Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-04-22
2001-04-10
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C257S292000, C348S308000
Reexamination Certificate
active
06215113
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to semiconductor imaging devices and in particular to CMOS active pixel sensors capable of motion detection.
BACKGROUND OF THE INVENTION
Complementary metal-oxide-semiconductor (CMOS) active pixel sensor (APS) can be utilized to convert optical images into corresponding electronic signals, and can be used as the sensing part in digital still image camera systems or digital video camera systems. The main advantage of CMOS APS compared to the conventional charge-coupled device (CCD) imager is that the CMOS APS can be fabricated in standard CMOS process. Therefore, the CMOS APS imager and other CMOS circuits can be integrated into a single chip. Consequently, the cost, power consumption, and complexity of the whole system can be greatly reduced.
In many digital camera applications, motion detection is a desirable function, and especially in security applications. The simplest way to detect motion is from the difference of two consecutive image frames. Large difference between two consecutive frame means there is a motion between the images. Therefore, if the image sensor is capable of direct frame difference readout, it can be used to detect motion.
Another main application of image sensors is the digital video camera. Due to the large amount of data in video signals, video compression is essential in limited transmission bandwidth and storage size. In video signals, two consecutive image frames tend to be very similar, because a short period of time elapses therebetween, and thus the amount of information in the frame difference is much less than that in the digital camera applications. If the frame difference between two consecutive image frames in video signals can be readout directly from the image sensor, video compression can be achieved by removing the temporal redundancy.
In order to obtain the frame difference from traditional image sensors, a frame buffer is needed to store the previous frame data, and then take the difference between the current frame data and the previous frame data. In this approach, the frame buffer will occupy a large amount of chip area. If the image sensor can provide direct frame difference output, the complexity and area of the chip can be reduced.
Traditional photo diode CMOS APS can not provide frame difference directly. Photo gate APS with intra pixel charge transfer has an operating mode for frame difference output. However, when the sensor is operating in this mode, the exposure time must be the frame period, which can not be easily changed. In real applications, the environment luminance change significantly, and it is essential to effectively control the exposure time to get good quality images.
Therefore, a CMOS APS image sensor with direct frame difference output for motion detection and video compression applications and with electronic shutter to control the exposure time is desirable.
SUMMERY OF THE INVENTION
This invention proposes a CMOS APS circuit with direct frame difference output. The circuit includes a photo diode, a reset switch, two sample-and-hold circuits, and two readout circuits. Each sample-and-hold circuit includes a switch as an electronic shutter and a node for charge storage. Each readout circuit includes a MOS transistor as a source follower and a row select switch.
Since there are two sample-and-hold circuits in the CMOS APS circuit, the current frame image data and the previous frame image data can be stored in the CMOS APS circuit. The output data is read out differentially from the two readout circuits, so that the frame difference can be obtained directly. When the sensor is operated in an intra frame mode, the sample-and-hold circuit associated with the previous frame data is reset to a reference voltage, and the current frame image data can be read out from the differential readout lines.
This invention is suitable for video oriented applications such as motion detection, video compression, and moving object segmentation systems. Furthermore, low supply voltage operation has been considered in the design of the CMOS APS circuit, and it is suitable for low power applications such as portable systems.
In one embodiment of the present invention, a CMOS active pixel image sensor cell comprises:
a photo diode including a region of a N-type conductivity formed on a P-type substrate, the P-type substrate being adapted to connect to a first power supply VSS;
a MOS transistor as a reset switch, a gate of the MOS transistor being adapted to connect to a reset control signal, a source thereof being adapted to connect to a second power supply VDD, and a drain thereof being connected to the N-type region of the photo diode;
two MOS transistors as electronic shutter switches, each gate of said two MOS transistors being adapted to connect to a shutter control signal, and each drain thereof is connected to the N-type region of the photo diode;
two source follower NMOS transistors, two gates of said two source follower NMOS transistors being connected separately to two sources of said two MOS transistors, and two drains of said two source follower NMOS transistors being adapted to connect to VDD; and
two switch NMOS transistors as row select switches, two drains of said two switch NMOS transistors being connected separately to two sources of said two source follower NMOS transistors, both gates of said two switch NMOS transistors being adapted to connect to a row select signal, and each source of said two switch NMOS transistors being an output node.
The active pixel image sensor cell may further comprises two capacitors, two terminals of the two capacitors are connected separately to said two gates of said two source follower transistors, and another two terminals of said two capacitors are connected to a fixed voltage.
Preferably, said two capacitors are two NMOS transistors, two gates of said two NMOS transistors are connected separately to said two gates of said two source follower transistors, two sources and two drains of said two NMOS transistors are all adapted to connected to VSS.
Preferably, the reset switch and the electronic shutter switches in the active pixel image sensor cell are PMOS transistors.
In another embodiment of the present invention, a CMOS active pixel image sensor cell comprises:
a photo diode including a region of a P-type conductivity formed on a N-type well region, the N-type well region being adapted to connect to a first power supply VDD;
a MOS transistor as a reset switch, a gate of the MOS transistor being adapted to connect to a reset control signal, a source thereof being adapted to connect to a second power supply VSS, and a drain thereof being connected to the P-type region of the photo diode;
two MOS transistors as electronic shutter switches, each gate of said two MOS transistors being adapted to connect to a shutter control signal, and each drain thereof is connected to the P-type region of the photo diode;
two source follower PMOS transistors, two gates of said two source follower PMOS transistors being connected separately to two sources of said two MOS transistors, and two drains of said two source follower PMOS transistors being adapted to connect to VSS; and
two switch PMOS transistors as row select switches, two drains of said two switch PMOS transistors being connected separately to two sources of said two source follower PMOS transistors, both gates of said two switch PMOS transistors being adapted to connect to a row select signal, and each source of said two switch PMOS transistors being an output node.
The active pixel image sensor cell constructed according to the another embodiment of the present invention may further comprises two capacitors, two terminals of the two capacitors are connected separately to said two gates of said two source follower transistors, and another two terminals of said two capacitors are connected to a fixed voltage. Preferably, said two capacitors are two PMOS transistors, two gates of said two PMOS transistors are connected separately to said two gates of said two source follower transistors, tw
Chen Liang-Gee
Ma Shyh-Yih
Fish & Richardson P.C.
Lee John R.
National Science Council
Pyo Kevin
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