Abstract:
An image sensor ( 10 ) which can be fabricated using conventional CMOS processes uses a comparator circuit ( 18 ) at each pixel ( 14 ) having a first input coupled to a photodetector ( 16 ) and a second input coupled to a ramp signal generator ( 30, 32 ). The ramp signal generator ( 30, 32 ) is comprised of a counter ( 32 ) and a D/A conversion circuit ( 30 ) with the analog output of the D/A conversion circuit ( 30 ) forming an analog ramp input to the comparator circuit ( 18 ). A counter circuit ( 32 ) can be used to drive the digital side of the D/A conversion circuit ( 30 ) and configured to count from 0 to 2 n −1 to 0, N being the resolution of the photodetector ( 16 ). The output of the D/A conversion circuit ( 30 ) causes comparator circuit ( 18 ) to flip when the ramp signal is equal to the value of the output from the photodetector ( 16 ). The comparator circuit ( 18 ), in turn, drives a load signal to a register ( 38 ) which stores the counter values  32  from pixel ( 14 ) at the instant the comparator  18  flips. In this way an A/D conversion of the image data takes place.

Description:
This application claims priority under 35 USC §119(e)(1) of Provisional Application No. 60/083,477, filed Apr. 29, 1998. 
    
    
     TECHNICAL FIELD 
     The present invention relates in general to an image signal processing system and more specifically to an image sensor circuit and related method of converting image signals from their analog to digital equivalent by placing a comparator circuit at each pixel in the image sensor array. 
     BACKGROUND OF THE INVENTION 
     Solid state imaging systems have been in use for a number of years in high-tech devices such as medical instruments, satellites and telescopes. More recently, imagers have been employed in a wide array of mainstream applications such as digital cameras, camcorders and scanners. Most of these applications use Charge-Coupled Devices (“CCD”) to build the underlying solid state image sensors. 
     For various reasons, CCD-based image sensors are limited or impractical for use in many consumer applications. First, CCDs require at least two polysilicon layers with a buried-channel implant to-achieve their high performance, meaning that they cannot be fabricated using standard CMOS fabrication processes. Second, the level of integration that can be achieved with CCD-based imagers is low since they can not include the devices necessary to integrate them with other devices in the application. Finally, the circuits used to transfer data out of the image array to other devices on the system board, such as Digital Signal Processors (“DSPs”) and other image processing circuits, have a large capacitance and require voltages higher than the other circuits. Since the currents associated with charging and discharging these capacitors are usually significant, a CCD imager is not particularly well suited for portable or battery operated applications. 
     As such, less expensive image sensors fabricated out of an integrated circuits using standard CMOS processes are desirable. Essentially, with a CMOS type imager sensor, a photo diode, photo transistor or other similar device is employed as a light detecting element. The output of the light detecting element is an analog signal whose magnitude is approximately proportional to the amount of light received by the element. CMOS imagers are preferred in some applications since they use less power, have lower fabrication costs and offer higher system integration compared to imagers made with CCD processes. Moreover, CMOS imagers have the added advantages that they can be manufactured using processes similar to those commonly used to manufacture logic transistors. 
     An important signal processing circuit is the analog to digital convertor (“ADC”). In the last few years, CMOS imagers have been developed with the ADC on the imager itself. The optimal place for the ADC is immediately after the photosensor, i.e., on the pixel itself. An example of a prior CMOS image sensor is described in the article entitled “A 128 by 128 Pixel CMOS Area Image Sensor With Multiplex Pixel Line A/D Conversion”, IEEE 1996 Custom Integrated Circuits Conference, Yang, David X. D., Fowler, Boyd, Gamal, EL Abbas. In their article, the authors describe an image sensor consisting of an array of pixel blocks wherein each block further consists of a group of four nearest neighbor pixels sharing a single Analog to Digital (“A/D”) convertor. 
     A limitation inherent to such sensors is the use of over-sampling A/D conversion methods which require a clock rate well above the image frame rate. The need to keep the conversion rate high in such image sensors requires a substantial amount of drive current making the sensor impractical for many mainstream applications including battery powered or portable devices. 
     Another problem common to prior art CMOS image sensors is the amount of fixed pattern noise due to beta variations from pixel to pixel which can often be seen with the naked eye. Other undesirable features of prior art CMOS image sensors include large comparator offsets, high complexity of the A/D conversion circuitry, high power dissipation and the inability to achieve a non-linear response for certain applications. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, the disadvatages and problems associated with the prior CMOS image sensors are substantially reduced or eliminated. 
     According to one embodiment, disclosed is an image sensor which can be fabricated using a conventional CMOS process. A comparator circuit is placed at each pixel having a first input connected to a photodiode, photo transistor or other similar light detecting element, and a second input connected to a ramp signal generator. Since the comparator circuits are relatively simple devices, they are small enough to be fabricated within individual pixel cells and are effective at eliminating the noise associated with prior art conversion techniques. 
     In one embodiment, the ramp signal generator comprises a Digital to Analog Conversion “DAC” circuit that drives one input of the comparator circuit. A counter can be coupled to the digital side of the DAC circuit and configured to count from 2 N−1  to 0, N representing the resolution of the light detecting element. 
     In one embodiment, the pixels are arranged in a two-dimensional array of columns and rows. For each row of pixels, the counter drives the DAC circuit which, in turn, outputs an analog signal proportional to the value received from the counter. A converter and register are connected so that the output of the counter drive the register, and the load signal of the register is connected to the output of the comparator circuit. The output of the DAC circuit is fed to the comparator circuit which flips when the ramp signal equals the value of the light detecting element. The counter value loaded into the register at the time the comparator flips is the digital representation of the analog output of the light detecting element. 
     In another embodiment, the ramp signal can be generated by other means, such as a capacitor fed by a constant current source, the output of the capacitor being followed by a unity gain voltage buffer whose output is the ramp signal. 
     In another embodiment, when a nonlinear response is desired, a programmable memory means such as ROM, EEPROM, or RAM may be employed to store values corresponding to the desired response curve. The values may be loaded into the DAC circuit and converterd as herein described to obtain a non-linear response. 
     According to another embodiment, each comparator circuit is reset to eliminate variations in comparator offsets by resetting the pixels to their settling or zero light value prior to the analog digital conversion sequence. 
     According to another embodiment, a single D/A conversion circuit is associated with the entire pixel array which may consist of N by M pixels. The D/A conversion circuit drives only a single row at a time with the corresponding comparator circuit in that row driving the associated register loads at one time. 
     Other advantages of the present invention, including specific implementations, are understood by reference to the following detailed description taken in conjunction with the appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a block diagram of a CMOS A/D per pixel imager for a 2×2 array according to one embodiment; 
     FIG. 2 is a schematic diagram of a circuit for single pixel at row M column N; 
     FIG. 3 is a block diagram for the registor load signal pre-charge circuit; 
     FIG. 4 is a block diagram of a sample D/A conversion circuit with output weighting for each bit D 1 :D 8 ; 
     FIG. 5 is a chip-level block diagram of a per pixel imager according to one embodiment; 
     FIG. 6 is a timing diagram for the pixel readout sequence corresponding to a single row within the imager; and 
     FIG. 7 is a timing diagram for the RESET and EXPOSE signal sequences. 
     Numerals in the figures refer to corresponding parts in the description unless otherwise indicated. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a block diagram of the CMOS A/D per pixel imager  10  according to a preferred embodiment of the invention. As shown, a pixel array  12  comprises a plurality of pixels  14  arranged in a two-dimensional array of columns and rows. Within each of the individual pixels  14 , a photodetector  16  is predisposed to receive light when exposed to a light emitting source. The photodetector  16  can be a photo-diode, photo-transistor or other similar device as is known to those of ordinary skill. 
     The output from the photodetector  16  is a first input to the comparator circuit  18 . A second input into the comparator circuit  18  comes from the Digital to Analog (“D/A”) conversion circuit  30  whose output on the comparator circuit side is an analog signal. As shown, a counter circuit  32  is coupled to the D/A conversion circuit  30  through the bus  34 . Preferably, the width of bus  34  is n bits corresponding to the resolution of the photodetector  16 . A clock signal  36  is applied to the counter  32  which determines the counting rate of the counter  32 . The output from the counter  32 , in turn, drives the D/A conversion circuit  30  which controls the magnitude and rate of change of the analog signal output from the D/A conversion circuit  30 . The speed of clock signal  36  determines the slope of the analog signal. 
     For each pixel  14 , a comparator circuit  18  is attached to both the photodetector  16  and the D/A conversion circuit  30  as shown. In one embodiment, the output of the D/A conversion circuit  30  is a ramp signal that reaches the value of the output of the photodetector  16  at some point in time. 
     The output of each comparator circuit  18  acts as a load signal for registers  38  through switch  20 . The registers  38  load the value of the counter circuit  32  at the moment that the output of the D/A conversion circuit  30  equals or exceeds the output of the photodetector  16 . At this time, a conversion of the analog signal from the photodetector  16  is converted to its digital equivalent as represented by the count of counter  32  on signal  34 . This is done for all pixels  14  in columns  1  through N in a rom M at the same time in parallel. The conversion is done for each row sequentially, preferably for all pixels for  14  in columns  1  through N in a row M at the same time in parallel. Thus the conversion is done for each row sequentially. 
     Preferably, the D/A conversion circuit  30  drives only one row of pixels within the array  12  at a time. It should be understood, however, that more or less pixels within the array  12  may be read during a given cycle and that the reading sequences may vary. Thus, a single pixel or the entire image may be read during a single cycle according to various embodiments. 
     As shown, a pixel RESET signal  26  is coupled to each of the photodetectors  16  within the array  12 . Pixel RESET  26  provides a way of canceling offsets to limit the differences, or offsets, among individual comparator circuits  18  and photodetectors  16  within the pixel array  12 . The offsets in the comparator circuits  18  are canceled and photodetects  16  by pixel RESET  26  greatly reduce or even eliminate the fixed pattern noise associated with pixel cells  14 . Preferably, the pixel RESET  26  places the pixels  14  in the array  12  at their settling or zero light value prior to activation of the D/A conversion circuit  30 . While this technique permits each comparator  18  within individual pixels  14  to be adjusted to compensate for offsets in both  16  and  18 , it should be understood that additional methods of fixed pattern noise cancellation may be utilized. 
     Turning next to FIG. 2, a schematic diagram for an individual pixel  14  of the pixel array  12  is shown having two voltage bias inputs V bias  and V low . The bias voltages (V bias , V low ) are the same for every pixel. Preferably, V low  is the minimum expected voltage output achievable by an individual comparator circuit  18 . V bias , on the other hand, should be one CMOS threshold voltage below the supply rail V dd . 
     As shown, a photo-diode  16  is used as the photodetecting element. The EXPOSE signal  24  connects the photo-diode  16  to comparator  18  through transistor M 2 . Preferably, a pixel RESET  26  is asserted prior to the EXPOSE signal  24 . As shown, a pair of complementary NMOS transistors (M 3 , M 4 ) may be used in the comparator circuits  18  to receive the outputs from the photo-diode  16  and D/A conversion circuit  30 . 
     For D/A output signals greater than the output from the photo-diode  16 , the output at row M column N is a logical zero. As a D/A output is decreased to a value equal to the photo-diode output, the inverter formed by transistors M 7  and M 8  is flipped. 
     While the row signal is an NMOS passgate  50  which can easily pull the pixel output LOW, it is not sufficient for pulling it HIGH. As such and according to one embodiment, every column output  40  is pre-charged high by the corresponding register  38  and then pulled low when the comparator  18  flips. 
     An example of a register load signal pre-charge circuit suitable for this purpose is shown in FIG.  3  and denoted generally as  70 . An individual register  38  is pre-charged using the pull-up transistor  72  to bring the column line output  40  HIGH prior to flipping of the comparator circuit  18 . This permits the pixel  14  to pull the column line  40  LOW at the appropriate time as determined by the row select switches  20  and the clock cycle  36  of the counter circuit  32 . Preferably, all pixels  14  in a row are read at the same time by loading the outputs from the corresponding comparators  18  into the registers  38  during a single cycle. In one embodiment, row select signals (Row 1 , Row 2 ) are provided to activate the read sequence after RESET. Other methods of reading the pixel outputs can be devised within the scope of the invention. 
     Register  38  can be designed to loads D IN  on the rising edge of LD. If so, once LD is HIGH, it will not load new data until LD goes LOW and then goes HIGH again. 
     FIG. 4 is a circuit diagram for a suitable D/A convertor circuit  30  for use in an image sensor according to one embodiment. The D/A convertor circuit  30  must be monotonic with a continuously decreasing output. The output of the D/A convertor circuit  30  is dependent on the digital bit stream sequence D 1 :D 8  delivered by the counter circuit  32 . The output of the D/A convertor circuit  30  can be non-linear, as long as it is still monotonic, if the bit stream D 1 :D 8  is other than step-wise linear. 
     In one embodiment, the counter circuit  32  comprises a programmable memory means such as a ROM, RAM, or EEPROM device which drive the waveform output from the transistors Q 1  through Q 8  to be linear, non-linear or other desired shape. Transistors Q 1  through Q 8  can be weighted to give the desired output for each bit in the bit input stream sequence D 1 :D 8 . It should be understood, however, that the D/A conversion circuit  30  of FIG. 4 is but one example of a signal generator suitable for driving pixels  14 . For example, a ramp signal generator could be used instead of the D/A conversion circuit  30  to provide step output that drives the pixels  14  in the array  12 . Other methods may be utilized as will be apparent to those of ordinary skill. 
     Turning to FIG. 5, an integrated circuit chip CMOS imaging sensor according to one embodiment is shown and denoted generally as  100 . The N×M image array  120  of the chip  100  consist of M rows and N columns of pixels  14  configured as described above in FIG. 1 with respect to pixel array  12 . For the example shown in FIG. 5, a 4×M imager chip is illustrated with 4 registers  38  being driven by the image array  120 . M can be equal to N, but this is not necessary. 
     The imager chip  100  incorporates the control logic  102  and shift register  104  which comprise the input/output interface for the imager chip  100  and allow external control of the chip  100  and general communications with external processing systems via the control/clock line  106  and data out line  108 , respectively. As the registers  38  are loaded with pixel array data from the image array  120 , individual image bits are passed to the NP bit shift register  104  where N represents the number of columns and P represents the number of bits in the output sequence from the counter  32 . The NP bit shift register  104  allows the imager chip  100  to deliver the image data to an outside system using output bus  108 , while the next row of image data is being read out of the image array  120 . 
     As shown, the control logic  102  receives control signals  106  which operate the imager chip  100  including the counter circuit  32 , the NP bit shift register  104  and registers  38 . In one embodiment, the control lines  106  consist of a clock, RESET and EXPOSE signals. The control lines  106  are used to operate various features of the chip  100  including the cancellation of offsets, the EXPOSE time cycle of the image array  120 , and the frequency rate of the clock. It should be understood, however, that various other functions of the imager chip  100  may be controlled via the control lines  106  depending on the application in which the chip  100  is used. 
     The control logic  102  is shown coupled to the NP bit shift register  104  and the registers  38  to control the shifting of data to the outside world via bus  106 . Preferably, the outputs from all pixels in the image array  120  are switching so that the D/A convertor  30  drives only one row of pixels at a time and thus, only one comparator circuit  18  within a column will drive a register load signal  22  at a time. The reading sequence for pixels in the array  120  can vary depending on how the outputs from the individual pixel cells  14  are loaded into the registers  38 . 
     FIG. 6 is a timing diagram for the pixel readout sequence of an arbitrary pixel in the array  120  at column N and row M. As shown, the RESET line  26  and EXPOSE line  24  stay LOW the entire time. The output from the D/A convertor circuit  30  is the only analog signal in the system, as all other signals including row M, register RESET and column N are digital. 
     Column N is the output of the pixel in column N and row M and is set HIGH by the register RESET  26 . Assuming the counter circuit  32  counts from its maximum value down to 0, the D/A output goes from its highest value to its lowest value. The column N goes LOW when the D/A output equals the voltage stored on the gate M 3 , as shown in FIG.  2 . This permits the pixels  14  along row M to drive the corresponding register load signal  22  at time T 4 . 
     A timing diagram for the RESET and EXPOSE signal sequences is shown in FIG.  7 . In contrast to the signal sequence of FIG. 6, the signals row  1  through row M stay LOW the entire time while the counter output (2 N −1) remains at its maximum value throughout. As indicated, the only critical times are T 4  and T 5 . For T 4 , with the signal pixel RESET going LOW before the EXPOSE signal goes HIGH. Likewise, the EXPOSE signal goes HIGH as soon as possible after the pixel RESET goes LOW. T 5  is the expose cycle time of the image array  120 . In one embodiment, the time T 5  can be adjusted according to light level of the image to be captured. Thus, the brighter the light, the shorter the exposure time. 
     While the invention has been described in conjunction with preferred embodiments, it should be understood that modifications will become apparent to those of ordinary skill in the art and that such modifications are intended to be included within the scope of the invention and the following claims.