Abstract:
An image sensor array that uses an over saturation sensor to detect when one of the pixels is being oversaturated. When that happens, the power supply to that pixel is prevented from affecting power supplies to the other pixels, by regulating the power supply to the pixel.

Description:
[0001]    This application claims priority from Provisional application No. 61/654,258, filed Jun. 1, 2012, the entire contents of which are herewith incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    CMOS image sensors may have a column parallel readout using an array of programmable gain amplifiers or PGAs. The array of PGAs reads a row of pixels simultaneously and stores the results in Sample and Hold structures which are typically capacitors. Ideally, the signal being stored by a given column&#39;s PGA should be independent of any external conditions such as power or ground supply variation. 
         [0003]    When the power supply does show variation, it can induce image artifacts known as “streaking” or “ghosting”. A sample of such an image artifact is shown in  FIG. 1 . This kind of image artifact can occur when a certain number of PGAs are over saturated, and draw excess power to maintain. The power drawn by the oversaturated PGAs effect the power supply, and thus cause the values of the other PGAs in the same row to deviate. The deviation is shown in  FIG. 1  as dark regions, as compared to regions where no PGAs are saturated. 
       SUMMARY 
       [0004]    A recognition of the present application is a cause of such artifacts, and a special circuit and technique that avoids these artifacts from occurring. 
         [0005]    Embodiments describe a voltage clamping circuit that regulates a current draw of a two-stage amplifier by detecting when that amplifier is oversaturated. The purpose of regulating the current draw for the amplifier is such to maintain the power supply profile across an array of amplifiers; used in a CMOS image sensor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    In the Drawings: 
           [0007]      FIG. 1  shows an example image of a ghosting artifact; 
           [0008]      FIG. 2  shows a pixel array of the CMOS image sensor as used according to embodiments; 
           [0009]      FIG. 3  shows a block diagram of a clamp circuit according to embodiments is used with the image sensor readout; and 
           [0010]      FIG. 4  shows details of the clamp circuit that is used. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 2  illustrates a block diagram of the CMOS image sensor that has a column parallel readout array. The pixel array  200  includes a group of individual pixels forming an image sensor. A row driver  205  drives the pixel array to readout one row at a time into the column parallel programmable gain amplifiers. The PGA array is shown as  210 , driven by the PGA supply  215 . The column that is read-out is sampled by the sample and hold capacitors, then A/D converted by an A/D converter  220  and eventually output. 
         [0012]    The PGA supply  215 , as described above, inherently changes based on conditions and especially the based on the amount of loading. When the power supply changes, this change can cause artifacts. 
         [0013]    The inventors have recognized that the artifacts that are caused are due to the following: 
         [0014]    Limited power supply rejection of the circuit. 
         [0015]    Power supply resistance across the column array. 
         [0016]    The signal being sampled will depend on changes in the power supply voltage and current variation. 
         [0017]    A typical output stage of a PGA amplifier has an output node that can swing up a level to as high as the Vsupply rail when the value of the pixel being sampled is oversaturated. When this occurs, the current bias M 8 _PGA gradually reduces and eventually cuts off depending on the how much power is being consumed. 
         [0018]    This change in current draw changes the power supply profile across the PGA array. The amount of current drawn by each element of the PGA array therefore depends on the number of PGA&#39;s that are in the over saturated condition. The inventors found that because this number can vary, the power supply profile correspondingly varies. 
         [0019]    According to an embodiment, a circuit is described which maintains the same power supply profile across the array regardless of the scene being imaged, by preventing oversaturated PGAs from effecting the power supply. To do so, an embodiment uses a circuit that detects when the PGA is in the oversaturated condition, and the circuit regulates the current draw of that amplifier to maintain a constant current to that amplifier. 
         [0020]      FIG. 3  shows a block diagram of a typical image sensor column readout circuit with a PGA. The pixel  300  is formed of a photosensor  305  that is connected selectively by a selectable switch to a source follower  315 . A row select transistor  316  selects when the output is produced. There is also a reset transistor  317  that resets the pixel. 
         [0021]    The PGA  320  is shown as including an amplifier  325  with a variable capacitance  326  in the feedback between the output and input of the amplifier, and another variable capacitance  327  in the input. The ratio between the capacitances of  326  and  327  can be varied to change the amplification rate in the charge domain. 
         [0022]    A PGA clamp circuit  330  is attached to the output of the PGA circuit  320 . The PGA clamp circuit uses a comparator  335  which compares the output signal  328  from the PGA amplifier with a reference  340  called PGA clamp. This reference represents the value beyond which or above which the output will be considered to be oversaturated. A current mirror  345  is turned on by the output of the comparator  335 . When enabled, the current mirror provides a current bias to regulate the supply. This prevents the output from clamping, by substituting the current mirrored output for the other normal output. 
         [0023]      FIG. 4  shows an implementation of the PGA clamp circuit with the output stage of the PGA Amplifier. The amplifier output stage is shown as  400 , where a supply voltage  405  is driven by the bias level  407  to create a bias output  408 . 
         [0024]    The comparator  335  is formed of M 0 , M 1 , M 2 , M 3  and M 4 . When the comparator detects that the PGA output has exceeded the reference voltage shown as VPGA clamp, it steers a matching current thru M 5 , M 6 , and M 7  to limit the PGA output swing and maintain the current draw thru Vsupply. 
         [0025]    M 8  is used only to enable or disable the PGA clamp circuit. The current biases, Vbiasn and Vbiasp are mirrored from the same current seed to match the currents (M 7 , Ibias_clamp) and (M 8 _PGA, Ibias_output). 
         [0026]    Although only a few embodiments have been disclosed in detail above, other embodiments are possible and the inventors intend these to be encompassed within this specification. The specification describes specific examples to accomplish a more general goal that may be accomplished in another way. This disclosure is intended to be exemplary, and the claims are intended to cover any modification or alternative which might be predictable to a person having ordinary skill in the art. For example, other kinds of circuits can be controlled in this way. 
         [0027]    Those of skill would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the exemplary embodiments. 
         [0028]    The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein, may be implemented or performed with a general purpose processor, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. The processor can be part of a computer system that also has a user interface port that communicates with a user interface, and which receives commands entered by a user, has at least one memory (e.g., hard drive or other comparable storage, and random access memory) that stores electronic information including a program that operates under control of the processor and with communication via the user interface port, and a video output that produces its output via any kind of video output format, e.g., VGA, DVI, HDMI, displayport, or any other form. This may include laptop or desktop computers, and may also include portable computers, including cell phones, tablets such as the IPAD™, and all other kinds of computers and computing platforms. 
         [0029]    A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. These devices may also be used to select values for devices as described herein. 
         [0030]    The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, using cloud computing, or in combinations. A software module may reside in Random Access Memory (RAM), flash memory, Read Only Memory (ROM), Electrically Programmable ROM (EPROM), Electrically Erasable Programmable ROM (EEPROM), registers, hard disk, a removable disk, a CD-ROM, or any other form of tangible storage medium that stores tangible, non transitory computer based instructions. An exemplary storage medium is coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in reconfigurable logic of any type. 
         [0031]    In one or more exemplary embodiments, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium. Computer-readable media includes both computer storage media and communication media including any medium that facilitates transfer of a computer program from one place to another. A storage media may be any available media that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer. 
         [0032]    The memory storage can also be rotating magnetic hard disk drives, optical disk drives, or flash memory based storage drives or other such solid state, magnetic, or optical storage devices. Also, any connection is properly termed a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. The computer readable media can be an article comprising a machine-readable non-transitory tangible medium embodying information indicative of instructions that when performed by one or more machines result in computer implemented operations comprising the actions described throughout this specification. 
         [0033]    Operations as described herein can be carried out on or over a website. The website can be operated on a server computer, or operated locally, e.g., by being downloaded to the client computer, or operated via a server farm. The website can be accessed over a mobile phone or a PDA, or on any other client. The website can use HTML code in any form, e.g., MHTML, or XML, and via any form such as cascading style sheets (“CSS”) or other. 
         [0034]    Also, the inventor(s) intend that only those claims which use the words “means for” are intended to be interpreted under 35 USC 112, sixth paragraph. Moreover, no limitations from the specification are intended to be read into any claims, unless those limitations are expressly included in the claims. The computers described herein may be any kind of computer, either general purpose, or some specific purpose computer such as a workstation. The programs may be written in C, or Java, Brew or any other programming language. The programs may be resident on a storage medium, e.g., magnetic or optical, e.g. the computer hard drive, a removable disk or media such as a memory stick or SD media, or other removable medium. The programs may also be run over a network, for example, with a server or other machine sending signals to the local machine, which allows the local machine to carry out the operations described herein. 
         [0035]    Where a specific numerical value is mentioned herein, it should be considered that the value may be increased or decreased by 20%, while still staying within the teachings of the present application, unless some different range is specifically mentioned. Where a specified logical sense is used, the opposite logical sense is also intended to be encompassed. 
         [0036]    The previous description of the disclosed exemplary embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these exemplary embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.