Coded data generation or conversion – Analog to or from digital conversion – Analog to digital conversion
Reexamination Certificate
2002-08-29
2004-03-09
Tokar, Michael (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Analog to digital conversion
C341S122000, C341S143000, C378S098700, C378S098800
Reexamination Certificate
active
06703959
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a signal detecting method and a signal detecting device using an integrating amplifier for detecting charge signals by correlated double sampling.
2. Description of the Related Art
Conventionally, image information retrieving apparatuses, which retrieve image information by use of photoelectric converter elements such as CCD's or photomultipliers or solid state image detectors, have been utilized in various fields.
Particularly in the field of medical science, there has been disclosed a solid state radiation image detector capable of recording radiation image information on an electric accumulator as an electrostatic latent image by means of accumulating electric charges in an amount corresponding to a dosage of X-rays irradiated with a radiation image capturing apparatus or the like as electric charges for a latent image, and capable of retrieving the radiation image information by scanning with a laser beam or a line light source as retrieving light.
Moreover, in the above-mentioned image information retrieving apparatus, radiation image capturing apparatus and the like, integrating amplifiers are generally used for detecting retrieved image signals because the integrating amplifiers are processible into integrated circuits and generate a relatively small amount of noise. Such an integrating amplifier is designed to initiate accumulation of electric charges when switched to an accumulator mode, to discharge the accumulated electric charges when switched to a reset mode and thereby to output electric signals corresponding to the amount of the electric charges.
Here, immediately after the integrating amplifier is switched to the accumulator mode, an offset called a charge feedthrough attributable to capacitance of a switch inside the integrating amplifier is outputted as shown in FIG.
8
. The charge feedthrough is not always constant but is fluctuant; therefore, it is not possible to obtain accurate signal components corresponding to the image information if processing is applied that subtracts an uniform offset. In this context, processing called correlated double sampling is applied in order to eliminate the influence of charge feedthrough. Correlated double sampling refers to processing, which can eliminate the influence of the charge feedthrough by means of measuring a difference between an electric signal to be outputted immediately after the integrating amplifier is switched to the accumulator mode and an electric signal to be outputted immediately before the integrating amplifier is switched to the reset mode, and by defining the difference as a signal component.
Now, there are two methods of correlated double sampling, namely, analog correlated double sampling (ACDS) and digital correlated double sampling (DCDS).
The analog correlated double sampling uses a readout circuit as illustrated in FIG.
9
and is operated under control corresponding to a timing chart as shown in FIG.
10
. The readout circuit includes an integrating amplifier
1
, first signal retaining means
2
for retaining an electric signal to be outputted immediately after switching to an accumulator mode, second signal retaining means
3
for retaining an electric signal to be outputted immediately before switching to a reset mode, a differentiator circuit
4
for finding a difference between the electric signals retained in the first signal retaining means
2
and in the second signal retaining means
3
and for outputting the difference as a signal component, and an A/D converter
5
for converting the signal component into a digital signal.
As shown in
FIG. 10
, a first electric signal to be outputted immediately after the integrating amplifier
1
is switched to the accumulator mode is retained by the first signal retaining means
2
first, and after passage of a sufficient time period for accumulating electric charges in the integrating amplifier
1
, a second electric signal to be outputted immediately before the integrating amplifier
1
is switched to the reset mode is retained by the second signal retaining means
3
. Then, the difference between the first electric signal and the second electric signal is determined as the signal component with the differentiator circuit
4
and the signal component is converted and outputted by the A/D converter
5
. Note that the timing chart illustrated in
FIG. 10
shows the control timing for detecting signals for each pixel.
Meanwhile, the digital correlated double sampling uses a readout circuit as illustrated in FIG.
11
and is operated under control corresponding to a timing chart as shown in FIG.
12
. The readout circuit includes an integrating amplifier
6
, signal retaining means
7
for retaining a first electric signal to be outputted immediately after switching to an accumulator mode and a second electric signal to be outputted immediately before switching to a reset mode, and an A/D converter
8
for converting the signals retained by the signal retaining means
7
into digital signals.
As shown in
FIG. 12
, the first electric signal to be outputted immediately after the integrating amplifier
6
is switched to the accumulator mode is retained by the signal retaining means
7
first, and the retained first electric signal is outputted by the A/D converter
8
as a first digital signal. Then, after passage of a sufficient time period for accumulating electric charges, the second electric signal to be outputted immediately before the integrating amplifier
1
is switched to the reset mode is retained by the signal retaining means
7
. Thereafter, the retained second electric signal is converted into a second digital signal and outputted by the A/D converter
8
. Note that a difference between the first digital signal and the second digital signal is computed by software loaded on an image processing apparatus or the like to be connected subsequent to the A/D converter
8
.
However, the analog correlated double sampling has the following problem. Specifically, when radiation image information is read out of the radiation image detector while accumulating the electric charge one-on-one for each pixel in the integrating amplifier, for example, the first signal retaining means
2
and the second signal retaining means
3
must retain the first electric signal and the second electric signal during digitalization of the signal component by the A/D converter
5
. Accordingly, accumulation of an electric charge corresponding to a subsequent pixel cannot be initiated before A/D conversion of a first pixel is completed. Therefore, a long processing time period is required for obtaining a digital signal for one pixel, because the required time period combines a time period for accumulating the electric charge and a time period for A/D conversion.
Similarly, the digital correlated double sampling has the following problem. When the radiation image information is read out of the radiation image detector while accumulating the electric charge one-on-one for each pixel in the integrating amplifier, for example, the signal retaining means
7
must retain the second electric signal until digitalization of the second electric signal is completed. Accordingly, accumulation of an electric charge corresponding to a subsequent pixel cannot be initiated beforehand. Therefore, a long processing time period is required for obtaining a digital signal for one pixel as similar to the above-described case with the analog correlated double sampling, as the required time period combines a time period for accumulating the electric charge and a time period for A/D conversion.
Moreover, in the case of retrieving the radiation image by scanning the retrieving light as previously described, the time period for accumulating the electric charge needs to be prolonged because speed of response to the retrieving light is slow. Nevertheless, such processing time is preferred to be as short as possible.
Furthermore, in the case of retrieving the radiation image by scanning the retrieving ligh
Fuji Photo Film Co. , Ltd.
Nguyen Linh V
Tokar Michael
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