JP2002051264A - Correlative double sampling circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a correlative double sampling circuit for solid-state image pickup element capable of attaining the reduction of production costs and miniaturization. SOLUTION: This correlative double sampling circuit is provided with a charge sensitive amplifier 50 for converting an electric charge signal inputted from a solid-state image pickup element to a voltage, a low-pass filter(LPF)/ amplifier part 11 for removing a noise from the voltage signal and amplifying it, a sample/hold(S/H) part 53 for sampling the amplified voltage signal, an A/D converter 54 for converting the sampled analog signal to a digital signal, and a latch 55. Plural capacitors 5 and 12 are arranged in parallel on the input terminal side of an operational amplifier 8 in the LPF/amplifier part 11 and a connection between the capacitors is changed over by switches 13 and 14. The capacitors 5 and 12 to be changed over by the switches variably determine the time constant of the LPF part and the gain of the amplifier part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a correlated double sampling circuit used for reading out a signal from a solid-state imaging device using a solid-state imaging device including an X-ray image sensor.
The present invention relates to a correlated double sampling circuit capable of reducing the number of components and changing the gain of an amplifier.

[0002]

2. Description of the Related Art Generally, low-frequency noise is superimposed on a signal output from a solid-state imaging device using a solid-state imaging device, which adversely affects image quality. A correlated double sampling circuit that reduces noise by the correlated double sampling method is used.
Conventionally, a correlated double sampling circuit for a CCD, which is a typical example of a solid-state imaging device, includes, for example, Japanese Patent Application Laid-Open No. 8-31.
There is one described in US Pat. On the other hand, in addition to normal noise, thermal noise of the data line in the X-ray image sensor is added to the output signal of the X-ray image sensor, which is one of the solid-state imaging devices. A low-pass filter must be inserted. Prior art relating to this is described in, for example, the document "Signal and Noise Analysi
s Using TransmissionLine Model for Larger Area Fla
t-Panel X-Ray Imaging Sensors ”SPIE Vol.3659.

FIG. 6 shows a block diagram of a conventional correlated double sampling circuit for reading a signal from an X-ray conversion element. The correlated double sampling circuit includes a charge-sensitive amplifier section 50, a low-pass filter section 51, an amplifier section 52, a sample hold section 53, an AD converter 54, and a latch 5
5 is comprised. The charge input from the X-ray conversion element according to the received X-ray dose is converted into a voltage by the charge-sensitive amplifier unit 50 in FIG. 6, noise is removed by the low-pass filter unit 51, and the voltage signal from which the noise is removed Is the amplifier 52
, And then sampled by the sample and hold unit 53 and held for a predetermined time. The held analog signal is converted into digital data by the AD converter 54, and the converted digital data is held by the latch 55.

FIG. 7 shows a specific circuit of the first half block in the correlated double sampling circuit of FIG. The operational amplifier 63 and the capacitor 62 and the reset switch 61 connected in parallel between the inverting input terminal and the output terminal of the operational amplifier 63 make the charge-sensitive amplifier unit 50 of FIG. 6 comprises an operational amplifier 70, a capacitor 67 connected to the inverting input terminal thereof, a capacitor 69 connected in parallel between the inverting input terminal and the output terminal, and a reset switch 68. Are respectively constituted. Between the low-pass filter unit 51 and the amplifier unit 52,
A buffer 66 is provided. The gain of the amplifier section 52 is
The gain is determined by the capacitance ratio of the capacitors 67 and 69. If the capacitances of both capacitors are the same, the gain becomes 1.

[0005]

In the X-ray image sensor panel, since a large number of X-ray conversion elements are arranged in a vertical and horizontal matrix, an output signal is converted into digital data. A double sampling circuit is also required according to the number of elements. In particular, a large-screen X-ray image sensor panel usually requires a resolution of 1000 × 1000 pixels or more, and thus requires a correlation double sampling circuit of 1000 or more. However, in the above-described conventional correlated double sampling circuit, the low-pass filter section 51 and the amplifier section 52 are separately formed, and the buffer 66 is interposed between the two. If 1000 double sampling circuits are used for a large-screen X-ray image sensor panel, there is a problem that manufacturing costs increase and the size of the apparatus increases.

When a moving image of a human body is taken with X-rays,
It is necessary to shorten the irradiation time in order to prevent a line disturbance, and if that is the case, the contrast of the displayed image is reduced. Therefore, it is necessary to increase the gain of the amplifier unit 52. However, in the above-described conventional correlated double sampling circuit, the gain cannot be increased because the capacitance of the capacitors 67 and 69 for adjusting the gain is fixed, so that a normal X-ray image sensor panel can be used for capturing a moving image of a human body. There is a problem that they cannot be shared.

Accordingly, an object of the present invention is to simplify the correlated double sampling circuit by integrating the low-pass filter section and the amplifier section, and to reduce the manufacturing cost by changing the capacitance of the capacitor in the amplifier section. It is an object of the present invention to provide a correlated double sampling circuit for a solid-state imaging device that can reduce the size of an apparatus and can also be used for capturing a moving image of a human body.

[0008]

In order to achieve the above object, according to the present invention, a low-pass filter section for removing noise from an input signal, an amplifier section for amplifying a signal passing through the low-pass filter section, In a correlated double sampling circuit including a sample and hold unit that samples a signal passed through an amplifier unit and holds the signal for a predetermined time and an AD converter that converts an output signal of the sample and hold unit into a digital signal, The capacitor for determining the constant also serves as a capacitor for determining the gain of the amplifier section.

In the correlated double sampling circuit according to the first aspect of the present invention, since one capacitor is also used to determine the time constant of the low-pass filter section and the gain of the amplifier section, the capacitance of this capacitor is changed to change the low-pass filter section. The frequency band of the noise to be removed by the filter unit can be changed, and the gain of the amplifier unit can be changed. Then, the input signal from the solid-state imaging device or the like is subjected to noise removal by a low-pass filter unit, then amplified by an amplifier unit, sampled and held at a predetermined rate by a sample hold unit, and the held signal is converted by an AD conversion unit. Is converted into a digital signal and output. Therefore, the correlated double sampling circuit that performs the same function as the conventional one can be simplified by also using the capacitor, so that the manufacturing cost can be reduced and the device can be downsized.

A correlated double sampling circuit according to claim 2 is
The amplifier section includes a plurality of capacitors arranged in parallel on an input terminal side, and a switch for switching connection between these capacitors. By switching the switches, the gain of the amplifier section is made variable, and the low-pass The time constant of the filter is made variable.

In the correlated double sampling circuit according to the second aspect, the connection between a plurality of capacitors arranged in parallel on the input terminal side of the amplifier section is switched by a switch. In addition, the frequency band of the noise to be removed by the low-pass filter unit and the gain of the amplifier unit can be changed only by switching the switch. When the input signal is a moving image of the human body from the X-ray conversion element, the switch is switched to a side where the gain is greater than when the subject is an object, and the X-ray irradiation dose to the human body is reduced. It is possible to obtain a display image having the same good contrast as that of the object while suppressing it.

A correlated double sampling circuit according to claim 3 is
The low-pass filter section and the amplifier section are integrated.

In the correlated double sampling circuit according to the third aspect, since the low-pass filter section and the amplifier section are integrated, the circuit can be further simplified as compared with the case where the capacitor is also used, the manufacturing cost is reduced, and the size of the apparatus is reduced. Can be further improved. Therefore, when used in an X-ray image sensor panel or the like that requires a large number of solid-state imaging devices requiring a correlated double sampling circuit, a remarkable effect of reduction in manufacturing cost and size of the device can be achieved.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the illustrated embodiments. FIG. 1 shows an example of a correlated double sampling circuit according to the present invention. This correlated double sampling circuit is different from the correlated double sampling circuit except that a low-pass filter section 2 and an amplifier section 3 are integrated into a low-pass filter / amplifier section 1. 7, the same members are denoted by the same reference numerals. The correlated double sampling circuit includes a charge-sensitive amplifier section 50 including an operational amplifier 63, a capacitor 62, and a reset switch 61; a low-pass filter / amplifier section 1;
It is composed of a D changing section 54 and a latch 55.

In the low-pass filter / amplifier 1, the capacitor 65 and the buffer 66 in FIG. 7 are omitted, and the capacitor 5 is also used as a capacitor for determining the time constant of the low-pass filter 2 and a capacitor for determining the gain of the amplifier 3. I have. That is, the time constant of the low-pass filter unit 2 including the resistor 4 and the capacitor 5 is
And the gain of the amplifier section 3 including the operational amplifier 8, the reset switch 6, and the capacitors 5 and 7 is determined by the capacitance ratio of the capacitors 5 and 7. It increases as the capacity of the capacitor 5 increases.

FIG. 2 is a timing chart showing the operation of the correlated double sampling circuit of FIG. 1. The upper part shows ON / OFF of the reset switch 61 of the charge-sensitive amplifier unit 50, and the middle part shows the reset switch 6 of the amplifier unit 3. , And the lower row shows the sampling period of the sample and hold section 53, respectively. First, in a period T1 in FIG.
Are turned on, the input and output of the operational amplifiers 63 and 8 are both short-circuited and reset. In the subsequent period T2, the reset switch 61 is turned off to start the operation of the operational amplifier 63 of the charge-sensitive amplifier unit 50, while the reset switch 6 is kept on and the reset state of the operational amplifier 8 of the amplifier unit 3 is changed. Continuously, the offset noise and the reset noise of the operational amplifier 63 are absorbed.

In the period T3, the reset switch 6 is turned off, the operation of the operational amplifier 8 is started, and input is made from each X-ray conversion element constituting the large-screen X-ray image sensor panel according to the received X-ray dose. The charge is converted into a voltage by the charge-sensitive amplifier unit 50. By taking this period T3 long enough, the reset noise of the operational amplifier 8 is absorbed. The voltage signal output from the charge-sensitive amplifier unit 50 includes thermal noise caused by the data line of the X-ray conversion element as described above. The voltage signal that has been removed by the filter unit 2 and from which noise has been removed is amplified by the amplifier unit 3 including the capacitors 5 and 7 and the operational amplifier 8. In the period T4, the voltage signal amplified by the amplifier unit 3 is sampled by the sample and hold unit 53,
It is held until the D conversion ends. The digital data for one element after the AD conversion is held in the latch 55.

As described above, in the above embodiment, unlike the conventional example, the capacitor 5 is used not only for determining the time constant of the low-pass filter section 2 and for determining the gain of the amplifier section 3 but also for the low-pass filter section 2. Since the amplifier unit 3 is integrated with the low-pass filter / amplifier unit 1, the same functions as those in the conventional example can be achieved, and the manufacturing cost of the correlated double sampling circuit can be reduced and the size of the apparatus can be further reduced. Therefore, when used for an X-ray image sensor panel or the like that requires a large number of correlated double sampling circuits because it has a large number of X-ray conversion elements, the effects of remarkable reduction in manufacturing cost and downsizing of the device can be achieved.

FIG. 3 shows another embodiment of the correlated double sampling circuit according to the present invention. In this correlated double sampling circuit, a capacitor 12 is arranged in parallel with the capacitor 5 of FIG.
Switches 13 and 14 are provided to switch the connection between the capacitors 5 in parallel or to connect the capacitor 12 between the output side of the capacitor 5 and the reference potential.
Since the configuration is the same as that of the embodiment of FIG. 1 except that a new low-pass filter / amplifier unit 11 is provided, the same members are denoted by the same reference numerals and description thereof will be omitted. And switch 1
By switching between 3, 14, the gain of the amplifier section and the time constant of the low-pass filter section can be changed simultaneously.

That is, the capacitances of the capacitor 12 and the capacitor 5 are set to the same value, the switch 13 is turned off, and the switch 14 is turned off.
Is turned on, the low-pass filter / amplifier unit 1 of FIG.
Therefore, the same gain as that of the low-pass filter / amplifier unit 1 shown in FIG. 1 can be obtained by the same operation. To increase the gain of the amplifier, switch 1
3 is turned on and the switch 14 is turned off. Then, the capacitors 5 and 12 are connected in parallel, and the gain of the amplifier section is determined by the ratio of the combined capacity of the capacitors 5 and 12 and the capacity of the capacitor 7, so that the gain is increased by the addition of the capacitor 12. At the same time, since the time constant of the low-pass filter is determined by the product of the combined capacitance of the resistor 4 and the capacitors 5 and 12, the time constant increases by the addition of the capacitor 12.
Due to the increase in the time constant of the low-pass filter unit and the gain of the amplifier unit, the effect of the low-pass filter / amplifier unit 11 for removing thermal noise caused by the data line of the X-ray conversion element also increases, and A sufficient noise removal effect can be exhibited even with a weak signal input.

That is, the correlated double sampling circuit of the embodiment of FIG. 3 can remove low frequency noise, reset noise, offset noise, and thermal noise caused by the data line in the input signal as in the embodiment of FIG. 1, the gain of the amplifier unit and the time constant of the low-pass filter, which are fixed in the embodiment of FIG. 1, can be varied. Therefore, this correlated double sampling circuit is incorporated in the X-ray image sensor panel,
When the level of the input signal is low or the noise is high, the switches 13 and 14 are switched from the normal gain side to the high gain side to maximize the capability of the X-ray image sensor panel which is one of the solid-state imaging devices. Can be demonstrated.

FIG. 4 is a block diagram of an X-ray imaging device as an example of a solid-state imaging device using the correlated double sampling circuit described in FIG. This X-ray imaging apparatus has 2880 vertical x 2880 horizontal X-ray conversion elements for converting incident X-rays into electric charges.
An X-ray image sensor panel 23 which is arranged in a matrix and a gate driver circuit 21 which specifies an address of an X-ray conversion element to be accessed via a gate line
And a signal conversion circuit 22 that extracts a charge signal from the accessed X-ray conversion element via a data line and converts the charge signal into a digital signal, and controls a gate driver circuit 21 and a signal conversion circuit 22 to supply a drive signal to the former. , The digital signal from the latter is taken into the built-in memory,
It is composed of a control circuit 24 which performs image processing such as variation processing and defect processing of the line conversion elements. A TFT (thin film transistor) for extracting generated charges is provided below the X-ray conversion elements arranged in a matrix as shown in the partial detailed view of FIG. On the other hand, the correlated double sampling circuits of FIG. 3 are provided in the signal conversion circuit 22 of FIG.

The signal processing in the X-ray imaging apparatus is performed as follows. That is, the gate driver circuit 21 selects only one of the 2880 gate lines, only two of which are representatively indicated by 38 and 39 in FIG. Therefore, if the gate line 38 is selected, the TFT
Are turned on, only the charges generated by the X-ray conversion element 34 are read out to the data line 40, and only the charges generated by the X-ray conversion element 36 are read out to the data line 41. The signals read to the data lines 40 and 41 are respectively sampled by the correlated double sampling circuit described in FIG. 3 provided for each data line in the signal conversion circuit 22 and converted into digital signals. The data is sent to the circuit 24 and stored in a built-in memory. The control circuit 24 performs the above-described image processing on the digital signal stored in the memory, outputs the processed image signal to a display device such as a CRT or a liquid crystal display device, and displays the X-ray on the screen of the display device.
A line image is displayed.

Since the above X-ray imaging apparatus is provided with a large-screen X-ray image sensor panel 23 having 2880 × 28880 X-ray conversion elements, 2880 correlated double sampling circuits are required in the signal conversion circuit 22. However, since the circuit shown in FIG. 3 is used as the correlated double sampling circuit, which can reduce the number of parts compared to the conventional example shown in FIG. Can be realized.

A further advantage of the X-ray imaging apparatus of FIG. 4 provided with the correlated double sampling circuit of FIG. 3 is that it can capture a moving image of a human body in which X-ray damage due to long-time irradiation becomes a problem. It takes a long time to irradiate a moving image compared to a still image of a human body, and it is necessary to reduce the amount of X-ray irradiation in order to avoid X-ray damage to the human body due to long-time irradiation. However, a decrease in the amount of X-ray irradiation naturally results in a decrease in the contrast of a display image, and the display image becomes unclear. Therefore, correlated double sampling for amplifying a voltage signal obtained from an output signal of the X-ray conversion element. It is essential to increase the gain of the circuit. Therefore, the switch 1 of the low-pass filter / amplifier unit 11 is provided by each correlated double sampling circuit (see FIG. 3) in the signal conversion circuit 22 of the X-ray imaging apparatus (see FIG. 4).
3 on, switch 14 off, capacitors 5, 12
Are connected in parallel, the time constant of the low-pass filter section is increased, and the gain of the amplifier section is increased. As a result, as described above with reference to FIG. 3, a large-level output signal without noise can be obtained from a small-level input signal including noise such as thermal noise. It is possible to reproduce a clear X-ray moving image of a human body.

In the case where the subject of the moving image is an article having no fear of X-ray obstruction, the switch 13 of the low-pass filter / amplifier section 11 is turned off and the switch 14 is turned on to reduce the time constant of the low-pass filter section. Imaging is performed with a normal X-ray irradiation amount by lowering the gain of the amplifier section. Thus, in the X-ray imaging apparatus having the correlated double sampling circuit shown in FIG. 3, when the level of the input signal is small or the noise is large, the switches 13 and 14 are switched from the low gain side to the high gain side, The ability of the X-ray imaging apparatus can be maximized.

The signal conversion circuit 22 of FIG. 4 including a large number of correlated double sampling circuits is used for actual manufacturing.
Since it is difficult to assemble and assemble individual components because the number of components is large and the accuracy is required, the entire signal conversion circuit 22 including the correlated double sampling circuit is implemented as an LSI. At this time, it is difficult to put 2880 input signals into one chip due to the restriction on the number of terminals of the package. Therefore, in this embodiment, 128 input lines are used as one chip. The one-chip LSI thus formed has a size of 14.6 mm × 7.9 mm using a 0.35 μm CMOS process, and can be economically manufactured by the current semiconductor manufacturing technology. As a result, in this embodiment, a remarkable cost reduction was realized as compared with the related art.

In the above embodiment, the case where the correlated double sampling circuit of the present invention is applied to an X-ray imaging apparatus has been described. However, the correlated double sampling circuit is applicable to other solid-state imaging devices such as a CCD (charge coupled device). It can also be applied to devices.

[0029]

As apparent from the above description, the low-pass filter section, the amplifier section, the sample-and-hold section, and the AD
The correlated double sampling circuit of the present invention with the converter
Since one capacitor is also used to determine the time constant of the low-pass filter unit and the gain of the amplifier unit, the correlated double sampling circuit that performs the same function as the conventional one is simplified by using the same capacitor, and the manufacturing cost is reduced. In addition, the size of the apparatus can be reduced.

The correlated double sampling circuit according to claim 2 is
Since the connection between a plurality of capacitors arranged in parallel on the input terminal side of the amplifier section is switched by a switch, the frequency band of the noise to be removed by the low-pass filter section only by switching the switch and the gain of the amplifier section When applied to an X-ray imaging device which is one of solid-state imaging devices in which X-ray conversion elements are arranged in a matrix, the switch is switched to a side where the gain is increased, and the While suppressing the X-ray irradiation dose, it is possible to obtain a moving image having the same good contrast as that obtained when the subject is an object.

The correlated double sampling circuit according to claim 3 is
Since the low-pass filter unit and the amplifier unit are integrated, the circuit can be further simplified as compared with the case where only the capacitor is used, and the manufacturing cost and the size of the device can be further reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of a correlated double sampling circuit of the present invention.

FIG. 2 is a timing chart showing the operation of the correlated double sampling circuit of FIG.

FIG. 3 is a diagram showing another embodiment of a correlated double sampling circuit according to the present invention.

FIG. 4 is a graph showing X using the correlated double sampling circuit shown in FIG. 3;
It is a block diagram of a line image device.

5 is a partial detailed view of an X-ray image sensor panel of the X-ray image apparatus of FIG.

FIG. 6 is a block diagram of a conventional correlated double sampling circuit.

FIG. 7 is a block diagram of a conventional correlated double sampling circuit including details of the first half block of FIG. 6;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1,11 Low-pass filter / amplifier part 2 Low-pass filter part 3,52 Amplifier part 6,61,68 Reset switch 8,63 Operational amplifier 13,14 Switch 21 Gate driver circuit 22 Signal conversion circuit 23 X-ray image sensor panel 24 Control circuit 30 TFT 34,35,36,37 X-ray conversion element 38,39 Gate line 40,41 Data line 50 Charge sensitive amplifier section 51 Low pass filter section 52 Amplifier section 53 Sample hold section 54 AD converter 55 Latch

 ──────────────────────────────────────────────────続 き Continued on front page (72) Inventor Shinichi Tanaka 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka F-term (reference) 5C024 AX11 CX06 CX41 HX05 HX13 HX17 HX23

Claims (3)

[Claims] A low-pass filter section for removing noise from an input signal; an amplifier section for amplifying a signal passing through the low-pass filter section; a sample-and-hold section for sampling a signal passing through the amplifier section and holding the signal for a predetermined time; In a correlated double sampling circuit having an AD converter for converting an output signal of the sample hold unit into a digital signal, a capacitor for determining a time constant of the low-pass filter unit also serves as a capacitor for determining a gain of the amplifier unit. Features a correlated double sampling circuit. 2. The correlated double sampling circuit according to claim 1, wherein the amplifier section includes a plurality of capacitors arranged in parallel on an input terminal side, and a switch for switching a connection between the capacitors. A correlated double sampling circuit wherein the gain of the amplifier section is made variable and the time constant of the low-pass filter is made variable by switching this switch. 3. The correlation double sampling circuit according to claim 1, wherein said low-pass filter section and said amplifier section are integrated.
Double sampling circuit.