Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2001-03-29
2004-09-28
Epps, Georgia (Department: 2873)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S2140AG, C348S300000
Reexamination Certificate
active
06797932
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a charge amount detection circuit for use in an image sensor such as an X-ray sensor such as using and relates to a two-dimensional image sensor using such a charge amount detection circuit.
BACKGROUND OF THE INVENTION
First, the following description deals with an image sensor having a general two-dimensional matrix structure with reference to
FIGS. 1 through 9
that are diagrams for explaining the present invention and FIG.
13
.
The image sensor can be used in an X-ray diagnosis apparatus when it is functioned as an X-ray sensor for detecting X-rays, for example.
In an image sensor
48
shown in
FIG. 1
, is provided with a photoelectric conversion layer
54
and a bias electrode
52
on a glass substrate
50
. The photoelectric conversion layer
54
is formed by a thin film made of amorphous selenium or other materials. The bias electrode
52
is formed by a metal film that transmits the X-rays, for example a conductive layer such as gold. On the surface of the photoelectric conversion layer
54
side of the glass substrate
50
, pixel electrodes
56
that are provided in a matrix manner, a storage capacitor (pixel capacitance)
17
, switching devices
18
, scanning lines
10
(column), and data lines
12
(row). The scanning lines
10
and the data lines
12
are connected with a scanning driver (gate driver)
14
and a reading circuit
16
, respectively.
Thus, the image sensor
48
is mainly composed of a photoelectric conversion layer
54
and an accumulation capacitor
17
, i.e., is composed of (a) a photoelectric section for converting photons such as X-rays into charges and storing the charges and (b) a reading circuit (charge amount detection circuit)
16
for reading a signal relating to the stored charges from the photoelectric section.
The pixel electrode
56
is connected with the data line
12
through the switching device
18
. The switching operation of the switching device
18
is carried out in response to a voltage sent by the scanning driver
14
through the scanning line
10
. In the case of a thin film transistor (hereinafter referred to as TFT) that is generally used as the switching device
18
, a source of the TFT is connected with the pixel electrode
56
, a drain of the TFT is connected with the data line
12
, and a gate of the TFT is connected with the scanning line
10
. In the following description, it is assumed that the TFT is used as the switching device
18
.
FIG. 2
is a cross-sectional view taken along line A—A in FIG.
1
. An auxiliary electrode
60
is provided so as to face the pixel electrode
56
through an insulating film
58
. The storage capacitor
17
is formed by the pixel electrode
56
, the auxiliary electrode
60
, and the insulating film
58
provided therebetween. The auxiliary electrode
60
is wired so that a common reference voltage (Vref) is applied to all pixel electrodes
22
. The bias electrode
52
can apply a high voltage (for example, several thousands of voltages) to the pixel electrode
56
.
When X-ray photons
68
are incident on the image sensor
48
from the bias electrode
52
side, the X-ray photons
68
that have transmitted the bias electrode
52
generates electron-hole pairs in the photoelectric conversion layer
54
. In the case where a positive voltage is applied to the bias electrode
52
side, the holes move toward the pixel electrode
56
so as to arrive at the pixel electrode
56
located in a position corresponding to the position on which the photon
68
are incident. In the case where a negative voltage is applied to the bias electrode
52
side, the electrons move toward the pixel electrode
56
so as to arrive at the pixel electrode
56
located in a position corresponding to the position on which the photon
68
are incident. The holes or electrons that have arrived at the pixel electrode
56
are stored by the capacitor
17
. The charges having positive polarity or negative polarity that have been stored by the capacitor
17
(hereinafter referred to as signal charges) are outputted to the data line
12
in response to the switching-on of the switching device
18
of TFT, and the charged amount (signal charge amount) is read out by the reading circuit
16
that is connected with the data lines
12
.
When the scanning driver
14
outputs a voltage of a high level to a target scanning line
10
, all the TFTs connecting with the scanning line
10
turn on. The signal charges stored by each capacitor
17
flows out to the corresponding data line
12
. The scanning driver
14
consecutively outputs a voltage of a high level to the respective scanning lines
10
, thereby resulting in that the data of all the pixel electrodes
56
are read out. Thus, the image data of one page are read out.
The following description deals with the reading circuit
16
which is used in the image sensor
48
.
FIG. 3
is a circuit diagram showing a basic structure of a charge sensitive amplifier (hereinafter referred to as CSA)
20
used for reading out the charge amount. In an operational amplifier
20
a
, an inverted input terminal and an output terminal are connected with each other through a feedback capacitor
20
b
so as to form a negative feedback circuit. A reset switch
20
c
is connected in parallel with the feedback capacitor
20
b
so that the resetting is carried out by discharging the charges stored in the feedback capacitor
20
b
. The data line
12
is connected with the inverted input terminal of the operational amplifier
20
a
, and a non-inverted input terminal is connected with a reference voltage GND.
FIG. 4
is an equivalent circuit for reading one pixel
22
including the switching devices
18
and the capacitor
17
.
FIG. 5
is a graph showing the timing for the reading operations and the output voltage of the CSA
20
in FIG.
4
.
In
FIG. 4
, it is assumed that the pixel
22
indicates a pixel connected with a scanning line
10
i
and a data line
12
j
. The scanning line
10
i
corresponds to a scanning line
10
of the i-th column and the data line
12
j
corresponds to a data line
12
of the j-th row. Note that Cdl indicates a capacitance of the data line
12
j
. In
FIG. 5
, G(i) indicates a voltage outputted to the scanning line
10
i
, and Rst indicates a reset signal outputted to the reset switch
20
c.
According to the reading operation, first, the reset switch
20
c
turns on (period A). This causes the charges that have been stored in the feedback capacitor
20
b
in the previous operation to be discharged so as to carry out the resetting. As a result, the output voltage of the CSA
20
reduces to the reference voltage GND, i.e., zero. Then, Rst becomes a voltage of a low level (period D), a voltage of a high level is outputted to G(i) so that the switching device
18
of TFT turns on. The signal charge (−Q) stored in the capacitor
17
flows out to the data line
12
j
. The operational amplifier
20
a
operates so that all the signal charge (−Q) that have flowed out to the data line
12
j
are collected to an electrode of the input side of the feedback capacitor
20
b
. Thus, the same amounts of charge (+Q) having negative polarity are come out on an electrode of the output side of the feedback capacitor
20
b
. Finally, the CSA
20
outputs a voltage obtained by dividing the charge Q that corresponds to the signal charge by the capacitance of the feedback capacitor
20
b
(period B). By reading such a voltage, it is possible to detect the signal charge as a voltage. After a little while is passed since a voltage of a low level is outputted to G(i) of this column (period C), the Rst is reset again for another reading operation of the next column, thereby resulting in that the output voltage of the CSA
20
returns to the reference voltage GND.
The following description briefly deals with a voltage reading method that is so called as a correlated double sampling (hereinafter referred to as CDS). If the circuit system shown in
FIG. 4
is perfect, the voltage that has been read during the period C must co
Okada Hisao
Takahashi Masayuki
Conlin David G.
Epps Georgia
Harrington Alicia
Manus Peter J.
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