Patent Publication Number: US-11032501-B2

Title: Low noise image sensor system with reduced fixed pattern noise

Description:
PRIORITY CLAIM 
     This application claims priority to U.S. Provisional Application 62/680,007, entitled “Low Noise Image Sensor System with Reduced Fixed Pattern Noise”, filed Jun. 4, 2018, the disclosure of which is incorporated herein in its entirety. 
    
    
     BACKGROUND 
     Technical Field 
     This disclosure is directed to electronic systems, and more particularly, to image sensor systems. 
     Description of the Related Art 
     High quality image sensor systems have often utilized a single slope analog to digital converter (ADC) architecture due to its good uniformity, power efficiency and compactness. More recently, Successive Approximation Register (SAR) ADC architectures have been adopted for image sensor systems due to their speed benefit. The speed of a SAR ADC originates from the binary operation of a digital to analog converter (DAC) within the SAR ADC. Furthermore, column multiplexing techniques are sometimes implemented in image sensing systems due to the relatively wider pitch of the unit ADC relative to the pixel itself. Due to the uniqueness of SAR operation compared to single slope ADCs, SAR ADC based image sensors may introduce new and various critical non-idealities that degrade image quality mandating new approaches to mitigate these adverse impacts. 
     SUMMARY 
     Various embodiments of an image sensing system and methods for operating the same are disclosed. In one embodiment, an image sensing system includes a plurality of pixel circuits, a multiplexer configured to select one of the pixel circuit and provide analog pixel data therefrom, and a successive approximation register (SAR) analog-to-digital converter (ADC) configured to convert the analog pixel data into digital data. The SAR ADC includes a capacitive digital-to-analog converter (CDAC) configured to convert contents of the SAR into a corresponding analog signal for comparison, by a comparator, with the analog pixel data. The CDAC includes a two-dimensional array of circuit elements. A control circuit in the image sensing system is configured to cause random ones of the circuit elements of the CDAC to be selected for generation of the corresponding analog signal. 
     In one embodiment, the image sensing system includes a plurality of pixel units each including the various components (pixel circuits, multiplexer, SAR ADC, etc.) as described above. During operation and within a given frame, each pixel circuit may be selected to provide pixel data. In one embodiment, the order in which the pixel circuits may be selected to provide pixel data is random rather than sequential. Within the CDAC, the selection of circuit elements for converting the contents of the SAR to a corresponding analog signal may also be randomized. In an embodiment, the various bit positions of the SAR and corresponding circuit elements of the CDAC may be subdivided into groups, with the selection of circuit elements being randomized within these groups, and thus orthogonal with respect to each other. 
     In one embodiment, the analog pixel data may be provided to an input of the comparator as a continuous analog circuit, with no sample-and-hold circuitry coupled to that comparator input. Furthermore, a dithering signal may be applied to the same comparator input that receives the analog signal from the CDAC, with these signals applied concurrently. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description makes reference to the accompanying drawings, which are now briefly described. 
         FIG. 1  is a diagram illustrating one embodiment of an image sensing system. 
         FIG. 2  is a diagram illustrating one embodiment of a group of multiplexed pixel units of an image sensing system. 
         FIG. 3  is a diagram of one embodiment of a capacitive digital-to-analog converter (CDAC). 
         FIG. 4  is a diagram illustrating one embodiment of an apparatus coupled to convey a dithering signal to the input of an amplifier concurrent with another input signal. 
         FIG. 5  is a flow diagram illustrating one embodiment of a method performing an analog-to-digital conversion in a SAR ADC having a CDAC. 
         FIG. 6  is a flow diagram illustrating one embodiment of a method for selecting pixel circuits in the processing of pixel data. 
         FIG. 7  is a flow diagram illustrating one embodiment of a method for performing a comparison in which a dithering signal is applied along with an input signal to one input of the comparator. 
         FIG. 8  is an illustration of one embodiment of a pixel circuit and compensation driver. 
         FIG. 9  is a flow diagram illustrating one embodiment of a method for operating a pixel circuit. 
         FIG. 10  is a diagram illustrating one embodiment of an apparatus for controlling the slope of a power enable signal and a diagram illustrating operation of the same. 
         FIG. 11  is a diagram illustrating one embodiment of an integrate-and-reset pre-amplifier used in an embodiment of a comparator. 
         FIG. 12  is a table illustrating repeat sampling patterns usable in one embodiment of an image sensing system. 
         FIG. 13  is a flow diagram of one embodiment of a method for powering up and powering down a comparator using a slope-controlled power enable signal. 
         FIG. 14  is a flow diagram illustrating one embodiment of a method for selectively repeating a sampling process for particular ones of a number of pixel circuits. 
         FIG. 15  is a block diagram of one embodiment of an example system. 
     
    
    
     Although the embodiments disclosed herein are susceptible to various modifications and alternative forms, specific embodiments are shown by way of example in the drawings and are described herein in detail. It should be understood, however, that drawings and detailed description thereto are not intended to limit the scope of the claims to the particular forms disclosed. On the contrary, this application is intended to cover all modifications, equivalents and alternatives falling within the spirit and scope of the disclosure of the present application as defined by the appended claims. 
     This disclosure includes references to “one embodiment,” “a particular embodiment,” “some embodiments,” “various embodiments,” or “an embodiment.” The appearances of the phrases “in one embodiment,” “in a particular embodiment,” “in some embodiments,” “in various embodiments,” or “in an embodiment” do not necessarily refer to the same embodiment. Particular features, structures, or characteristics may be combined in any suitable manner consistent with this disclosure. 
     Within this disclosure, different entities (which may variously be referred to as “units,” “circuits,” other components, etc.) may be described or claimed as “configured” to perform one or more tasks or operations. This formulation [entity] configured to [perform one or more tasks], is used herein to refer to structure (i.e., something physical, such as an electronic circuit). More specifically, this formulation is used to indicate that this structure is arranged to perform the one or more tasks during operation. A structure can be said to be “configured to” perform some task even if the structure is not currently being operated. A “credit distribution circuit configured to distribute credits to a plurality of processor cores” is intended to cover, for example, an integrated circuit that has circuitry, that performs this function during operation, even if the integrated circuit in question is not currently being used (e.g., a power supply is not connected to it). Thus, an entity described or recited as “configured to” perform some task refers to something physical, such as a device, circuit, memory storing program instructions executable to implement the task, etc. This phrase is not used herein to refer to something intangible. 
     The term “configured to” is not intended to mean “configurable to.” An unprogrammed FPGA, for example, would not be considered to be “configured to” perform some specific function, although it may be “configurable to” perform that function after programming. 
     Reciting in the appended claims that a structure is “configured to” perform one or more tasks is expressly intended not to invoke 35 U.S.C. § 112(f) for that claim element. Accordingly, none of the claims in this application as filed are intended to be interpreted as having means-plus-function elements. Should Applicant wish to invoke Section 112(f) during prosecution, it will recite claim elements using the “means for” [performing a function] construct. 
     As used herein, the term “based on” is used to describe one or more factors that affect a determination. This term does not foreclose the possibility that additional factors may affect the determination. That is, a determination may be solely based on specified factors or based on the specified factors as well as other, unspecified factors. Consider the phrase “determine A based on B.” This phrase specifies that B is a factor that is used to determine A or that affects the determination of A. This phrase does not foreclose that the determination of A may also be based on some other factor, such as C. This phrase is also intended to cover an embodiment in which A is determined based solely on B. As used herein, the phrase “based on” is synonymous with the phrase “based at least in part on.” 
     As used herein, the phrase “in response to” describes one or more factors that trigger an effect. This phrase does not foreclose the possibility that additional factors may affect or otherwise trigger the effect. That is, an effect may be solely in response to those factors, or may be in response to the specified factors as well as other, unspecified factors. Consider the phrase “perform A in response to B.” This phrase specifies that B is a factor that triggers the performance of A. This phrase does not foreclose that performing A may also be in response to some other factor, such as C. This phrase is also intended to cover an embodiment in which A is performed solely in response to B. 
     As used herein, the terms “first,” “second,” etc. are used as labels for nouns that they precede, and do not imply any type of ordering (e.g., spatial, temporal, logical, etc.), unless stated otherwise. For example, in a register file having eight registers, the terms “first register” and “second register” can be used to refer to any two of the eight registers, and not, for example, just logical registers 0 and 1. 
     When used in the claims, the term “or” is used as an inclusive or and not as an exclusive or. For example, the phrase “at least one of x, y, or z” means any one of x, y, and z, as well as any combination thereof. 
     In the following description, numerous specific details are set forth to provide a thorough understanding of the disclosed embodiments. One having ordinary skill in the art, however, should recognize that aspects of disclosed embodiments might be practiced without these specific details. In some instances, well-known circuits, structures, signals, computer program instruction, and techniques have not been shown in detail to avoid obscuring the disclosed embodiments. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The present disclosure is directed to various embodiments of an image sensor system. Various optimizations disclosed herein may allow an image sensor system to operate at greater speeds with less power consumption while reducing/minimizing the various sources of noise that can otherwise degrade image quality. 
     In various embodiments, an image sensor system according to this disclosure includes pixel units each having a number of pixel circuits. The pixel circuits are coupled to provide pixel data to a comparator of a SAR ADC, via a multiplexer. In one embodiment, the pixel data is provided to the comparator as a continuous analog signal, as no sample-and-hold circuit is provided on that comparator input, which may reduce the impact of aliased thermal noise on the image quality. The other comparator input may receive, for a basis of comparison, an analog signal generated by a SAR ADC. 
     The SAR ADC may include a CDAC having a number of circuit elements used in generating the analog signal. For a given iteration, not all of the circuit elements are used. Furthermore, for any given iteration, the circuit elements of the CDAC may be randomly selected for use in generating the analog signal, and the random selection may occur within subsets that are independent (and thus orthogonal) from one another. Random selection of the elements may help to average out image noise that may result from an issue known as integral nonlinearity (INL). 
     Another nonlinearity issue, differential nonlinearity (DNL), may be addressed through the application of a dithering signal to the same input as the analog signal output from the CDAC. In some embodiments, a separate DAC may be provided to generate the dithering signal, while in other embodiments, the circuitry for generating the dithering signal may be merged into the existing CDAC. Application of the dithering signal, which may be performed on a per-frame basis, may randomize systematic linearity errors in the CDAC and may improve DNL performance. 
     In some embodiments, the order in which pixel circuits are selected to provide pixel data for conversion to digital may be randomized. For example, if pixel circuits are arranged in columns, the selection of columns may be randomly shuffled from one frame to the next. The random selection of pixel circuits, through column shuffling or some other mechanism, may help reduce fixed patter noise that can otherwise reduce image quality. 
     Embodiments in which the pixel circuits include compensation circuitry are possible and contemplated. For example, a portion of the pixel circuitry may be replicated to provide a compensation circuit, or a simple buffer or amplifier circuits may be used. The main portion of the pixel circuit may be coupled to a source follower configured to amplify pixel data conveyed onto a column node (the column node being coupled directly to ADC or to a multiplexer which is in turn coupled to an ADC). The compensation circuit portion of the pixel circuit may be capacitively coupled to the column node, and may provide a disturbance voltage thereto. The disturbance voltage applied to the column node concurrent with the pixel data (via the source follower) may reduce the settling time of the column line. This may in turn reduce residue patterns that can result from clock feedthrough during operation of the pixel circuits. 
     Various power saving featured may be implemented in embodiments of the image sensor system disclosed herein. In one embodiment, the comparator of the ADC may be powered up when an ADC input is to be processed and powered down when input is not processed. A power enable signal may be slope-controlled such that the powering up and powering down of the comparator is likewise controlled. Powering the comparator up and down in a controlled manner may in turn reduce the occurrence and magnitude of power supply disturbances (e.g., voltage droops) from sudden changes in current demand (e.g., current surges). In image systems having multiple ADCs, the powering up and down of comparators in each may also be staggered relative to one another, further minimizing power supply disturbances. It is noted that many of the embodiments discussed herein utilize correlated double sampling (CDS), and as such, a given comparator may be powered up and down twice in performing a quantization process of pixel data provided from a corresponding pixel circuit. 
     With respect to the comparator of an ADC, various embodiments may include therein an integrate-and-reset pre-amplifier. This type of pre-amplifier may be optimized to the fundamental limit for a target signal-to-noise ratio (SNR) for the specific image sensor output signal by signal processing techniques regardless of ADC type. 
     To achieve good noise performance within power budgets, one embodiment may perform the quantization process using a weighted repeat with respect to some pixel circuits. That is, for selected ones of the pixel circuits, the quantization process may be repeated one or more times. The specific ones of the pixel circuits for which quantization may be determined through various mechanisms, such as trial and error. Performing the weighted repeat may allow for, e.g., reduced fixed pattern noise and reduced random noise for the fixed number of quantization. 
     Various embodiments of an image sensing system may implement some or all of the various method and apparatus embodiments discussed above and in further detail below. 
     Basic Image Sensor System: 
     Turning now to  FIG. 1 , a diagram illustrating one embodiment of a portion of an image sensor system is disclosed. In the embodiment shown, image sensor system  20  is implemented on an integrated circuit (IC) package having a stacked arrangement, including a top chip  12  and a bottom chip  13 . It is noted however that the physical arrangement of a top chip and bottom chip as shown here is not intended to be limiting, but rather is one possible physical implementation. However, the various method and apparatus embodiments discussed herein may be implemented in any suitable manner. Furthermore, certain details of the image sensor system are not shown in  FIG. 1 , but will be illustrated in other figures discussed below or will otherwise become apparent upon reading this disclosure. 
     Although not explicitly shown here (for the sake of simplicity) sensors may capture image data (e.g., video or still frame pictures). The image information may be conveyed to pixel drivers  77  (which it is noted, are separate from the pixel drivers in pixel circuits  103  discussed below). These pixel drivers may provide image information for each pixel to pixel circuits  103  in top chip  12 . Each of the pixel circuits  103  in the embodiment shown is in turn coupled to provide pixel data to an ADC  205 . Current sources Ic may be provided in some embodiments to provide current on respective signal paths from which pixel data is conveyed form a pixel circuit  103  to an ADC  205 . In various embodiments, the ADCs  205  may be implemented as SAR ADCs. 
     Each of the ADCs may convert pixel data, received in analog form, into a corresponding digital value. The digital values generated by the various ADCs  205  may in turn be provided to ADC output buffers  121 , which provide temporary storage. From there, the digital values corresponding to sampled pixel data are conveyed to sensor logic core/control circuitry  111 . The sensor logic core of this functional circuit unit may perform functions such as ordering of pixel data relative to other pixels in the captured frame. The control circuitry portion of this functional circuit block may generate and provide control signals for performing a number of different control functions, many of which are discussed below. These control functions include (but are not limited to) pixel circuit selection, CDAC circuit element selection, dithering signal generation, controlling the sampling operation, controlling the powering up and powering down of circuits and the manner in which it occurs, and so on. 
     Digitized pixel data may be conveyed from sensor logic core/control circuitry  111  to output interface  171 , and subsequently to other processing circuitry which may utilize the image data in any suitable manner. 
       FIG. 2  is a diagram illustrating one embodiment of a pixel unit  200 . Although not explicitly shown in  FIG. 1 , each of the pixel circuits  103  shown therein may be associated with particular one of the SAR ADCs  205 . As shown here, each of these pixel circuits may be arranged in, e.g., columns that are in turn coupled to a multiplexer  209 . Each column may include DC shift capacitors, which includes Csh 1 -Csh 8  in this particular example. These capacitors may block DC components of the pixel data conveyed on the column line (which may be alternatively referred to as a column node) mitigating the pixel&#39;s DC offset impact. Accordingly, pixel circuits  103  in this embodiment are AC coupled to SAR ADC  205 , via multiplexer  209 . 
     The number of pixel circuits  103 , and corresponding shift capacitors in a pixel unit  200  may vary from one embodiment to the next. Each column is also coupled to a reset switch (SR 1  through SR 8  are shown in this example). These switches may be closed when it is desired to reset a given column line, such as between quantization of the pixel circuits. 
     Multiplexer  209  in the embodiment shown is a one-hot multiplexer including a number of switches, SS 1 -SS 8 . Other types of multiplexers may be implemented in lieu of this particular embodiment. When a given one of the switches is closed, pixel data from the correspondingly coupled pixel circuit may be conveyed from the column line, through the corresponding DC shift capacitor to SAR ADC  205 . Control of the switches SS 1 -SS 8  may be performed based on control signals generated from control circuitry, such as that in sensor logic core/control circuitry  111 . 
     SAR ADC in the embodiment shown includes a comparator  221 , which is configured to compare the respective amplitudes of the signals received on its inputs. Each of the inputs is coupled to a corresponding reset switch (SR 9 , SR 10  as shown), which may be used to reset these circuit nodes similar to the resetting of the column nodes that may be performed. These inputs may be reset between samples in order to eliminate any memory of previously processed pixel data. 
     A first one of the inputs to comparator  221  is coupled to receive the pixel data. As specified in the drawing, the connection between the output of the multiplexer and the corresponding input of the comparator excludes sample and hold circuitry. Since no sample and hold circuit is provided here, the analog pixel data conveyed from the multiplexer on the second input as a continuous but almost constant analog signal after sufficient pixel settling. By not sampling the pixel data prior to its input into comparator  221 , the impact of aliased thermal noise on the image may be reduced. 
     The other input to comparator  221  in the embodiment shown is a corresponding analog signal provided from CDAC  231 . This signal corresponds to the digital value from SAR  233 . As the contents of SAR are updated for each comparison, the corresponding analog signal from CDAC  231  is similarly updated for the next comparison/approximation performed. When a conversion is complete, the contents of SAR  233  may be conveyed to, e.g., output buffers  121  as shown in  FIG. 1 . 
     Low Noise Image Sensing with Reduced Fixed Pattern Noise (FPN) 
     As previously noted, embodiments of an image sensing system in which pixel data is provided to the input of comparators implemented in corresponding SAR ADCs as continuous but almost constant analog signals (and thus, without a sample and hold circuit performing a sample and hold operation) are possible and contemplated within the scope of this disclosure. As further noted, this may result in a reduction of aliased thermal noise which can reduce image quality. The present disclosure contemplates additional embodiments that may reduce noise that can have an adverse impact on a final image. 
       FIG. 3  illustrates one embodiment of a CDAC  231  and a correspondingly coupled SAR  233 . CDAC  231  as implemented here includes an array of circuit elements, each of which is referred to here as an instance of cell  307 . An example of cell  307  is shown in  FIG. 3  includes a multiplexer  317  which may convey a value from either the R line or the C line to inverter  327  (shown here in a transistor implementation). Depending on the input values and the selection, the terminal capacitor Cc coupled to the output of multiplexer  317  is driven to the voltage refp, or discharged to the voltage refn. Capacitor Cc may charge or discharge accordingly, with the value Vx conveyed as part of the analog output signal from CDAC  231 . 
     During a given conversion, not all cells are used. Instead, only certain cells are selected for use in converting the digital value stored in SAR  233  into a corresponding analog signal. In the embodiment shown, the utilized cells may be randomly selected from one iteration (e.g., for one instance of pixel data) to the next. This type of random selection may result in the averaging out of linearity errors that might otherwise be present due to mismatches of various circuit elements in CDAC  231 . 
     The random selection may further be segmented into subsets, with the randomization within subsets being orthogonal with respect to the randomization within the other subset. In one embodiment, SAR  233  is subdivided into a first portion of storage locations coupled to provide bits to portions of the array of circuit elements associated with most significant bits (MSBs) of a digital value to be converted, and a second portion of storage locations coupled to provide bits to a portion of the array of circuit elements associated with a least significant bit (LSB) of the digital value to be converted. In this particular example, SAR  233  is a 10-bit SAR. In this example, 10-bit SAR  233  that is segmented into 6-bit unary (bits B 9 -B 4 ) and 4-bit binary (bits B 3 -B 0 ), although this segmentation may vary from one embodiment to the next dependent upon area and target linearity budgets. For each input from pixel, cells  307  corresponding to the unary portion are randomly selected by the multiplexers  313  and grouped to form most significant bits (MSBs) output from CDAC  231 . For the next input, the unary portion elements are shuffled and grouped again forming the same MSB nominal portions with different cells  307  being selected, breaking fixed pattern nonlinearity errors in the binary CDAC. In summary, cells  307  corresponding to the unary section are shuffled and the linearity errors are averaged over multiple frames to spread the INL error peaks at specific codes into other codes. 
     For the proposed scheme, extra randomization orthogonal to the MSB sections are introduced to the least significant bit (LSB) sections. As shown in the  FIG. 3 , replicas of LSB sections are instantiated and one of them is selectively chosen, with corresponding bits conveyed to the selected cells from SAR  233  through demultiplexer  303 . This way extra dynamic element shuffling further reduces fixed pattern errors in CDAC  231  in the average sense over multiple frames. In this embodiment, wherein the random selection of cells is performed in subsets orthogonal to one another (for the purposes or randomization) is referred to is two-dimensional orthogonal dynamic element matching, or 2DO-DEM. It is noted however that this particular randomization scheme is but one possible way. Generally speaking, any suitable randomization scheme for cell selection including data-directed methods may be performed within the scope of this disclosure. 
     Control of the randomization as discussed above may be controlled by random number generator (RNG)/logic  309 . These functions include random number generation which is provided to generate corresponding selection signals provided to the multiplexers  313  as well as to demultiplexer  303 . In one embodiment, RNG/logic  309  is a unit of the control circuitry implemented in sensor logic/control circuitry  111  of  FIG. 1 . However, this circuitry may be implemented separately as well. 
       FIG. 4  illustrates one embodiment of an apparatus coupled to convey a dithering signal to the input of an amplifier concurrent with another input signal. It is noted that, for the sake of illustration, only a portion of the circuitry of comparator  221  is shown. These portions include transistors M 41 -M 43 , capacitor Cin, and common mode reset switches Scm 1  and Scm 2 , which may reset the gate terminals of their respectively coupled devices to a common mode voltage, Vcm. 
     In the embodiment shown, comparator  221  is coupled to receive pixel data, Vpix, via a multiplexer on a first input, via multiplexer  209 . On the second input, the analog signal generated by CDAC  231  may be received. Additionally a dithering signal may also be applied to this input concurrent with application of the analog signal from CDAC  231 . Generally speaking, any suitable signal generation circuitry arranged to generate a dithering signal may be provided, with the corresponding input of the comparator is being coupled to receive the dithering signal. In this particular embodiment, a second DAC  405  is provided, its output coupled to the same input of comparator  221  as is the output of CDAC  231 . Control signals upon which the dithering signal is generated are provided from the control circuitry in sensor logic core/control circuitry  111  of  FIG. 1  for this particular embodiment. It is noted that embodiments are possible and contemplated wherein the DAC circuitry used to generate the dithering signal may be integrated into CDAC  231 , rather than as a separate DAC as shown here. However, as noted above, any suitable circuitry for generating a dithering signal may be provided in any embodiment falling within the scope of this disclosure. 
     The dithering signal applied to the input of comparator  221  as shown in  FIG. 4  may be a random signal. Application of a random dithering signal improves differential nonlinearity (DNL) performance of the SAR ADC, particularly at small input signal ranges associated with a camera application. The dithering technique discussed herein may randomize systematic linearity errors in CDAC  231  in addition to improving DNL. Furthermore, this particular embodiment applies the dithering signal before quantization on a per-frame basis. Accordingly the dithering signal, which is the same for a given frame, is removed after performing CDS without the introduction of extra noise in the output image, while also effectively minimizing small structured errors resulting from unit mismatch in CDAC  231 . 
       FIG. 5  is a flow diagram illustrating one embodiment of a method performing an analog-to-digital conversion in a SAR ADC having a CDAC. Method  50  as disclosed herein may be provided by any of the corresponding hardware embodiments of this disclosure. Additional hardware embodiments not explicitly disclosed herein may also perform method  50 , and thus may also fall within the scope of this disclosure. 
     Method  50  is generally directed to converting the analog pixel data into digital pixel data using a SAR ADC. The conversion of data includes the use of a CDAC, and thus includes randomly selecting CDAC circuit elements from a first subset (block  502 ). As shown above, the CDAC includes a two-dimensional array of circuit elements, while the converting includes the CDAC converting a digital value stored in a SAR of the SAR ADC into a corresponding analog signal. Thus, in this embodiment, converting the digital value into the corresponding analog signal comprises the control circuit causing random ones of the circuit elements being selected for generation of the corresponding analog signal, including those circuit elements of the first subset. Randomly selecting circuit elements of the first subset includes randomly selecting circuit elements associated with MSBs of the digital value stored in the SAR. Method  50  further comprises randomly selecting CDAC circuit elements from a second subset, wherein the selection of elements of the second subset is orthogonal with respect to selection of elements of the first subset (block  504 ). This in turn includes randomly selecting circuit elements associated with an LSB of the digital value stored in the SAR for performing the conversion to analog, and thus randomly selecting circuit elements associated with the LSB is performed orthogonally with respect to randomly selecting circuit elements associated the MSBs. 
     Upon selection of circuit elements of the first and second subset, method  50  further includes converting contents of the SAR to an analog signal, using the selected elements of the first and second subsets (block  506 ). Converting the data includes providing the corresponding analog signal (from the CDAC) to a first input of a comparator, providing the analog pixel data from the one of the plurality of pixel circuits to a second input of the comparator. This includes receiving the analog pixel data from the selected pixel circuit as a continuous signal, with no sample and hold performed (per block  508 , with no sample and hold circuitry coupled to the corresponding comparator input). The conversion also includes writing a result of comparing the corresponding analog signal and the analog pixel data from an output of the comparator to the SAR (block  510 ). This may be done in a manner of successive approximations, in accordance with the operation of a SAR ADC. 
     In addition to the random selection of circuit elements within the CDAC when performing a conversion of pixel data into the digital domain, further randomization may be achieved by randomly selecting pixel circuits for which pixel data is to be converted. Returning briefly to  FIG. 2 , the selection of pixel circuits  103  within a pixel unit may be randomized. For the embodiment illustrated in  FIG. 2 , this may be referred to as column shuffling, although the disclosure herein is not limited to this type of randomization in the selection of pixel circuits. 
       FIG. 6  is a flow diagram illustrating one embodiment of a method for selecting pixel circuits in the processing of pixel data. Method  60  as shown in  FIG. 6  is generally directed to the random selection of pixel circuits for the quantization of corresponding pixel data, and may apply to the embodiment of  FIG. 2 , as well as to other embodiments not explicitly discussed herein (but may nevertheless fall within the scope of this disclosure). 
     Method  60  includes randomly selecting a first pixel circuits for conversion of pixel data from analog to digital (block  602 ). In various embodiments, this includes randomly selecting the one of the plurality of pixel circuits to provide corresponding analog pixel data for digital conversion using a SAR ADC. Furthermore, randomly selecting may comprise a control circuit (such as that shown in  FIG. 1 ) causing a multiplexer to select the one of the plurality of pixel circuits. After data has been quantized (e.g., converted from analog to digital) for the first selected pixel circuit, the method may randomly select the next pixel circuit for conversion of pixel data from analog to digital (block  604 ). After this conversion is complete, if the number of unselected pixel circuits is greater than 1 (block  606 , yes), the next pixel circuit may be selected per block  604 . This loop may continue until the number of unselected pixel circuits is one (block  606 , no). Thereafter, pixel data from the last unselected pixel circuit is converted from analog to digital (block  608 ), and the method thereafter proceeds to the next frame (block  610 ) and repeats. 
     The randomization described above may reduce the effects of fixed pattern noise (FPN) which may otherwise result if the pixel circuits are selected in repetitive sequence. Randomly shuffling the sequence may reduce the FPN error to random noise. 
       FIG. 7  is a flow diagram illustrating one embodiment of a method for performing a comparison in which a dithering signal is applied along with an input signal to one input of the comparator. Method  70  may be performed using any of the various hardware embodiments discussed herein, as well as others that, by virtue of ability to perform the method, fall within the scope of this disclosure. 
     Method  70  includes applying an analog signal corresponding to SAR contents from a CDAC to a first comparator input (block  702 ). The method further includes generating a dithering signal and applying the dithering signal to the first input of the comparator concurrent with providing the corresponding analog signal (block  704 ). The pixel data is applied to a second comparator input as a continuous analog signal (block  706 ). Based on the signals provided to the first and second inputs, a comparison of their respective amplitudes is performed (block  708 ), with the results of the comparison provided to the SAR as part of the successive approximation performed in converting pixel data from analog to digital. Thereafter, method  70  proceeds to the next comparison (block  710 ) and repeats beginning with block  702 . 
     As previously discussed, the dithering signal may, in one embodiment, be generated by a DAC that is separate from the CDAC, or provided via extra circuitry integrated into the existing CDAC. Any other suitable signal generation circuitry may be used as well. The use of the dithering signal may improve DNL performance and thus reduce degradation to images produced by the image sensing system. 
     Circuit for Clock Feedthrough Compensation in Image Sensing Systems: 
       FIG. 8  is an illustration of one embodiment of a pixel circuit. Generally speaking, various embodiments of a pixel circuit as disclosed herein include a first driver circuit coupled to receive an analog pixel data signal, a transfer signal and a reset signal, a source follower transistor having a source terminal coupled to a column node and a gate terminal, wherein the gate terminal of the source follower is coupled to the first driver circuit and a second driver circuit coupled to receive the transfer signal and the reset signal, wherein the second driver circuit which may be shared by each column is capacitively coupled to the column node through a first capacitor. 
     The schematic shown in the left hand portion of  FIG. 8  generally illustrates an embodiment of a pixel circuit without compensation. In the embodiment shown, driver circuit includes a reset transistor, M 1 , and a transfer gate, M 2 . The reset transistor M 1  is coupled to receive a reset signal (RST), and when active, pulls the node Vfd up toward Vdd. This reset may be performed between the iterations of quantizing pixel data received and conveyed by driver  105  (shown in the box delineated by dashed lines). Assertion of the transfer gate (TG) signal activates transistor M 2 , thereby allowing pixel data (Vppd) to pass through the gate to node Vfd. This node is coupled to a gate terminal of transistor M 3 , which is coupled as a source follower to the column line (which may alternatively be referred to as the column node). Source follower M 3  may amplify the pixel data that is conveyed onto the column line, where it is subsequently passed through a multiplexer to a comparator input when the pixel data from this circuit is to be quantized. Capacitors Cppd and Cfd represent capacitances existing between their respective nodes and a return path. 
     The right hand portion of  FIG. 8  illustrates an embodiment of a pixel circuit  103  as implemented in, e.g., the pixel unit shown in  FIG. 2  and a compensation circuit  106 . Compensation driver  106  in this embodiment is coupled to the column lines by capacitors Ccal 1  and Ccal 2 , although a single capacitor may be used instead. These capacitors may be adjustable or trimmable depending on the pole from the pixel driver path and zero from the compensation driver path. The capacitors block any DC voltage from compensation circuit  106 , and thus the circuit is AC coupled to the column line. 
     As with the column lines shown in  FIG. 1 , that shown in  FIG. 8  include a current source, Ic, to provide current on the respective signal path. For simplicity, typical pixel row selection switch between source follow (M 3 ) and column line is not shown. 
     The effect of the operation of compensation driver is to provide a correlated disturbance voltage to the column line, which in turn results in faster settling time. In particular, the disturbance voltage may drive the column line (or column node) to its steady state value faster owing to the introduced zero in the feedforward path, as a result of the disturbance voltage, than would otherwise occur in the absence of the compensation driver. For a pixel driver such as that shown in the left-hand side of  FIG. 8 , the transitions resulting from assertion of the RST and TG signal may result in the column signal, Vcol, become disturbed from an effect known as clock feedthrough. This can introduce a non-uniform CDS residue pattern for that occurs for a certain number of pixels and corresponding column lines due to incomplete settling. The accumulated result of this may be increased FPN. By reducing the required settling time, as a result of the compensation driver, lower power may be consumed for a given noise budget, and/or noise may be reduced for a given timing budget. Further accuracy of the disturbance voltage can be attained through tuning the adjustable capacitors shown here (or single adjustable capacitor if only one is present). 
       FIG. 9  is a flow diagram illustrating one embodiment of a method for operating a pixel circuit. Method  90  may be performed by various embodiments of the pixel circuit disclosed herein, as well as other embodiment not explicitly disclosed. 
     Method  90  begins with the assertion of a reset signal and performing a reset of a main pixel driver and a compensation pixel driver (block  92 ). In one embodiment, the main and compensation pixel drivers may have a similar or identical circuit topology, although their coupling to a column node may be different. Performing the reset includes conveying a first voltage to a gate terminal of a source follower transistor, wherein a source terminal of the source follower transistor is coupled to a column node (or column line), and conveying a second voltage to the column node, the second driver circuit being capacitively coupled to the column node. Referring back to  FIG. 8 , conveying the first voltage may comprise pulling the Vfd node of the main pixel driver toward Vdd. While the compensation circuit provides controlled disturbance that is pulled toward Vdd with a potential DC offset, the capacitance coupled between it and the column line blocks the transfer of DC components, and thus the second voltage is primarily an AC component related to the switching of the corresponding reset transistor. 
     Note that the proposed clock feedthrough scheme can be applied to similar pixel topologies of which performance is impacted by the clock feedthrough regardless of the ADC architecture. 
     Method  90  further includes asserting a transfer gate signal into both the main and compensation driver circuits (block  94 ). As a result of the assertion of the transfer gate signal, a voltage corresponding to the pixel data is conveyed to the gate terminal of a source follower coupled between the main driver and the column node (block  96 ). This results in another voltage being conveyed to the column node, the voltage corresponding to clock feedthrough amplified by the source follower. The method further includes conveying, responsive to assertion of the transfer gate signal, a disturbance voltage, via the capacitor(s), from the compensation driver to the column node (block  98 ). The disturbance voltage may include a version of the clock feedthrough voltage and controlled AC components of the transfer gate signal, and may cause settling to a steady state value to occur faster on the column node. 
     High Speed, Low Power Image Sensing: 
       FIG. 10  is a diagram illustrating one embodiment of an apparatus for controlling the slope of a power enable signal and a diagram illustrating operation of the same. In the embodiment shown, comparator  221  is shown as being coupled to an RLC circuit (including a power supply, resistor R, inductor L, and a capacitor C), for the sake of illustration. The comparator  221  is part of an ADC, such as that shown in  FIG. 1 . 
     Comparator  221  in the embodiment shown includes a current source, I 1 , which may be implemented as a transistor or other suitable circuitry. This current source is coupled to receive a power enable signal from DAC  241 . In turn, DAC  241  may receive control signal from control circuitry in, e.g., sensor logic core/control circuitry  111 . These control signals may be used to control the slope of the power enable signal to provide a power up and power down sequence with minimum disturbance to the supply. DAC  241  may generate the power enable signal by converting the control signals from their received digital values into an analog signal. Comparator  221  may thus power up and power down in a manner that reduces power supply noise, as the current demand does not change as fast. This can reduce voltage droops and power supply noise that can affect image quality. 
     As shown in the timing diagram, the power enable signal may be used to cycle the power of the comparator during a quantization cycle. As previously noted, one embodiment of the image sensing system in which comparator  221  is implemented performs correlated double sampling, or CDS, in which data from pixel circuits is quantized twice. This may be performed under the control of the control circuitry discussed above. Within the first quantization cycle, comparator  221  may be powered on to process the rest input signal, powered down after the first quantization, powered up again for the pixel image signal, and then powered down. Accordingly, comparator  221  may be powered down during a settling time of a column line, and powered up to process the pixel output, powered down for the next settling time, and powered up for the next pixel output. Thereafter, the comparator  221  may be powered down again and remain so until the next CDS cycle. As can be seen in the diagram, the slope of the power enable signal is shaped such that the increase in current draw is smooth during the power up portion, while the decrease in current draw is smooth during the power down portion. Accordingly, power supply transients resulting from the powering on and off of comparator  221  are minimized if not eliminated. As a result, power supply noise from this power cycling is correspondingly minimized or eliminated. 
     In addition to controlling the shape of the power enable signal, the control circuitry in the system may also monitor for ringing that may result from powering up and powering down comparator  221 . Responsive to this monitoring, the control circuitry may adjust the control signals and thus, adjust the slope of the power enable signal provided to the various instances of comparator  221  to reduce or eliminate the ringing in real time. Other factors may be monitored for as well, with the control circuitry making adjustments to the control signals, and thus the slope of the power enable signal, as needed. 
     Conventional power cycling, in which the slope of the power signal is not shaped, based on the response or characteristic of the distributed supply and package may result in increased power supply disturbance due to surge currents. The power cycling shown in the timing diagram as shown here may result in a reduction of both fixed pattern and random noise in the image. 
     In addition to the noise benefits described above, cycling the power to comparator  221  may result in reduced power consumption. In image sensing systems having a significant number of ADCs, this reduction can be significant, as the comparators may actually spend more time powered down than powered up, given that they are powered up only during processing pixel data. 
     Another power cycling feature that may be performed is to stagger the power enable signals to different comparators to prevent multiple comparators (or too many) from powering up at substantially the same time. 
     Note that the proposed system response based power enable signal slope shaping can be applied to any low power ADC architectures. 
       FIG. 11  is a diagram illustrating one embodiment of an integrate-and-reset pre-amplifier used in an embodiment of a low power comparator optimized for the image sensor outputs. In the embodiment shown, pre-amplifier  227  includes an amplifier having transistors M 5  and M 6  having respective gate terminals for receiving input signals In 1  and In 2 . These signals may be the pixel data and analog value output from the CDAC as discussed above. A current source Io is coupled to draw current through M 5  and M 6 . The output nodes of pre-amplifier  227  are Out+ and Out−, on the drain terminals of M 5  and M 6 , respectively. Capacitors CL are coupled between each of the drain terminals and power supply node Vdd. A pair of reset switches (‘Reset’) are provided across the capacitors. These switches, when closed, may perform a reset by discharging any potential different across their respective capacitor CL. This reset may be performed periodically or as needed. 
     In image sensing systems, the overall readout power consumption may be dominated by a comparator pre-amplifier implemented in the ADC. In the embodiment shown in  FIG. 11 , the comparator pre-amplifier power may be optimized to the fundamental limit for the target signal-to-noise ratio (SNR) based on the signal processing technique. More specifically, the matched filter theory predicts that the SNR of the amplifier with input signal characteristic specific to the image sensor system (i.e. approximately DC signal) depends only on the average current applied (i.e. available energy). Therefore, the optimal comparator pre-amplifier in the image sensor ADC in terms of SNR for the given power may be the integrator and reset type structure as shown in  FIG. 11 . With this comparator pre-amplifier structure, the overall ADC power in the image sensor system may achieve minimum power for the given SNR specification. 
     Note that the pre-amplifier based on integrate-and-reset can be employed for any ADCs for low power image sensor applications. 
       FIG. 12  is a table illustrating repeat quantization patterns usable in one embodiment of a low power image sensing system for target RN and FPN under the constraint of fixed total quantization number. The quantization patterns shown here may be performed under the control of the control circuitry discussed above. The bit positions correspond to pixel circuits and therefore, to pixel data from these pixel circuits. In the embodiment shown, two different possible repeat patterns are shown. In the first pattern, labeled here as Rpt 4 , a quantization cycle (which may include performing CDS) is performed only once for bits B 9 , B 8 , B 7 , B 5 , and B 4 . Meanwhile, for B 6  and B 3 -B 0 , quantization is repeated once for each, and thus performed twice overall in each of these positions. This pattern is referred to herein as a weighted repeat pattern, where some bits are weighted and thus their sampling process is repeated at least once. 
     In the second pattern, Rpt 8 , repeat quantization is performed for the same bits, although bits B 3  and B 2  are weighted more than the others. In this example, quantization for B 3  and B 2  are repeated three times, for a total of four quantization each for these bits. Generally speaking, the number of repeats for a given position may be repeated any suitable number of times while reducing repeats in other bits to maintain total number of quantization. 
     Generally speaking, the weighted repeat methodology discussed herein may be performed in a wide variety of patterns without changing total number of quantization (hence, total energy). The weighted repeat may be used during the SAR ADC process to improve not only random noise but also bias (hence, FPN) with a small incremental hardware for the given power budget. One simple example to introduce redundancy is repeating the SAR processing multiple times for the least significant bit (LSB) to improve random noise performance. However, the blind repeat of LSB for SAR ADC in the image sensor system can cause larger fixed pattern noise. Accordingly, additional and/or different bits may be selected for repeat quantization in accordance with the various embodiments disclosed here. It is further noted that the control circuitry discussed above can change the repeat patterns if conditions warrant. Furthermore, patterns may be implemented wherein quantization of some bits are repeated once, while other bits undergo repeat quantization more than once. 
       FIG. 13  is a flow diagram of one embodiment of a method for powering up and powering down a comparator using a slope-controlled power enable signal. Method  130  may be performed with any of the various hardware embodiments discussed herein. Image sensing system capable of performing method  130  not explicitly discussed herein may nevertheless fall within the scope of this disclosure. 
     Method  130  begins with the conveying of pixel data as an analog signal to a comparator (block  131 ). The pixel data may be conveyed from a pixel circuit that is selected according to control signals (e.g., selection signals) provided by control circuitry. The pixel data may be conveyed through a multiplexer to the comparator of a SAR ADC. The control circuitry may also power up the comparator to which the pixel data is conveyed, using a slope controlled power enable signal (block  132 ). The shape of the power enable signal may, in one embodiment, be controlled based on control signals conveyed from the control circuitry to a DAC. 
     With the DAC powered on, the comparison may be performed (block  133 ), the result thereof being stored in a SAR of a SAR DAC. Upon completing the quantization, the control circuit may cause the comparator to be powered down again under the control of the slope controlled power enable signal (block  134 ). Thereafter, the method proceeds to the next quantization (block  135 ) and repeats. 
       FIG. 14  is a flow diagram illustrating one embodiment of a method for selectively repeating a quantization process for particular ones of a number of pixel circuits. Method  140  may be performed by any of the embodiments of an image sensing system disclosure herein, as well as others not explicitly disclosed herein but nevertheless falling within this disclosure&#39;s scope. 
     Method  140  may be performed using either of the repeat patterns shown in  FIG. 12 , or other patterns not explicitly discussed herein. The method begins with the selection of a pixel circuit for performing a quantization process of pixel data (block  141 ). If the pixel circuit is one that is designated for a repeat (block  142 , yes), the quantization process (which may include CDS) maybe repeated the designated number of times (block  144 ). If the pixel is not one designated for repeat quantization (block  142 , no), one quantization process of the pixel data is performed (block  143 ). After quantization of the designated pixel data is complete, the next pixel circuit is selected, and the method repeats from block  142 . 
     Turning next to  FIG. 15 , a block diagram of one embodiment of a system  150  is shown. In the illustrated embodiment, the system  150  includes at least one instance of an integrated circuit  10  coupled to external memory  158 . The integrated circuit  10  may include a memory controller that is coupled to the external memory  158 . The integrated circuit  10  is coupled to one or more peripherals  154  and the external memory  158 . A power supply  156  is also provided which supplies the supply voltages to the integrated circuit  10  as well as one or more supply voltages to the memory  158  and/or the peripherals  154 . In some embodiments, more than one instance of the integrated circuit  10  may be included (and more than one external memory  158  may be included as well). 
     The peripherals  154  may include any desired circuitry, depending on the type of system  150 . For example, in one embodiment, the system  150  may be a mobile device (e.g. personal digital assistant (PDA), smart phone, etc.) and the peripherals  154  may include devices for various types of wireless communication, such as WiFi, Bluetooth, cellular, global positioning system, etc. The peripherals  154  may also include additional storage, including RAM storage, solid-state storage, or disk storage. The peripherals  154  may include user interface devices such as a display screen, including touch display screens or multitouch display screens, keyboard or other input devices, microphones, speakers, etc. In other embodiments, the system  150  may be any type of computing system (e.g. desktop personal computer, laptop, workstation, tablet, etc.). 
     Various embodiments of the IC  10  and/or peripherals  154  may include circuitry for implementing one of the various image sensing systems discussed above. As shown in  FIG. 1 , portions of the system may be on different chips, although implementations wherein the entirety of the image sensing system is implemented on a single chip are possible and contemplated. 
     The external memory  158  may include any type of memory. For example, the external memory  158  may be SRAM, dynamic RAM (DRAM) such as synchronous DRAM (SDRAM), double data rate (DDR, DDR2, DDR3, LPDDR1, LPDDR2, etc.) SDRAM, RAMBUS DRAM, etc. The external memory  158  may include one or more memory modules to which the memory devices are mounted, such as single inline memory modules (SIMMs), dual inline memory modules (DIMMs), etc. 
     Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.