Abstract:
The apparatus and method provide a readout technique and circuit for increasing or maintaining dynamic range of an image sensor. The readout technique and circuit process each pixel individually based on the magnitude of the readout signal. The circuit includes a gain amplifier amplifying the readout analog signal, a level detection circuit for determining the signal&#39;s magnitude, a second gain amplifier applying a gain based on the signal magnitude and an analog-to-digital converter digitizing the signal and a circuit for multiplying or dividing the signal. The method and circuit allow for a lower signal-to-noise ratio while increasing the dynamic range of the imager.

Description:
This application is a divisional application of application Ser. No. 11/088,964, filed Mar. 24, 2005 now U.S. Pat. No. 7,297,917, which is hereby incorporated herein by reference in its entirety. 

   FIELD OF THE INVENTION 
   The invention relates to an image sensor and more particularly to a readout technique and circuit for increasing or maintaining dynamic range in an image sensor. 
   BACKGROUND OF THE INVENTION 
   Imaging devices, including charge coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) imagers, are commonly used in photo-imaging applications. 
   A CMOS imager circuit includes a pixel array, each one of the pixels including a photosensor which may be one of a photogate, photoconductor or a photodiode overlying a substrate for accumulating photo-generated charge in the underlying portion of the substrate. A readout circuit is connected to each pixel and includes at least an output field effect transistor formed in the substrate and a charge transfer section formed on the substrate adjacent the photogate, photoconductor or photodiode having a sensing node, typically a floating diffusion node, connected to the gate of an output transistor. The imager may include at least one electronic device such as a transistor for transferring charge from the underlying portion of the substrate to the floating diffusion node and one device, also typically a transistor, for resetting the node to a predetermined level prior to charge transference. 
   In a CMOS imager, the active elements of a pixel perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the floating diffusion node; (4) resetting the floating diffusion node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion node. CMOS imagers of the type discussed above are generally known as discussed, for example, in U.S. Pat. No. 6,140,630, U.S. Pat. No. 6,376,868, U.S. Pat. No. 6,310,366, U.S. Pat. No. 6,326,652, U.S. Pat. No. 6,204,524 and U.S. Pat. No. 6,333,205, assigned to Micron Technology, Inc., which are hereby incorporated by reference in their entirety. 
     FIG. 1  illustrates a block diagram for a CMOS imager  10 . The imager  10  includes a pixel array  20 . The pixel array  20  comprises a plurality of pixels arranged in a predetermined number of columns and rows. The pixels of each row in array  20  are all turned on at the same time by a row select line and the pixels of each column are selected for output by a column select line. A plurality of row and column lines are provided for the entire array  20 . 
   The row lines are selectively activated by the row driver  32  in response to row address decoder  30  and the column select lines are selectively activated by the column driver  36  in response to column address decoder  34 . Thus, a row and column address is provided for each pixel. The CMOS imager  10  is operated by the control circuit  41 , which controls address decoders  30 ,  34  for selecting the appropriate row and column lines for pixel readout, and row and column driver circuitry  32 ,  36 , which apply driving voltage to the drive transistors of the selected row and column lines. 
   Each column contains sample and hold circuitry (S/H)  38  comprising sampling and holding capacitors and switches associated with the column driver  36  that read and store pixel reset signal V rst  and a pixel image signal V sig  for selected pixels. A differential signal (V rst −V sig ) is produced by differential amplifier  40  for each pixel, which is digitized by analog-to-digital converter  45  (ADC). The analog-to-digital converter  45  supplies the digitized pixel signals to an image processor  50 , which forms and outputs a digital image output. 
   As previously noted, a typical CMOS image sensor includes an array of pixels  20 , arranged in a predetermined number of columns and rows. Generally, each pixel photosensor generates charge for a same amount of time and has an associated dynamic range. The dynamic range for an image sensor is commonly defined as the ratio of its largest non-saturating signal to the standard deviation of the noise under dark conditions. Dynamic range refers to the range of incident light that can be accurately sensed by an image sensor in a single frame of pixel data. It is desirable to have an image sensor with a high dynamic range in order to image scenes that generate high dynamic range incident signals, such as indoor rooms with windows to the outside, outdoor scenes with mixed shadows and bright sunshine, night-time scenes combining artificial lighting and shadows, and many others. 
   Dynamic range is limited on an upper end by the charge saturation level of the photosensor, and on a lower end by noise imposed limitations and/or quantization limits of the analog-to-digital converter used to convert the analog pixel output signals to digital signals. When the dynamic range of an image sensor is too small to accommodate the variations in light intensities of the imaged scene, image distortion occurs. 
   What is needed, therefore, is an image sensor, and methods of fabricating and operating the image sensor to achieve an improved dynamic range, which may be implemented using conventional CMOS processing techniques. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention, in its apparatus and method aspects illustrated in the exemplary embodiments described herein, provides a readout technique and circuit for increasing or maintaining dynamic range of an image sensor. The readout technique and circuit process each pixel individually based on the magnitude of the readout signal. The circuit includes a gain amplifier amplifying the readout analog signal, a level detection circuit for determining the signal&#39;s magnitude, a second gain amplifier applying a gain based on the signal magnitude and an analog-to-digital converter digitizing the signal and a circuit for multiplying or dividing the signal. The method and circuit allow for a lower signal-to-noise ratio while increasing the dynamic range of the imager. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other aspects of the invention will be better understood from the following detailed description of the invention, which is provided in connection with the accompanying drawings, in which: 
       FIG. 1  is a schematic diagram of a conventional imager; 
       FIG. 2  is a block diagram illustrating a pixel readout circuit according to an exemplary embodiment of the invention; 
       FIG. 3  is a block diagram illustrating a pixel readout circuit according to another exemplary embodiment of the invention; and 
       FIG. 4  shows a processor system incorporating at least one imaging device constructed in accordance with an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the following detailed description, reference is made to the accompanying drawings, which form a part hereof and illustrate specific embodiments in which the invention may be practiced. In the drawings, like reference numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. 
   The term “pixel” refers to a picture element unit cell containing a photo-conversion device and transistors for converting electromagnetic radiation to an electrical signal. It should be appreciated, however, that the invention is not limited to any particular pixel type or configuration. 
   Typically, a high resolution analog-to-digital converter  45  ( FIG. 1 ) is necessary to capture high dynamic range images in an image sensor  10 . The signal to nose ratio (SNR) is usually limited by photon noise to approximately 40-45 dB. In accordance with exemplary embodiments of the present invention, and as described in more detail below, a lower resolution analog-to digital converter may be utilized, yet the invention provides a high dynamic range. Thereby, the dynamic range can be increased further by maintaining the higher resolution analog-to-digital converter. The invention achieves high dynamic range images by proper amplification of the output signal level of each individual pixel prior to analog-to-digital conversion. 
   The invention described herein may be applied to both differential (e.g., Vrst−Vsig) and single ended signals. Although the figures may indicate that the signal is differential, the invention is not limited to differential signals. 
     FIG. 2  illustrates an exemplary pixel array readout chain  55  connected to sample hold circuitry  38 , which is connected to one selected column of the pixel array  20  according to the invention. The sample hold circuit  38  samples and holds the output from each selected pixel column in the array  20 . A reset signal is sampled and held in a first capacitor  31  when sample hold reset control signal SHR is generated. A photo signal is sampled and held in a second capacitor  33  when the sample hold control signal SHS is generated. 
   The sampled signals are processed by a first gain amplifier  54  of the readout chain  55 , which amplifies the signal to the fullest extent possible while retaining a low noise performance. The amount of amplification of the signal is limited by the power supply rails. The gain GAIN 1  of the first gain amplifier  54  may be programmable, for example, by an image processor  50  of  FIG. 1 , to compensate for image exposure and integration time. The magnitude of the signal output from the first gain amplifier  54  is determined by a level detection circuit  56 . The level detection circuit  56  quantizes the pixel signal into levels (e.g., small, mid level or high). The number of different signal levels is variable and is determined based on the specific readout chain circuit  55  implementation. Gain decision logic  60  uses the signal magnitudes (e.g., small, mid-level, high) to set the gain GAIN 2  of the second gain amplifier  70 . The level detection circuit  56  and the gain decision logic  60  comprise a signal level determining circuit. 
   The gain GAIN 2  settings of the second gain amplifier  70  depend on the resolution of analog-to-digital converter  80 , as well as the desired overall dynamic range. Examples of the GAIN 2  settings for amplifier  70  that could be used include 0.5, 1, 2, 4 and 8, depending on the number of signal levels which could be infinite. If the output of the first gain amplifier  54  is determined to be low, the gain GAIN 2  of the second gain amplifier  70  is set high (e.g., 8). Conversely, if the output of the first gain amplifier  54  is determined to be high, the gain GAIN 2  of the second gain amplifier  70  is set low (e.g., 0.5). The second gain amplifier  70  processes the differential signal Vrst−Vsig from amplifier  54 . The analog-to-digital converter (ADC 2 )  80  digitizes the signal. After the signal is digitized, it is multiplied or divided by a factor which is also set by gain decision logic  60  and allows for an overall dynamic range increase without increasing the resolution of the analog-to-digital converter  80 . The multiplication factor, for example, could be an inverse factor of the gain GAIN 2  applied in the second gain amplifier  70 . The multiplier factors are set based on the determined signal magnitude. Both the number of multipliers or divisors and the multiplier or divisor factors themselves may be set based on the specific imager application. 
   A description of an exemplary implementation of the readout circuit described above now follows. 
   The signals from each pixel stored in pixel array  20  are processed row-by-row and within each row, by the sample hold circuit  38 , which is common to each pixel column. Each pixel outputs reset Vrst and photo Vsig signal outputs which are sampled and held in response to respective control signals SHR and SHS, into respective capacitors  31 ,  33 . These signals are applied as a differential signal Vrst−Vsig to amplifier  54 , which amplifies the differential signal by a gain GAIN 1 . The level of the signal output from the first gain amplifier  54  is determined by the level detection circuit  56 . The levels assigned by the level detection circuit  56  could be low, 0.0V-0.25V, mid level, 0.25V-0.5V, and high, 0.5V-1V, corresponding to differential input signals. The signal level and the number of levels of analog-to-digital converter  56  can be set based on the implementation. If the gain decision logic  60  determines that the pixel output is high, a gain GAIN 2  of 0.5 is applied in the second gain amplifier  70 . Next, the signal is digitized into 8-bits by the analog-to-digital converter  80 . The digital signal processed by analog-to-digital converter  80  is digitally multiplied by 2 4 . When the pixel output is low, a gain GAIN 2  of 8 is used in the second gain amplifier  70  and the signal is digitized into 8-bits in the analog-to-digital converter  80 . The result is digitally multiplied by 2 0 . Alternatively, if the pixel output is mid level, a gain GAIN 2  of 2 is used in the second gain amplifier  70  and the signal is digitized into 8-bits in the analog-to-digital converter  80 . The signal is then digitally multiplied by 2 2 . The gains and multiplier factors applied in this example are only exemplary. The embodiments of the present invention allow for implementation different gains and multiplier factors to be used in the readout circuit based on the resolution of the analog-to-digital converter  80 , desired dynamic range and the implementation application. 
   In this example, the readout chain  55  obtains a signal-to-noise ratio based on 8 bits, but a dynamic range of 12 bits. The number of bits implemented in the analog-to-digital converter  80  is application specific. When the image is digitized by a conventional 8-bit analog-to-digital converter, details in the dark spots are lost. However, the implementation of the readout circuit  55  allows for an image digitized by an 8-bit analog-to-digital converter  80  to keep the details in the darkest spots. This readout method allows for the digitizing of images and obtaining of a high dynamic range with a relatively low resolution analog-to-digital converter. Therefore, complex and costly high resolution memory storage on a single integrated circuit or on a different chip than the processor. It is also possible to integrate the CPU  702 , RAM  710  and imaging device  708  on the same integrated circuit chip. 
   The above described invention can be implemented in any system providing an analog-to-digital conversion in which a dynamic range higher than the signal to noise ratio is desired. An exemplary implementation is in a swallowable pill as described in copending U.S. application 2003-0210439, herein incorporated by reference. 
   It should be appreciated that other embodiments of the invention include a method of fabricating the circuits of the invention as illustrated in  FIGS. 2 and 3 . For example, in one exemplary embodiment, a method of fabricating a pixel readout circuit comprises the acts of fabricating a first gain amplifier for amplifying a signal from a pixel array, fabricating a signal level determining circuit for determining a gain factor based on a level of the amplified signal, fabricating a second variable gain amplifier, connected to said signal level determining circuit, for amplifying the signal from the first gain amplifier using the determined gain factor and fabricating an analog-to-digital converter for digitizing an output of the second gain amplifier. 
   Furthermore, although the processes and device described above focus on increasing the dynamic range of an imager, the dynamic range may also be maintained with a lower resolution analog-to-digital converter by lowering other design constraints. For example, other design constraints that may be altered include power dissipation, bit rate after compression and the silicon area. 
   The processes and devices described above illustrate exemplary methods and typical devices of many that could be used and produced. The above description and drawings illustrate embodiments, which achieve the objects, features, and advantages of the present invention. However, it is not intended that the present invention be strictly limited to the above-described and illustrated embodiments. Any modification, though presently unforeseeable, of the present invention that comes within the spirit and scope of the following claims should be considered part of the present invention.