Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2001-05-29
2003-12-23
Pyo, Kevin (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C348S308000, C257S291000
Reexamination Certificate
active
06667468
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a solid state image sensor and, more particularly, to a black-level signal generation circuit employed in a CMOS (Complementary Metal-Oxide-Semiconductor) image sensor as a solid state image sensor.
2. Description of the Related Art
A CMOS image sensor can be fabricated by use of the CMOS process for logic LSIs. Therefore, the CMOS image sensor contains, on a single semiconductor chip, photo-sensor units or pixels and a signal processing circuit such as control logic, unlike a CCD-based image sensor. Furthermore, the CMOS image sensor has an advantage that it has lower noise than usual MOS-based image sensors, because each cell or pixel includes a photodetector and a charge-to-signal conversion circuit so that the conversion of the charge detected by the photodetector in response to light irradiated thereto into an electrical signal as well as the amplification of the signal thus converted are performed within each pixel.
This kind of image sensor is described, for example, by Sunetra K. Mendis et al., “A 128×128 CMOS Active Pixel Image Sensor for Highly Integrated Imaging System” (IEEE International Electron Devices Meeting 93, pp. 583-586) and by Roger A. Panicacci et al., “128 Mb/s Multiport CMOS Binary Active-Pixel Image Sensor” (1996 IEEE International Solid-State Circuits Conference pp. 100-101). Methods of reading out pixel information of an image sensor as described in these references are as follows.
In the Mendis et al. CMOS sensor, charge responsive to a light that has been illuminated to a photodetector for a given time is read out as an electrical signal onto a vertical signal line through a first source-follower consisting of an input transistor and a row-selection transistor. The potential of the vertical signal line is sampled on a capacitor by enabling a sample switch. Each potential at each capacitor is read out through second and third source-followers respectively, by enabling column-selection transistors attached to these source-followers. An image signal is created using these two kinds of voltage signals.
In the Panicacci et al. CMOS sensor, on the other hand, pixels are formed in the same way as the Mendis et al. pixels. However, the circuit of a readout portions has a signal detection capacitor whose one end is connected with a sample switch, the other end being connected with the gate of the input transistor of an output side source-follower. The photodiode is exposed to a light for a given time. Then, the voltage across this photodiode is read out into a column signal line through an input side source-follower. This voltage is sampled in the signal detecting capacitor by enabling the sample switch and is read out through the output side source-follower. Then, this voltage is compared with an externally applied threshold voltage. Required pixel information is obtained using this thresholded voltage.
Generally, a signal processing circuit of a image sensor needs a black-level signal forming a reference potential. Usual method of forming and way of using such a black-level signal are next described by referring to
FIG. 1
, which is a block diagram of the conventional MOS-based image sensor, in which a source-follower input transistor is included within each pixel. In
FIG. 1
, pixels are arranged in a plurality of rows and a plurality of columns to form an effective pixel array
111
. A vertical scanning circuit
113
selects each of a plurality of rows in the effective pixel array
111
successively. And, all pixels in selected row are reset at a time or read out into vertical signal lines at a time. Both of the resetting operation and the operation for reading out into vertical signal lines are under control of a vertical scanning circuit
113
.
Output signals from the selected row of pixels are stored in a readout circuit
115
, until they are delivered sequentially. This operation for delivering the signals from the readout circuit
115
sequentially is controlled by a horizontal scanning circuit
114
.
The output signal from the readout circuit
115
has an offset component corresponding to a black level, which represents a state in which there is no incident light. The amount of this offset component differs among different circuit configurations of pixels and readout circuit.
Therefore, the offset component corresponding to a black level is subtracted from each signal of pixels in the effective pixel array
111
in the manner described below. Referring again to
FIG. 1
, a light-shielded pixel array
112
covered with a light-shielding film to prevent incidence of light is arranged around the effective pixel array
111
. Shielded pixels are read out in the same way as effective pixels. Output signals from the shielded pixels are passed through the readout circuit
115
, then separated from the effective pixel signals, and then stored in an analog storage circuit
116
. In a signal level adjusting circuit
117
, an offset component stored in the analog storage circuit
116
is subtracted from the effective pixel signal.
Then, the output signal from this signal level adjusting circuit is applied to an A/D converter (not shown). At this time, an additional offset voltage may be added such that the black level of the output signal from the signal level adjusting circuit matches the level of the lower-voltage side of the input voltage range of the A/D converter. Also, in this case, the black-level signal held in the analog storage circuit
116
is utilized.
In some sensors, a black-level signal generation circuit may be disposed apart from the pixel array.
FIG. 2
is a block diagram schematically showing such image sensor accompanied with a black-level signal generation circuit, which is disclosed in Japanese Patent Publication Hei 5-23549. In this image sensor, a second photo-sensor cell
204
coated with a light-shielding film is provided besides photo-sensor cells constructing a photo-sensor array
201
. A black-level signal is read out from the photo-sensor cell
204
, being synchronized to the read-out timing signal of the photo-sensor array
201
, and stored in a storage circuit
205
. Then, a light information signal is obtained by subtracting the black-level signal stored in the storage circuit
205
from the output signal read out sequentially from the photo-sensor array
201
using a differential amplifier
202
.
In the conventional MOS-based image sensors according to the prior arts described above, it is necessary that the black-level signal and the effective pixel signal should be separated from each other and that the black-level signal should be stored in the analog storage circuit. Therefore, a timing signal generating circuit for producing a timing pulse for separating the black-level signal from the effective pixel signal and the analog storage circuit are necessitated.
FIG. 3
is a block diagram schematically showing an image sensor creating a black-level signal by an electric circuit, which is disclosed in the above Japanese patent publication. A light information signal is obtained by subtracting a black-level signal formed using a constant current source
203
constructed from a constant voltage source and a variable resistor from an output signal read out from a photo-sensor array
201
using a differential amplifier
202
. In the method, however, after the chip of the image sensor is completed or after the chip is incorporated into an imager, it is necessary to adjust the voltage value of the constant voltage source and/or the resistance value of the variable resistor for each individual commercial product of the chip to obtain a suitable level for the black-level signal.
SUMMARY OF THE INVENTION
Therefore, it is a main object of the present invention to provide a solid state image sensor having an improved black-level signal generation circuit.
It is another object of the present invention is to provide a CMOS image sensor that has a simplified black-level signal generation circuit.
It is still another object of the present inventio
Kurosawa Susumu
Muramatsu Yoshinori
McGinn & Gibb PLLC
NEC Electronics Corporation
Pyo Kevin
LandOfFree
MOS-based image sensor and method of forming black-level... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with MOS-based image sensor and method of forming black-level..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and MOS-based image sensor and method of forming black-level... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3128942