JPH09247353A - Electric charge detection circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a highly sensitive detection circuit without being affected by the output capacity and the wiring capacity of a switching element by connecting all the positive terminals of operation amplifiers to the same potential and all the negative terminals and output terminals in parallel with a resistance and a capacity. SOLUTION: A capacity C1 accumulating a signal electric charge is connected with all of one-side terminals of plural operation amplifiers through a switching element S1. All the positive terminals of the plural operation amplifiers are kept to substancially the same fixed potential, and all the output terminals and all the negative terminals of the plural operation amplifiers are connected by a capacitor C2. In addition all the output terminals and all one-side terminals of the plural operation amplifiers are connected to a resistance R2. In addition in stead of the resistance R2, a switching element can be used and the number of the operation amplifier is desirably between two to five.

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 of cadmium sulfide or amorphous silicon can have a comparatively large area, and a line sensor of an equal magnification contact type has been partially put into practical use in combination with a rod lens array.

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. 4 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 the voltage Vout1 is output to the output terminal of the operational amplifier.
Occurs. After that, the voltage at the output terminal decreases with a time constant t1.

[0005]

## EQU1 ## Vout1 = Q / C2 (1) t1 = R2 × C2 (2)

[0006]

However, depending on the output capacitance and wiring capacitance of the switching element (S1), a third capacitance (C
3) is present, there are problems that the rising of the output waveform is delayed, the sensitivity to signal charges is lowered, the output waveform is oscillated, and the noise output is increased.

[0007]

DISCLOSURE OF THE INVENTION As a result of intensive studies on the above-mentioned problems, the present inventors have found that the noise output can be reduced and the S / N ratio can be improved by configuring the charge detection circuit as follows. Then, the present invention has been completed. That is, the gist of the present invention is to connect the first capacitance for accumulating signal charges to all the negative terminals of a plurality of operational amplifiers via the first switching element, and to substantially connect all the positive terminals of the plurality of operational amplifiers. Keep the same constant potential
All output terminals of the plurality of operational amplifiers and all negative terminals of the plurality of operational amplifiers are connected by a second capacitor, and all output terminals of the plurality of operational amplifiers and all negative terminals of the plurality of operational amplifiers are connected to a second The charge detection circuit is characterized by being connected by the resistor or the second switching element. Further, it is desirable that the number of the operational amplifiers is between 2 and 5.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION In the charge detection circuit of the present invention, since the third capacitance is connected in parallel to a plurality of operational amplifiers, the noise generated at the output due to the influence of the capacitances decreases as the number of operational amplifiers increases. To do. On the other hand, since the signal charge is converted into a voltage by the second capacitance C2, the output does not decrease even if the number of operational amplifiers increases. Therefore, the S / N ratio of the output can be improved by increasing the number of operational amplifiers.

[0009]

FIG. 1 shows an embodiment in which three operational amplifiers are connected to the charge detection circuit of the present invention. However, C3 = 4
70 pF, R2 = 82 kΩ, C2 = 2 pF, C1 = 10
pF, and the operational amplifier is LM6365 (Nation
Al Semiconductor, and LC7938C (Sanyo Electric Co., Ltd.) was used as the switching element. The spectral distribution of the noise output is shown in FIG. It can be seen that the noise is reduced as compared with the charge detection circuit used conventionally.

Further, as shown in FIG. 2, a switching element S2 may be used instead of the second resistor R2 in FIG. In this case, the voltage at the output terminal of the operational amplifier 2 is maintained at Vout1 while S2 is in the OFF state, and is reset by turning S2 into the ON state. Further, a circuit for adjusting the offset may be added to each operational amplifier.

[0011]

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 circuit diagram of an embodiment.

FIG. 2 is a circuit diagram of an example.

FIG. 3 is a spectral distribution of noise output of the example.

FIG. 4 shows a charge detection circuit used conventionally.

Claims (2)

[Claims] 1. A first capacitance for accumulating signal charges is connected to all negative terminals of a plurality of operational amplifiers via a first switching element, and all positive terminals of the plurality of operational amplifiers are substantially the same. A constant potential is maintained, all output terminals of the plurality of operational amplifiers and all negative terminals of the plurality of operational amplifiers are connected by a second capacitor, and all output terminals of the plurality of operational amplifiers and all of the plurality of operational amplifiers are connected. A charge detection circuit characterized in that the negative terminal is connected by a second resistor or a second switching element. 2. The charge detection circuit according to claim 1, wherein the number of operational amplifiers connected in the detection circuit is between 2 and 5.