BACKGROUND 
     CMOS image sensors based on an active pixel design have gained wide acceptance in camera applications. In such sensors each pixel in the final image is generated by a pixel sensor that includes a photoreceptor that accumulates charge during an exposure. The accumulated charge is converted to a voltage by an output amplifier that is typically constructed from a source follower transistor that receives the charge at its gate and drives a bit line that is connected to the readout circuitry in the imaging array. The signal on the bit line is then digitized using an analog-to-digital converter (ADC) that is connected to the bit line. 
     In cameras that utilize global shutters, the image is projected on an array of pixel sensors after all of the photodiodes in the pixel sensors have been reset. All of the photodiodes accumulate charge in an amount that depends on the light intensity received by that photodiode. At the end of the exposure, the charge accumulated by each photodiode is transferred to a floating diffusion node in the pixel sensor, and the photodiode is isolated from the floating diffusion node, thereby ending the exposure. The charge remains on the floating diffusion node until the pixel sensor in question is readout. During the storage of charge on the floating diffusion node, the node accumulates additional electrons from noise sources in the array of pixels and the surrounding processing circuitry. The contribution of such “noise electrons” to the final image sensor intensity value depends on the amount of time the charge remains on the floating diffusion node. Compensation for this noise source remains a challenge in the search for designs that reduce the overall noise in CMOS imagers. 
     SUMMARY 
     The present invention includes an apparatus and method for forming a digital image. The apparatus includes a plurality of pixel sensors and a controller. Each pixel sensor includes a photodiode, a floating diffusion node that can be selectively connected to said photodiode or a reset voltage, and an ADC connected to the floating diffusion node, the ADC converting a voltage on the floating diffusion node to a digital value. Each pixel sensor also includes an output circuit that connects the ADC to a bit bus. The apparatus also includes a controller that causes the ADCs to operate in parallel to convert the voltages on the floating diffusion nodes to the digital values in a time that is less than the time needed for the floating diffusion node to acquire ten electron equivalents of noise. The optional apparatus includes circuitry that allows correlated double sampling to be performed in each of said pixel sensors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a prior art CMOS imaging array. 
         FIG. 2  is a schematic drawing of a typical prior art pixel sensor. 
         FIG. 3  is a schematic drawing of a pixel sensor that utilizes a distributed ADC according to one embodiment of the present invention. 
         FIG. 4  illustrates a pixel sensor according to another embodiment of the present invention. 
         FIG. 5  illustrates another embodiment of a pixel sensor according to the present invention. 
         FIG. 6  illustrates an imaging array according to one embodiment of the present invention. 
         FIG. 7  is a more detailed view of pixel sensor  41 . 
     
    
    
     DETAILED DESCRIPTION 
     The manner in which the present invention provides its advantages can be more easily understood with reference to  FIG. 1 , which is a block diagram of a prior art CMOS imaging array. Imaging array  40  is constructed from a rectangular array of pixel sensors  41 . Each pixel sensor includes a photodiode  46  and an interface circuit  47 . The details of the interface circuit depend on the particular pixel design. However, all of the pixel circuits include a gate that is connected to a row line  42  that is used to connect that pixel to a bit line  43 . The specific row that is enabled at any time is determined by a bit address that is input to a row decoder  45 . 
     The various bit lines terminate in a column processing circuit  44  that typically includes sense amplifiers and column decoders. Each sense amplifier reads the signal produced by the pixel that is currently connected to the bit line processed by that sense amplifier. The sense amplifiers may generate a digital output signal by utilizing an ADC. At any given time, a single pixel sensor is readout. The specific column that is readout is determined by a column address that is utilized by a column decoder to connect the sense amplifier/ADC output from that column to circuitry that is external to the imaging array. 
     Refer now to  FIG. 2 , which is a schematic drawing of a typical prior art pixel sensor. Pixel sensor  20  includes a photodiode  27 , which is preferably a pinned photodiode, that is coupled to a floating diffusion node  28  by gate  21 . During the exposure of the imaging array to the image being recorded, charge accumulates in photodiode  27 . The accumulated charge is transferred to floating diffusion node  28  by applying a signal to gate  21 . The charge transferred to floating diffusion node  28  is converted to voltage by the parasitic capacitor  29  associated with the gate of transistor  23 , which is connected as a source follower. Transistor  23  provides the gain needed to drive bit line  26  when pixel sensor  20  is connected to that bit line via a signal on row select line  25  that is coupled to the gate of transistor  24 . Prior to transferring charge from photodiode  27  to floating diffusion node  28 , the potential on gate  21  is reset to a predetermined potential via gate  22 . However, there are small variations in the final charge on floating diffusion node  28  after the reset. 
     A procedure known as correlated double sampling is used to compensate for these variations. The potential on floating diffusion node  28  is then measured by connecting pixel sensor  20  to bit line  26 . After this starting potential is measured, the charge that accumulated on photodiode  27  is transferred to floating diffusion node  28  and the potential on floating diffusion node  28  is again measured by connecting pixel sensor  20  to bit line  26 . The difference in the signal between the two potential measurements is the light intensity value that is reported for pixel sensor  20 . 
     As noted above, during the time the charge “sits” on floating diffusion node  28  prior to being readout, the charge is corrupted by electrons, or holes, that are generated in other pixels, the surrounding silicon, or the circuitry present in the decoders and other processing circuitry in the imaging array. Since the silicon in which the pixels are formed is exposed to light, photoelectrons and holes are generated in the underlying silicon. These noise charges migrate to the diffusion nodes and alter the charge on the diffusion nodes. The longer the photo-generated charge from the photodiode remains on floating diffusion node  28 , the greater the corruption of the signal from the pixel sensor. Since the pixel sensors are readout one row at a time, pixel sensors in the last row to be readout accumulate more noise than pixel sensors that are readout earlier in the readout process. This variation in storage time complicates attempts to correct for the noise. 
     The noise that is accumulated on the floating diffusion node can be characterized in terms of the change in voltage on the node that would arise from one noise electron or hole moving onto the floating diffusion node. This level of noise will be referred to as an electron equivalent of the noise level. If the noise can be reduced to the point that the change in voltage is much less than the change in voltage that would occur if one electron were added or subtracted from the charge on the floating diffusion node, further improvements will not provide any benefit. In fact, the charge that is transferred to the floating diffusion node for any given light exposure will have a statistical variation with a standard deviation on the square root of N, where N is the average number of photoelectrons that are generated for the light exposure. This statistical noise is most problematic at low values of N where a noise level equivalent to a few electrons can contribute errors that are of the same level as the statistical noise. 
     The present invention reduces storage dependent noise and its variation with storage time by digitizing all of the pixel sensor signals in parallel and performing the digitization in a time that is small compared to the time in which the noise on the floating diffusion node exceeds ten electron equivalents, and preferably less than the time needed to exceed three electron equivalents. In a still further embodiment, the storage time is less than the time in which the noise on the floating diffusion node exceeds one electron equivalent. In effect, the present invention provides an ADC in each pixel sensor that operates on the charge generated in that pixel sensor. Refer now to  FIG. 3 , which is a schematic drawing of a pixel sensor that utilizes a distributed ADC according to one embodiment of the present invention. Pixel sensor  100  includes a photodiode  197  that is connected to a transfer gate  191  and a reset gate  192  that operate in a manner analogous to that described above. After the pixels in the array containing pixel sensor  100  have been exposed, the charge that accumulated on each of the photodiodes is digitized in parallel. 
     Since the digitization is carried out in parallel, only the process with respect to pixel sensor  100  will be explained in detail. However, it is to be understood that each pixel in the imaging array carries out the same process. At the start of the digitization process, the charge stored on photodiode  197  is coupled to node  102  by applying a signal to transfer gate  191  in parallel with the corresponding gates in the other pixel sensors in the array. Counter  104  is then reset and begins to count clock pulses while the potential on the other input of the comparator  103  is increased. The potential on the ramp line is linearly related to the count that has accumulated in counter  104 . When the ramp potential is equal to the potential at node  102 , the comparator  103  generates a stop signal that is applied to counter  104  and causes the counter to stop counting clock pulses. Hence, counter  104  is left with a count that is related to the potential at node  102 . In this embodiment, the pixels are readout one row at a time using a row decoder that operates a set of row select switches  105  that connect the counter output to a bit bus  106  that is associated with the pixel sensors in the column in which pixel sensor  100  is located. The bit bus serves a function analogous to the bit lines described above, albeit the signal on the bit line is in digital format instead of the analog format described above. In this embodiment, the bit bus includes one line per bit in counter  104 . Hence, the counter is readout in parallel down bit bus  106 . While this embodiment utilizes a bit bus that reads out the counter bits in parallel, embodiments in which the bits in the counter are shifted down a single conductor bit line can also be constructed. 
     It should be noted that all of the data stored as an analog charge in the pixels is digitized at the same time; hence, the analog-to-digital conversion of the data is reduced to the time needed to digitize the charge in a single pixel sensor. Once the data has been digitized, the photodiodes can be reset by placing reset gate  192  and transfer gate  191  in the conducting state. A new exposure can then be commenced while the data stored in the counters is being readout. Since the data is in digital form, the readout time can be significantly less than the time needed to readout the analog signal from the source followers used in the above-described prior art systems. 
     As noted above, variations in the reset potential on node  102  can also lead to noise in the final image. Embodiments of the present invention that carry out a procedure analogous to the correlated double sampling procedures described above can be utilized to reduce this noise. Refer now to  FIG. 4 , which illustrates a pixel sensor according to another embodiment of the present invention. Pixel sensor  110  utilizes an up/down counter  114 . The direction of the count is controlled by a signal on control line  109  that is shared by all of the pixels in the imaging array. Prior to transferring the charge collected on photodiode  197  to node  102 , node  102  is reset via reset gate  192 . The potential on node  102  is then digitized and stored in up/down counter  114  in a manner analogous to that discussed above. In this phase of the readout, up/down counter  114  is reset to 0, and control line  109  directs up/down counter  114  to count up. After up/down counter  114  has been loaded with a count indicative of the potential on node  102  after the reset operation, the charge on photodiode  197  is transferred to node  102 . The transferred electrons reduce the voltage on node  102 . The voltage on node  102  is then digitized and subtracted from the count in up/down counter  114  by setting the control signal on control line  109  accordingly. When the signal on node  102  is less than the ramp voltage, the counting operation is stopped, and up/down counter  114  will be left with the corrected count indicating the charge transferred from photodiode  107  corrected for the reset noise on node  102 . Other methods for combining the measured reset voltage with the voltage produced by the charge collected by the photodiode could also be used. 
     It should be noted that the resetting of node  102  and the loading of up/down counter  114  can be performed while photodiode  197  is accumulating charge during the exposure. The reset operation is preferably performed near the end of the image exposure so that the node will be reset and up/down counter  114  will be loaded just prior to the end of the exposure. Hence, node  102  will not have had time to accumulate a significant number of noise electrons prior to the charge from photodiode  197  being transferred to node  102 . 
     While performing the digitization of the photodiode accumulated charge in parallel avoids the problems associated with the charge being stored on node  102  for varying amounts of time depending on the position of the pixel sensor in the array of sensors, to provide the desired overall noise reduction, the time between the resetting of node  102  and the completion of the digitization of the charge from node  102  should be as small as possible. In the following discussion, the time between the resetting of node  102  and the time the value on node  102  is digitized after the charge is transferred from photodiode  197  to node  102  will be referred to as the “digitization time”. 
     In one aspect of the present invention, the digitization time is set to be less than the time needed to accumulate ten electron equivalents of noise. In another aspect, the digitization time is set to be less than the time needed to accumulate three electron equivalents. In a still further aspect of the invention, the digitization time is set to be less than the time needed to accumulate one electron equivalent of noise. 
     In the above-described embodiments of the present invention, the ramp used by the digitization circuitry is typically a linear ramp and the same ramp is used for digitizing both the reset potential and the actual signal from the photodiode. However, embodiments in which the ramp is non-linear and/or different ramps are used during the double sampling procedure can provide additional benefits. Refer now to  FIG. 5 , which illustrates another embodiment of a pixel sensor according to the present invention. Pixel sensor  120  includes a memory  121  that is used to store a value from counter  122  and provide that value on bit bus  106  during the readout process. In this embodiment, the readout also includes a first phase in which node  102  is reset to a potential near V dd  by placing reset gate  192  in the conducting state while leaving photodiode  197  isolated from node  102 . The potential on node  102  is then digitized in a manner analogous to that described above. The result of the digitization is held in counter  122  until it is transferred to memory  121  prior to the start of the second phase. A first ramp is used during the digitization in the first phase. 
     In the second phase, the charge stored on photodiode  197  is transferred to node  102  and digitized using a second ramp. The final digital value corresponding to that charge is left in counter  122 . Again, all pixels in the camera area are processed in parallel using the same ramps. The values stored in the various pixel sensors are readout onto the corresponding bit buses  106  in two readout phases. In the first phase, the values stored in memory  121  are readout. In the second phase, the values stored in counter  122  are readout by row select circuit  125 . The two values are then combined to provide the pixel value representing the light exposure for the pixel in question that is corrected for variations in the reset potential on node  102 . 
     The optimal form for the ramp used to digitize the reset voltage on node  102  will, in general, be different from the ramp used to digitize the actual pixel value. The reset voltage on node  102  is ideally V dd ; however, due to noise in the system, the actual reset voltage differs slightly from this ideal value. Denote the smallest reset voltage that is expected by R min  and the largest by R max . Using a ramp that increases linearly from zero to a voltage slightly above V dd  is less than optimum, since the all of the pixels will generate counts that differ by a small number corresponding to the range of voltages around V dd . If the increase in the ramp voltage corresponding to one count in counter  122  is denoted by DV, the counts will range from R min /DV to R max /DV. In essence, all of the range below R min  is wasted. This results in a decrease in accuracy for the measured reset voltage and a waste of time while waiting for the ramp to increase to R min . As noted above, minimizing the time over which node  102  is subjected to noise is an important factor in reducing the overall noise in the image. 
     A better ramp for measuring the reset voltage would be one that starts at a value just below R min  and has its maximum count corresponding to a voltage at or just above R max . If the same number of counts are used, i.e., the full range of counter  122 , then the accuracy with which the reset voltage is known is significantly improved. 
     In one aspect of the present invention, the ramp used to digitize the signal from photodiode  197  is a non-linear, or a piecewise linear ramp. The value of the signal obtained by transferring the electrons stored on photodiode  197  to node  102  after node  102  has been reset can, in principle, vary from a value near zero to V dd . If this range is covered using a linear ramp, the digitization noise for signal values near zero can be significant. 
     For the purposes of this discussion, an ADC is defined to be a circuit that converts an analog signal between a minimum voltage and a maximum voltage to a digital value between 0 and N−1. For convenience, it will be assumed that the minimum voltage is 0, and the maximum voltage is V dd . All input voltages that are between 0 and V dd /N are converted to a digital value of 0, input voltages between V dd /N and 2V dd /N are converted to a digital value of 1, and so on. Hence, the signal value represented by any given output digital value may be in error by as much as ±V dd /(2N). This error will be referred to as digitization noise in the following discussion. The digitization noise depends on the number of steps provided by the ADC. Hence, the digitization noise could, in principle, be lowered by increasing N. However, there is a practical limit to the size of N. In addition, the size of counter  122  and memory  121  increases with the size of N, and hence, the fill factor for the pixel sensor decreases with increasing N, unless larger CMOS dies are used to construct the imaging array. 
     In the case of the ramp used to digitize the reset voltage, the digitization noise is (R max -R min )/2N. Since R max  is close to R min , the digitization noise in the reset voltage measurement is often negligible. Hence, the goal of utilizing a non-linear ramp applies mainly to the measurement of the signal generated by the photodiode in each pixel sensor. 
     In many applications, the error measurement of interest is the error as a percentage of the total pixel signal value. For example, the human eye cannot detect small percentage differences in intensity. Hence, in an image that is to be viewed by a human observer, the digitization noise will be masked by the lack of sensitivity of the eye once the noise is reduced to some predetermined percentage of the pixel signal value. Low light values correspond to higher signal values at node  102 , since the number of photoelectrons transferred to node  102  will be small, and hence, the potential at that node will remain near the reset potential. Accordingly, a ramp that has a smaller slope at values near V dd  than at values near zero provides the desired property. The ramp could be piecewise linear or a function that changes slope over time in a continuous manner. By altering the slope as a function of time, the range of the ADC can be extended without requiring larger counters or conversion times. Since the shape is known to the controller in the camera or imaging array, the conversion back to a linear intensity scale can be performed after the image is off-loaded from the pixel sensors. 
     Refer now to  FIG. 6 , which illustrates an imaging array according to one embodiment of the present invention. Imaging array  200  includes a plurality of pixel sensors such as pixel sensor  241 . The pixel sensors are organized as a plurality of rows and columns. The pixel sensors in each row are connected to a corresponding row bus  242 , and the pixel sensors in each column are connected to a corresponding column bus  243 . 
     Refer now to  FIG. 7 , which is a more detailed view of pixel sensor  241 . Pixel sensor  241  includes a photodiode  197  that can be connected to node  102  by transfer gate  191 . Node  102  is can also be connected to V dd  by reset gate  192 . The voltage on node  102  is digitized by ADC  234  in a manner analogous to that described above. Any of the embodiments discussed above can be utilized for ADC  234 . ADC  234  can include storage for the result of one of the conversions as well as the counter discussed above. ADC  234  is controlled from conductors in bus  231 . The specific control lines will depend on the specific embodiment of the ADC. Conductors from this bus also supply the control signals that operate transfer gate  191  and reset gate  192 . It should be noted that bus  231  is connected to all pixels in imaging array  200  and controls operations that are performed in parallel in each pixel sensor. This bus can be operated from controller  250  as is the case for imaging array  200  or from circuitry that is not part of imaging array  200 . To simplify  FIG. 6 , the connections between bus  231  and the individual pixel sensors have been omitted from the drawing. The circuitry that generates the ramp and clock signals is part of controller  250  in this embodiment. However, that circuitry could be external to controller  250 . 
     The readout operations are controlled by the row buses shown at  242 . Each pixel sensor  241  includes a row select circuit  233  that connects the digital value or values stored in the attached ADC to the corresponding column bus  243 . The exact structure of the row select circuits will in general depend on the number of storage elements in ADC  234 . For example, if ADC  234  includes both a counter and a memory as shown in  FIG. 5 , row select circuit will include gates for separately connecting the counter and the memory to column bus  243  in response to a signal on row bus  242 . The row buses are driven by a row controller  245 . Row controller  245  receives a row address from controller  250  and provides the relevant control signals to the row selectors in the row identified by that row address. 
     Imaging array  200  does not require an external shutter. Just prior to the beginning of an exposure, all of the pixel sensors can be reset by connecting the photodiodes in each pixel sensor to V dd  by placing transfer gate  191  and reset gate  192  in their conducting states. As long as the photodiodes are so connected, any photoelectrons generated by light from the image projected on imaging array  200  will be removed from the photodiodes. The exposure can then be started by isolating the photodiodes by closing transfer gates  191  in all of the pixels. During the exposure, node  102  can be held at V dd  by leaving reset gate  192  in the conducting state in all of the pixel sensors. Just prior to the end of the exposure, reset gate  192  is closed. If correlated double sampling is being implemented in imaging array  200 , the potential on node  102  is digitized by ADC  234  and stored in ADC  234 . Transfer gate  191  is then placed in the conducting state for a period of time sufficient to sweep all of the photoelectrons that have accumulated in the photodiode  197  onto node  102 . Transfer gate  191  is then closed and the voltage on node  102  digitized. The pixel sensors are then readout one row at a time by controller  250 . 
     Controller  250  can perform additional functions related to generating the actual image, or these functions can be performed by a separate controller that is external to imaging array  200 . For example, if correlated double sampling is utilized, controller  250  can correct for fluctuations in the reset voltage on node  102  if that correction has not been done in the individual pixel sensors. 
     In addition, controller  250  can correct for variations in the ADCs in imaging array  200 . The above-described embodiments assume that all of the ADCs are identical. However, there may be variations in threshold values in the comparators. For example, the comparators may include an amplification stage that provides the high input impedance needed to digitize the voltage on node  102  without significantly depleting the charge stored on node  102 . Differences in the amplification stages from pixel sensor to pixel sensor can result from different gains in the amplification stages, leading to differences in the amount by which the voltage on node  102  must be less than the ramp voltage to stop the counter in the ADC. These differences in threshold value can be measured by exposing the array to a uniform light source and measuring the counter outputs for varying lengths of exposures. A count increment or decrement for each pixel can then be derived. Controller  250  can store this calibration map and make the required count alterations at the end of each exposure. 
     The above-described embodiments of imaging arrays according to the present invention utilize an array of pixel sensors that are arranged as a plurality of rows and columns in which the digitized pixel values are readout one row at a time via a plurality of column buses. However, other arrangements could also be utilized. Typically, the final image will be readout one pixel value at a time from the controller in the imaging array. Since the pixel values have all been digitized and stored in the individual pixel sensors, there is no need to speedup the readout from the pixel sensors to the controller to reduce noise. In applications in which the time between exposures is sufficient to allow the readout of the pixel sensors to the final storage location for the image, little is gained by reading out the pixel sensors in parallel, since a row will be stored in the controller while the row is readout one value at a time to the image storage memory in the camera. In such embodiments, all of the pixel sensors can be readout on a single readout bus, one pixel sensor at a time. The specific pixel sensor that is readout at any given time is determined by a token that is passed from sensor to sensor on the bus. 
     The above-described embodiments of the present invention have been provided to illustrate various aspects of the invention. However, it is to be understood that different aspects of the present invention that are shown in different specific embodiments can be combined to provide other embodiments of the present invention. In addition, various modifications to the present invention will become apparent from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.
An apparatus and method for forming a digital image are disclosed. The apparatus includes a plurality of pixel sensors and a controller. Each sensor includes a photodiode, a floating diffusion node that can be selectively connected to said photodiode or a reset voltage, and an analog-to-digital converter (ADC) connected to the floating diffusion node, the ADC converting a voltage on the floating diffusion node to a digital value. Each pixel sensor also includes an output circuit that connects the ADC to a bus. The apparatus also includes a controller that causes the ADCs to operate in parallel to convert the voltages on the floating diffusion nodes to the digital values in a time that is less than the time needed for the floating diffusion node to acquire ten electron equivalents of noise. The optional apparatus includes circuitry that allows correlated double sampling to be performed in each sensor.