Abstract:
A unit cell ( 20 ) is disclosed that has an input node for coupling to an output of a detector (D 1 ) of electromagnetic radiation, such as IR or visible radiation. The unit cell includes a first capacitor (C intA ) switchably coupled to the input node for receiving a charge signal from the detector, and for integrating the charge signal during a first integration period, as well as a second capacitor (C intB )switchably coupled to the input node for integrating the charge signal during a second integration period. The unit cell further includes an output multiplexer ( 32, 34 ) for selectively coupling the first capacitor and the second capacitor to an output signal line ( 38 ) during respective charge signal readout periods. In the preferred embodiment a duration of the first integration period is one of greater than or less than the second integration period, and the first integration period is one of non-overlapping or overlapping with the second integration period, and vice versa. The first integration period can be interleaved with the second integration period, or vice versa.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates in general to detectors of electromagnetic radiation and, more specifically, this invention pertains to unit cell circuitry for integrating electrical charge generated by a detector of visible or infrared radiation (IR).  
         BACKGROUND  
         [0002]    In U.S. Pat. No. 5,043,820, issued on Aug. 27, 1991, entitled “Focal plane array readout employing one capacitive feedback transimpedance amplifier for each column”, Wyles et al. disclose a readout circuit for use with a focal plane array that employs a single transistor in each unit cell and a single capacitive feedback transimpedance amplifier (CTIA) to process the outputs of each column of detector elements ofthe array. The capacitive feedback transimpedance amplifiers extract the signals associated with the pixels along a particular row of the array. The disclosed invention permits high performance readouts to be constructed with little circuitry in the unit cells. For example, only a single minimum sized transistor switch is required in each unit cell to perform readout and reset functions for the array.  
           [0003]    Referring to FIGS. 1 and 2, in a disclosed embodiment the readout circuit includes an array of unit cells  1 , where each unit cell  1  includes a detector input circuit  1 A, a transistor (TR) and a single charge storage capacitor (CAP). Row address circuits  2  are coupled to the cells in each row of the array  1 , and a plurality of capacitive feedback transimpedance amplifiers  3  are coupled to the cells in each column of the array. The amplifiers process charge derived from the detector elements and stored in the charge storage capacitor (CAP) of each unit cell  1 . Column multiplexing circuits multiplex the output signals provided by each of the amplifiers. Column address circuits  4  are coupled to the column multiplexing circuits which couple output signals from each of the multiplexer circuits as the output from the readout circuit.  
           [0004]    The charge mode readout made possible by using the circuit shown in FIG. 1 is achieved by using a CTIA at each column output. The CTIA operates to hold the vertical busline connecting a column of unit cells  1  at a virtual ground, while extracting the charge from the unit cell integration capacitors. The charge mode output, direct injection input circuit generally provides the largest integration storage capacity in the smallest unit cell circuit area. General reference with regard to CTIAs can be made, as examples, to U.S. Pat. No. 4,978,872, “Integrating Capacitively Coupled Transimpedance Amplifier”, Morse, Gaalema, Keimel and Hewitt, and to U.S. Pat. No. 6,121,843, “Charge Mode Capacitor Transimpedance Amplifier”, Vampola and Hewitt.  
           [0005]    While well suited for use in many important applications, it has been found that the unit cell design illustrated in FIG. 1 does not lend itself to implementing a true “snapshot integrate-while-read” capability. The use of snapshot integration is desirable for operation in highly dynamic imaging systems to prevent the creation of image artifacts that can be generated by non-simultaneous scene-derived charge integration, sometimes referred to as “rolling frame integration”.  
           [0006]    This problem is related to a problem that can be experienced by IR sensors that operate in an IRST (IR Search and Track) mode of operation. In the IRST mode the sensors must accommodate a large dynamic range. A typical response to the wide dynamic range requirement is to obtain two signal samples that are closely-spaced in time with different integration or exposure times, e.g., a low gain sample with a short integration time and a high gain sample with a long integration time. In the conventional circuit having a single integration well per detector input, as in FIG. 1, the two integration times are separated by a full readout frame time (e.g., tens of milliseconds when operating with a 30 Hz frame rate). As can be appreciated, when the scene radiation is rapidly changing this amount of delay can be objectionable, and may result in ambiguous scene data being generated.  
           [0007]    In an attempt to provide a snapshot integrate-while-read capability with the conventional unit cell circuitry it has been known to add a sample/hold capacitor to each unit cell. While allowing snapshot integration, this approach also attenuates the input signal, and reduces the amount of charge that can be accumulated. Also, since charge division occurs between the integration capacitor and the sample/hold capacitor, a non-uniformity in the ratio of these two capacitors can result in an undesirable variation in gain over the two-dimensional array of unit cells.  
         SUMMARY OF THE PREFERRED EMBODIMENTS  
         [0008]    The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings.  
           [0009]    A unit cell is disclosed that has an input node for coupling to an output of a detector of electromagnetic radiation, such as IR or visible radiation. The unit cell includes a first capacitor switchably coupled to the input node for receiving a charge signal from the detector, and for integrating the charge signal during a first integration period, as well as a second capacitor switchably coupled to the input node for receiving a charge signal from the detector and for integrating the charge signal during a second integration period. The first capacitor has a value that is one of equal to or different than a value of the second capacitor. The unit cell further includes an output multiplexer for selectively coupling the first capacitor to an output signal line during a first charge signal readout period, and for selectively coupling the second capacitor to the output signal line during a second charge signal readout period. The first and second integration periods, and the first and second readout periods, occur within one frame period. In the preferred embodiment a duration of the first integration period is one of greater than or less than the second integration period.  
           [0010]    The first integration period is one of non-overlapping or overlapping with the second integration period, and vice versa, and the first integration period can be interleaved with the second integration period, or vice versa.  
           [0011]    In one mode of operation the first and second capacitors are switchably coupled to the input node during an integration period for simultaneously receiving and integrating the same charge signal from the detector.  
           [0012]    In the preferred embodiment of this invention the unit cell is one of a plurality of unit cells of an array of unit cells, where the array is a two dimensional array arranged in rows and columns, and where the output signal line comprises a column output signal line. In the preferred embodiment the column output signal line is coupled to an input of a capacitive transimpedance amplifier for converting a readout charge signal to a voltage. A threshold detector may be employed for detecting whether a readout charge signal exceeds a threshold indicating a saturation condition for enabling the other readout charge signal to be selected for use.  
           [0013]    The unit cell further includes switching circuitry for selectively resetting one or more of the input node and the first and second capacitors by selectively coupling one or more of the input node and the first and second capacitors to a reset voltage.  
           [0014]    Also disclosed is an array of unit cells, where each unit cell includes an input node for coupling to an output of a detector of electromagnetic radiation; a first capacitor switchably coupled to the input node for receiving a charge signal from the detector and for integrating the charge signal during a high gain integration period; a second capacitor switchably coupled to the input node for receiving a charge signal from the detector and for integrating the charge signal during a lower gain integration period; and an output circuit for coupling the first capacitor to a first output signal line during a charge signal readout period, and for coupling the second capacitor to a second output signal line during the charge signal readout period. The array further includes a first amplifier having an input coupled to the first output signal line, a second amplifier having an input coupled to the second output signal line, a comparator for comparing an output of the first amplifier to a saturation threshold signal, and an output multiplexer, coupled to an output of the comparator, for selecting one of the output of the first amplifier or the output of the second amplifier to be coupled to further circuitry. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The foregoing and other aspects of these teachings are made more evident in the following Detailed Description ofthe Preferred Embodiments, when read in conjunction with the attached Drawing Figures, wherein:  
         [0016]    [0016]FIG. 1 is circuit diagram of a conventional unit cell circuit having a single integration capacitor;  
         [0017]    [0017]FIG. 2 is a block diagram of a conventional two dimensional imager containing an input circuit array, row address circuitry, column amplifiers and column address circuitry;  
         [0018]    [0018]FIG. 3 is a circuit diagram of a high capacity direct injection (DI) input circuit in accordance with an embodiment of this invention, that contains two integration capacitors for enabling true snapshot integrate-while-read capability;  
         [0019]    [0019]FIGS. 4-7 each illustrate the use of the DI input circuit of FIG. 3 in one of a plurality of operational modes over an exemplary 30 Hz (33.3 microsecond) frame time; where  
         [0020]    [0020]FIG. 4 is a diagram that illustrates the use of the DI input circuit of FIG. 3 in a minimal latency, dual integration integrate-then-read (ITR) mode of operation;  
         [0021]    [0021]FIG. 5 is a diagram that illustrates the use of the DI input circuit of FIG. 3 in an interleaved (no latency), dual integration integrate-then-read mode of operation;  
         [0022]    [0022]FIG. 6 is a diagram that illustrates the use of the DI input circuit of FIG. 3 in a high capacity, dual integration integrate-then-read mode of operation;  
         [0023]    [0023]FIG. 7 is a diagram that illustrates the use of the DI input circuit of FIG. 3 in a minimal latency, dual integration integrate-while-read (IWR) mode of operation; and  
         [0024]    [0024]FIG. 8 shows the use of an optional per-column threshold detector and signal sample selection logic;  
         [0025]    [0025]FIG. 9A shows an embodiment where the input circuit is constructed as a buffered direct injection (BDI) or feedback enhanced direct injection (FEDI) circuit;  
         [0026]    [0026]FIG. 9B is a diagram of the detector, input circuit and the integration capacitors connected to one column busline;  
         [0027]    [0027]FIG. 10 shows an embodiment where the integration capacitors of FIG. 9B are disposed in a plurality of integrated circuit chips that are stacked one upon another;  
         [0028]    [0028]FIG. 11A shows a multi-mode input circuit embodiment with independent output bus lines; and  
         [0029]    [0029]FIG. 11B is a schematic diagram showing how the use of the independent column bus lines of FIG. 11A allows both high and low gain samples to be read out simultaneously. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    [0030]FIG. 3 is a circuit diagram of a high capacity, charge mode DI input unit cell circuit  20  in accordance with an embodiment of this invention. The unit cell circuit  20  is constructed to include a plurality of integration capacitors, such as two in the presently preferred embodiment, designated C intA  and C intB . A radiation detector, shown as a photovoltaic diode (D 1 )  22 , also referred to herein as a detector  22 , may or may not be located in the unit cell  20 . In general, the unit cell circuit  20  can be used with a monolithic detector  22  (where the detector and unit cell circuit share the same substrate), or with a hybrid detector  22  (where the detector and unit cell circuit are on different substrates that are interconnected by, for example, Indium or solder bumps, or by plated metal vias).  
         [0031]    In the illustrated embodiment the anode of D 1  is connected to a voltage V detcom  and the cathode is connected to the unit cell  20  via an input circuit embodied as a first transistor (an injection transistor)  24  that is driven by a signal V ig . The signal V ig  is adjusted to establish a suitable bias across the photovoltaic diode D 1 . The circuit node to which the cathode of D 1  is connected is also connected to a second transistor  26  for inputting a test signal TestIn under control of signal pTestEn. This test-related circuitry and signals are not required for operation of the unit cell  20  of this invention, and are shown in FIG. 3 for completeness. The injection transistor  24  that is driven by the signal V ig  is connected at an output side to third and fourth transistors  28  and  30  (pIntA and pIntB) that, when on, apply the input signal to C intA  and C intB  respectively. C intA  and C intB  are connected to a column output line  38  via fifth and sixth transistors, functioning as switches  32  and  34  (Row 13  selA and Row_selB), respectively, that provide an output multiplexer function for the unit cell  20 . The output side of the first transistor  24 , and the input sides of the third and fourth transistors  28  and  30 , are also connected to a seventh transistor  36  (pNoInt) that is used for applying zero volts (circuit ground) to C intA  and C intB  for resetting same to a known potential. Note that the pNoInt transistor  36  may also be employed as an anti-bloom device for D 1 . While described as a p-channel transistor, an n-channel transistor could be used as well (as is the case for the other transistors).  
         [0032]    In an illustrative and non-limiting embodiment of this invention the unit cell  20  is contained within a 25 micrometer square circuit area, and C intA  and C intB  are equal valued capacitors having a nominal value of about 740 femtofarad (fF). In this case each of C intA  and C intB  may store up to about 20.3×10 6  electrons of charge. In other embodiments C intA  and C intB  are not equal in value.  
         [0033]    In a most basic mode of operation, and at the beginning of an integration period, both D 1  and C intA  and C intB  are reset through the pNoInt transistor  36  to zero volts. Note that reset can also be accomplished through the two select transistors  32  and  34  via an optional reset transistor (not shown) that is attached to the column line  38 . At the end of the reset operation, the pNoInt transistor  36  is opened, and the photocurrent generated by D 1  integrates onto C intA  and C intB  through the closed pIntA and pIntB transistors  28  and  30 , respectively. Note that only one of the pIntA and pIntB transistors  28  and  30  need be closed during the integration period, storing photocurrent on only one of C intA  and C intB . At the end of the integration period the pIntA and pIntB transistors  28  and  30  are opened, leaving the integrated charge stored on at least one of C intA  and C intB . The integrated signal charge is then read out row by row by turning on one or both of the row select transistors  32  and  34 , thereby transferring the stored charge to the column line  38 . The column line  38  is connected to the input of a one-per-column charge sensing amplifier which converts the charge signal to a proportional voltage, which can occur in the manner discussed in the above-referenced U.S. Pat. No. 5,043,820.  
         [0034]    The unit cell  20  can be operated in a number of different modes in both an integrate-then-read (ITR) and an integrate-while-read (IWR) manner, as shown in FIGS. 4-7 and described in further detail below.  
         [0035]    [0035]FIG. 4 is a diagram that illustrates the use of the DI input circuit of the unit cell  20  in a minimal latency, dual integration ITR mode of operation. This mode enables both long and short integration samples to be obtained with a minimal latency (e.g., less than about 10 microseconds) between samples. This mode thus advantageously reduces tracker-mode artifacts that could potentially occur if the first sample was required to be read out prior to taking the second sample, where a full frame delay (e.g., 33.3 milliseconds) would occur between the first and the second samples.  
         [0036]    In the mode illustrated in FIG. 4, and after the initial reset period (where pNoInt is closed), transistor switch  28  is closed and transistor switch  30  is opened so that the photocurrent is first integrated onto C intA  for a short period of time (e.g., 100 microseconds, a low gain sample). After a second (short) reset period (e.g., less than 100 microseconds), transistor  28  is opened and transistor  30  is closed to integrate the photocurrent on C intB  for a longer period of time (e.g., less than about 5.3 milliseconds, a high gain sample). Having thus stored both the low and high gain charge samples, the output multiplexer is operated twice to first readout C intA  (during period Read 13  A), and then readout C intB  (during period Read_B). Note that during the readout period transistors  28  and  30  are open (off) and transistor switch  36  (pNoInt) is closed (on) for holding D 1  reset. The cathode of D 1  can be connected to, for example, five volts, assuming a five volt process and a detector diode having p-on-n polarity. It should be noted that this circuit will operate just as well with an n-on-p polarity diode, provided that the voltage polarity and the transistor types are reversed.  
         [0037]    [0037]FIG. 5 is a diagram that illustrates the use of the DI input unit cell circuit  20  of FIG. 3 in an interleaved (no latency), dual integration ITR mode of operation. Note that this mode is similar to that shown in FIG. 4, except that the integration times are interleaved rather than being sequential. The longer integration time (the high gain sample) is actually split into two periods separated by the shorter integration time (the low gain sample). This allows both the long and the short integration samples to be made with essentially zero latency. In this embodiment the photocurrent is first integrated onto C intB  for some portion, such as half, of the desired integration period. After an optional short reset period (e.g., less than about 10 microseconds) during which pIntA and pIntB transistors  28  and  30  are both open, and the pNoInt transistor  36  is closed (to clear the charge on the input node), transistor pIntA  28  is closed to integrate photocurrent onto C intA  for a significantly shorter period of time. After another optional short reset period, photocurrent is again integrated onto C intB , which still stores the photocurrent from the first integration period, for the remainder of the longer, high gain integration period. Having thus stored both the low and high gain charge samples, the output multiplexer is operated twice to first readout C intA  (during period Read 13  A), and then readout C intB  (during period Read 13  B), as was discussed above with respect to FIG. 4.  
         [0038]    [0038]FIG. 6 is a diagram that illustrates the use of the DI input unit cell circuit  20  of FIG. 3 in a high capacity, dual integration ITR mode of operation. This mode is useful when a higher charge capacity is desired for one of the two samples in the ITR mode, as both integration capacitors are used together and are charged in parallel from D 1 . In this non-limiting and exemplary case, combining both integration capacitors increases the charge storage capacity to over 40×10 6  electrons. Dual integration is still possible to obtain, as only one of the two charge samples need be read out, and the other integration capacitor can be reset and used to acquire another integration sample. In the example illustrated in FIG. 7 charge is initially stored onto both C intA  and C intB . Assuming an exemplary 100 microsecond Int 13  A integration period, this mode allows a flux of greater than, as a non-limiting example, 6.5×10 16  photons/cm 2  to be integrated without saturation. The charge stored on one ofthe integration capacitors (e.g., the charge stored on C intA ) is saved while the charge stored on C intB  is reset (transistors pIntB and pNoInt  30  and  36  are closed and transistor pIntA  28  is opened). Then, during the higher gain and longer integration period Int_B, transistor pIntB  30  is closed to allow the second sample to be integrated onto C intB . The output multiplexer is then operated twice to first readout C intA  (during period Read 13  A), and then readout C intB  (during period Read 13  B), as was discussed above with respect to FIGS. 4 and 5.  
         [0039]    In the IWR mode of operation, the two capacitors are used in a ping-pong fashion to allow the integration and the readout periods to be overlapped, as in the embodiment shown in FIG. 7. While photocurrent is being integrated on C intA , the charge stored on C intB  is read out, and vice versa. The two integration periods can be timed so that the latency between the two samples is small (e.g., less than about 10 microseconds).  
         [0040]    Referring to FIG. 8, where one sample represents a high gain signal and the other sample represents a lower gain signal, where the ratio of the integration times can be considered the gain ratio if C intA  and C intB  are equal in value, then a decision circuit can be used to detect a saturation condition in the high gain sample, allowing the lower gain sample to be selected for use. As an example, assume a plurality of the unit cells  20  coupled to the vertical column line  38  feeding the input of a CTIA  40 . The output of the CTIA  40  is a voltage (V OUT ) having a magnitude that is indicative of the amount of integrated photocurrent currently read out of one of C intA  or C intB  of a unit cell  20  of a currently selected row of unit cells. In this case the decision circuit may be embodied as a voltage comparator  42  for comparing V OUT  with some threshold voltage (fixed or programmable). The output of the comparator  42  indicates one of a saturated or a not-saturated condition, depending on whether V OUT  is less than or equal to or grater than the threshold voltage. If a saturated condition is indicated then the high gain sample can be discarded, and the lower gain sample used instead by downstream signal processing circuitry.  
         [0041]    While described in the context of presently preferred embodiments, those skilled in the art should appreciate that this invention is not limited for use with only these disclosed embodiments. As an example, FIG. 9A shows an embodiment where the input circuit is constructed as a buffered direct injection (BDI) or feedback enhanced direct injection (FEDI) circuit. This type of input circuit employs a feedback amplifier  23  to bias the gate of the injection transistor  24  in order to obtain a more stable detector bias voltage as the photodiode  22  output current increases. As another example, more than two integration capacitors may be employed per unit cell  20 , with a corresponding increase in the number of transistor switches. This can be seen in the embodiment of FIG. 9B, which is a diagram of the detector  22 , the input circuit (shown generically as an input circuit  24 A, e.g., the injection transistor  24  embodiment of FIG. 3 or the FEDI embodiment of FIG. 9A) and N integration capacitors connected to one column busline  38 .  
         [0042]    Further, this invention should not be construed as being limited to any of the above-mentioned integration periods, or frame times, or component values, or charge storage capacities. The use of the unit cell  20  is also not restricted to two dimensional arrays of unit cells, as linear unit cell arrays can benefit from this invention as well, as can individual unit cell embodiments. The unit cell  20  can be constructed using CMOS processes, or any desired integrated circuit fabrication process. Furthermore, FIG. 10 shows an embodiment where, as an example, the N integration capacitors of FIG. 9B are disposed in a plurality (N) of integrated circuit chips  50  that are stacked one upon another in a three dimensional (3D) manner. In this embodiment the interconnections between the chips  50  can be made using any suitable vertical interconnection means, such as Indium bump technology, solder balls, or plated metal vias.  
         [0043]    It is also within the scope of this invention to provide a column comparator to select the high or low gain signal values from the unit cells  20 . For example, FIG. 11A shows a multi-mode input circuit embodiment with independent output bus lines  38 A and  38 B, and FIG. 11B illustrates the use of the independent column bus lines  38 A,  38 B to enable both high and low gain samples to be read out simultaneously. In this embodiment the two unit cell output transistors  32  and  34  are connected to the separate column bus lines  38 A and  38 B, respectively. The charge stored on C intA  and C intB  may then be read out simultaneously by the readout multiplexer with the use of a dedicated column CTIA  40 A,  40 B for each bus line  38 A,  38 B, respectively. The comparator  42  is then arranged to compare the signal stored on C intB  (assumed for this case to be the high gain, longer integration time signal) to the threshold, and its output then selects, via multiplexer transistors  44 , which CTIA output to connect to additional, downstream circuitry. If the column-based comparator  42  senses a saturation condition on the high gain sample, the low gain sample is automatically selected. The simultaneous reading of both unit cell capacitors can decrease the read time in the integrate-then-read (ITR) mode by about a factor of two.  
         [0044]    In view of the foregoing, it should be realized that this invention should be accorded a scope that is at least commensurate with the elements of the following claims, and equivalents thereof.