected with the first resistance, the negative terminal of the first differential amplifier is connected with the constant potential with the second resistance, and the positive terminal of the second differential amplifier is kept at the constant potential. ,
The output terminal of the second differential amplifier and the negative terminal of the second differential amplifier are connected by the third capacitor, and the output terminal of the second differential amplifier and the negative terminal of the second differential amplifier are connected to the third terminal. Is a charge detection circuit characterized by being connected by the resistor or the second switching element.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION In a charge detection circuit of the present invention, a second differential amplifier, a third capacitor, a third resistor or a current-voltage conversion circuit composed of a second switching element is connected in parallel to a current-voltage conversion circuit. Since a circuit composed of one differential amplifier, a first resistor, a second resistor, and a second capacitor is connected, the current-voltage of the fourth capacitor, which is the output capacitance of the first switching element. The influence on the conversion circuit can be eliminated.

[0011]

【Example】

(Embodiment 1) Embodiment 1 of the charge detection circuit of the present invention is shown in FIG. In FIG. 1, S1 is LC7938C (Sanyo Electric Co., Ltd.)
LM6365N (manufactured by National Semiconductor Co.) was used for 1 switch and differential amplifiers 1 and 2 in the image sensor driver manufactured by National Instruments. R1 is 18Ω resistance, R2 is 10
Ω resistance, R3 is 82 KΩ resistance, C1 is input capacitance of switch S1, C2 is 330 pF capacitor, C3 is 2 pF capacitor, C4 is about 360 pF (LC7938C output capacitance + wiring capacitance + 330 pF capacitor: Assuming an actual image sensor drive circuit) And a 330 pF capacitor was added), the current value was 5 × 10 ^-9 amps.

While the switching element S1 is in the OFF state,
The signal charge is stored in the capacitor C1. This charge Q flows into the charge detection circuit by turning on the switching element S1, and when the output terminal of the second differential amplifier 2 is not affected by C4, the voltage Vout theoretically expressed by the equation (1) is obtained.
Occurs. After that, the voltage at the output terminal of the differential amplifier 2 decreases at the time constant t1 represented by the equation (2).

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

[0014]

(R2 × C2) × 0.9 ≦ R1 × C4 ≦ (R2 × C2) × 2.2 (3)

The output waveform in the circuit of FIG. 1 is shown in FIG. In FIG. 6, the time when the switch S1 is turned on is 0 in FIG.
From (nsec) to 800 (nsec), this cycle (accumulation time) is about 1.6 (msec). The output waveform has a steep rise as shown by 4 in FIG. 6, and an output (peak value) of about 3 V was obtained, and the influence of C4 could be eliminated.

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 kept constant while S2 is in the OFF state, and is reset by turning S2 into the ON state. In the present invention, a circuit or the like for adjusting the offset may be added to each of the first and second differential amplifiers 1 and 2.

(Second Embodiment) FIG. 3 shows a second embodiment of the charge detection circuit of the present invention. In FIG. 3, R1 is a 10Ω resistor, R2 is a 14Ω resistor, C2 is a 330 pF capacitor, and others are the same as in the circuit of FIG. The circuit operation is the same as in the first embodiment.
Here, it is desirable that the values of R1, C4, R2, and C2 satisfy the following equation.

[0018]

(3) (R2 × C4) × 0.5 ≦ R1 × C2 ≦ (R2 × C4) × 1.1 (4)

The output waveform in the circuit of FIG. 3 is shown in FIG. In FIG. 7, the operation of the switch S1 is similar to that of the first embodiment. The output waveform had a steep rise as shown by 5 in FIG. 7, and an output (peak value) of about 3 V was obtained, and the influence of C4 could be eliminated. Further, as in the first embodiment, as shown in FIG. 4, instead of R3 in FIG.
2 may be used. Furthermore, the first and second differential amplifiers 1,
A circuit for adjusting the offset for each of the two may be added.

(Comparative Example) FIG. 5 shows an example of a charge detection circuit which has been conventionally used. In FIG. 5, the differential amplifier 3 is an LM
6365N was used. Others are the same as the circuit of FIG.
The circuit operation is the same as in the first embodiment. The output waveform in the circuit of FIG. 5 is shown in FIG. In FIG. 8, the operation of the switch S1 is similar to that of the first embodiment. As shown by 6 in FIG. 8, the output waveform has a slow rising due to the influence of C4 and is an output (peak value) of about 2.5V.

[0021]

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 diagram showing an example of a charge detection circuit according to the present invention according to claim 1;

FIG. 2 is a diagram showing an example of a charge detection circuit according to the present invention according to claim 1;

FIG. 3 is a diagram showing an example of a charge detection circuit according to the present invention as defined in claim 2;

FIG. 4 is a diagram showing an example of a charge detection circuit according to the present invention as defined in claim 2;

FIG. 5 is an example of a charge detection circuit that has been conventionally used.

FIG. 6 is an output waveform of the charge detection circuit according to the present invention.

FIG. 7 is an output waveform of the charge detection circuit according to the present invention.

FIG. 8 is an output waveform of a charge detection circuit used conventionally.

[Explanation of symbols]

 1 1st differential amplifier 2 2nd differential amplifier 3 Differential amplifier in a prior art example 4-6 Output waveform S1, S2 Switching element C1-C4 Capacitance R1-R3 Resistance

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Kiyoshi Matsuda 1000 Kamoshida-cho, Aoba-ku, Yokohama-shi, Kanagawa Mitsubishi Chemical Corporation Yokohama Research Institute

Claims (2)

[Claims] 1. A first capacitance for accumulating a signal charge is connected to a positive terminal of a first differential amplifier and a negative terminal of a second differential amplifier via a first switching element,
The output terminal of the first differential amplifier and the positive terminal of the first differential amplifier are connected by a first resistor, and the output terminal of the first differential amplifier and the negative terminal of the first differential amplifier are connected to each other. The second differential amplifier connects between the negative terminal of the first differential amplifier and the constant potential with the second capacitor, and the positive terminal of the second differential amplifier is kept at the constant potential. The output terminal of the amplifier and the negative terminal of the second differential amplifier are connected by a third capacitor, and 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 resistor. A charge detection circuit characterized by being connected by a switching element. 2. A first capacitance for accumulating signal charges is connected to a positive terminal of a first differential amplifier and a negative terminal of a second differential amplifier via a first switching element,
The output terminal of the first differential amplifier and the positive terminal of the first differential amplifier are connected by the second capacitor, and the output terminal of the first differential amplifier and the negative terminal of the first differential amplifier are connected to each other. Connect with one resistance, connect between the negative terminal of the first differential amplifier and a constant potential with a second resistance, keep the positive terminal of the second differential amplifier at a constant potential, The output terminal of the amplifier and the negative terminal of the second differential amplifier are connected by a third capacitor, and 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 resistor. A charge detection circuit characterized by being connected by a switching element.