Abstract:
A self-adjusting adaptive input circuit with minimal excess noise and a linear charge-handling capacity exceeding 10 9  electrons to enable high-quality imaging at long wavelength infrared backgrounds and video frame rates is disclosed. An integration capacitor stores a charge produced from a photodetector. A self-adjusting current source skims a current during integration on the integration capacitor. The gate voltage of a skimming transistor is set via a programming transistor in order to set the skim level.

Description:
The present application is related to U.S. patent application Ser. No. 09/675,487, entitled “HIGH GAIN DETECTOR AMPLIFIER WITH ENHANCED DYNAMIC RANGE FOR SINGLE PHOTON READ-OUT OF PHOTODETECTORS”, filed Sep. 29, 2000; and is also related to U.S. patent application Ser. No. 09/675,483, entitled “ADAPTIVE AMPLIFIER CIRCUIT WITH ENHANCED DYNAMIC RANGE”, filed Sep. 29, 2000; the disclosures of which are herein incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to interface circuits for focal plane arrays (FPAs) and, specifically, to a self-adjusting adaptive amplifier circuit that uniquely provides high charge-handling capacity for optimally coupling IR detectors to multiplexing readouts in high-density staring FPAs. 
   2. Description of the Related Art 
   Optical sensors transform incident radiant signals in any spectral wavelength region, but most specifically in the near infrared (NIR; λ=0.8-2 μm), short wavelength IR (SWIR; λ=2.0-2.5 μm), medium wavelength IR (MWIR; λ=2.5-5 μm), and long wavelength IR (LWIR; λ=5-30 μm) bands into electrical signals that are used for data collection, processing, storage and display, such as real-time video. For high-quality imaging of various scenes without concern for ambient light, the MWIR and LWIR bands are often used interchangeably. However, MWIR infrared detector systems typically require sophisticated signal processing algorithms to accommodate the large dynamic changes in background information that result from the relatively high contrast and large solar influence of the scene radiation. Detectors operating in the preferred long wavelength infrared (LWIR) spectral band, on the other hand, can attain the same or greater thermal sensitivity with reduced signal processing complexity. This is especially true in the 8 to 12 μm wavelength atmospheric window, which is optimum for imaging many terrestrial scenes. As a result, infrared detection and tracking can be accomplished using smaller, more cost-effective sensors having LWIR focal plane arrays. 
   Unfortunately, the limited ability of the multiplexing readout circuits creates practical design constraints on LWIR focal plane arrays that in turn severely limit system performance. The result is degradation in signal-to-noise ratio by &gt;10× below the theoretical limit. In the readout portion of a focal plane array, each pixel has a preamplifier to couple the signal from each detector into the respective unit cell. The corresponding readout site must perform several functions that are difficult to simultaneously incorporate in the small amount of “real estate” typically available on such a signal-processing chip. Ideally, each detector/amplifier cell of an FPA should include the following: 1) a detector interface stage that provides low impedance at a uniform operating bias; 2) an integration capacitor with large charge-handling capability; 3) a stage for uniform suppression of the background if integration capacity is inadequate; 4) facility for low-power multiplexing and reset; 5) an output buffer capable of driving the bus line capacitance for subsequent multiplexing of the electrical signal at video rates; and 6) a sufficiently large transimpedance to enable sensor-limited rather than camera-noise-limited performance; i.e., the output-referred noise level of the shot noise at the lowest background must be easily measurable by conventional camera electronics. Note that any focal plane array (not just an LWIR array, but for instance a high operating temperature MWIR array) whose operation generates large amounts of charge to be integrated in the input cell will suffer from these multiplexer limitations even beyond the natural limits of the detector inputs. 
   Staring LWIR FPAs in formats up to 1024 by 1024 have now been demonstrated in the prior art. However, these LWIR devices are typically coupled to conventional MWIR readout circuits, which have several deficiencies that compromise system performance. The limited charge-handling capacity, for example, supports camera sensitivity no better than that achieved by a typical MWIR FPA. This obviates a key benefit of operation in the LWIR spectral band. Moreover, prior art devices limit capability for reducing pixel pitch and increasing pixel density. If the pixel pitch and detector/amplifier cell real estate are reduced in prior art devices, the performance limitations are further exacerbated. 
   Given the current photolithographic state-of-the-art and the limited chip area, there is insufficient detector/amplifier cell real estate to integrate even the most important features including the ability to directly handle all the charge that can be generated during the full frame time. Nevertheless, because small cells are necessary for FPAs with high pixel counts that can be used with compact optics, the readout circuit must be integrated in as little chip real estate as possible. Thus, there is a need for a compact amplifier having characteristics that are better optimized for use in staring LWIR FPAs. 
   U.S. Pat. No. 5,128,534 teaches the technique of biasing a capacitor with a variable voltage source to increase the amount of charge that can be effectively integrated on an integrating capacitor (of the typical size that can readily fit into a standard unit cell). This improves the signal-to-noise ratio of the focal plane array by clipping excess signal. The enhancement in capacity is a trade-off for a nonlinear dynamic range; this non-linearity is not optimum for LWIR imaging since the signal of interest is a small fraction of the background radiation. 
   U.S. Pat. No. 5,055,667 and U.S. Pat. No. RE34,802 also disclose nonlinear techniques for visible CCD imagers. These methods control the dynamic range of a CCD in a manner somewhat analogous to U.S. Pat. No. 5,128,534 by providing a sink region to dispose of excess charge from the photogate region by clipping any signal above the potential set by a control gate where the potential can be similarly modulated during the exposure period. Such predetermined fluctuation can facilitate various transfer functions including logarithmic behavior. Such characteristics are not optimum for LWIR imaging. 
   U.S. Pat. No. 5,382,977 proposes an alternative that effectively enhances the linear charge-handling capacity via electronic scanning. Since the total charges accumulated during a typical frame time ({fraction (1/60)} th  second) can exceed 10 10  carriers while the typical integration capacitor can only handle on the order of mid-10 7  carriers, this method accepts the ˜100× disparity between the two to extend the linear capacity to approximately 10 9  carriers. Unfortunately, all the available charges for each frame time are not used so maximum sensitivity is impossible. The inefficiency translates to a proportional reduction in the effective quantum efficiency of the imaging sensor. 
   U.S. Pat. No. 5,146,302 again teaches skimming in a CCD to enhance the linear charge-handling capacity. To improve efficacy, a sampling circuit comprising a tandem input is used to generate a prescribed skimming voltage. However, the non-uniformity in threshold voltages between the input gates of the main input and the sampling input sets a limit on the amount of charge that can be skimmed. This limit is typically no more than 3 times the instantaneous charge-handling capacity. The linear capacity is thus effectively increased to the order of 10 8  carriers. A further improvement of two orders of magnitude is still needed. 
   Finally, U.S. Pat. No. 6,064,431 discloses a circuit for skimming to linearly enhance the effective charge-handling capacity. This circuit, shown in  FIG. 1 , provides automatic control of the skimming charges to be transferred. Here the capability for predetermined sinking of excess signal, as taught by U.S. patent &#39;534, &#39;667 and &#39;802 noted above, is combined with conventional skimming, as taught by the &#39;302 patent, to further enhance the effective charge-handling capacity. In principle, the 2× to 3× enhancement of &#39;302 patent can be enhanced to yield several orders of magnitude enhancement in charge-handling capacity. Unfortunately, the improvement is again non-linear and thus not optimum for infrared applications. 
   SUMMARY OF THE INVENTION 
   In general, the present invention provides a self-adjusting adaptive input circuit with minimal excess noise and a linear charge-handling capacity exceeding 10 9  electrons to enable high-quality imaging at long wavelength infrared backgrounds and video frame rates. The invention supports various types of photovoltaic and photoconductive detectors, and largely reduces the fixed pattern noise relative to the prior art. The self-adjusting adaptive minimal noise input (SAAMI) amplifier circuit comprises either impedance-buffered or direct injection detector interface with self-nulling skimming of the background signal and self-adapting offset suppression. An integration capacitor stores a charge produced from a photodetector. A self-adjusting current source skims a portion of this charge so that the integration capacitor stores only the difference between the charge generated and the charge skimmed. The gate voltage of a skimming transistor is set via a programming transistor in order to set the skim level of the current source. 
   The net result is LWIR FPA sensitivity commensurate with a charge-handling capacity of 10 9  carriers and about 8 bits of dynamic range. In the LWIR context, the noise equivalent temperature difference that can be achieved is of the order of 0.003K, which is an order of magnitude better than is typically achieved. The front-end provides very uniform skimming at a minor cost in shot noise and a relative reduction in read noise. 
   The present solution can be integrated into the small pixel sizes normally used to amplify the infrared signal for subsequent processing or display. The resulting video has large margin over the background noise of the camera electronics. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
       FIG. 1  is a schematic diagram of a prior art technique for enhancing dynamic range as shown in U.S. Pat. No. 6,064,431; 
       FIG. 2  is a schematic diagram of a first embodiment of a self-adjusting adaptive amplifier with minimal noise input according to the present invention; 
       FIG. 3  is a schematic diagram of an embodiment of a self-adjusting adaptive amplifier with minimal noise input and impedance-buffered interface to the photodetector; and 
       FIG. 4  is a schematic diagram of an alternative embodiment of the self-adjusting adaptive amplifier with minimal noise input and high accuracy for programming the skimming current. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventor for carrying out the invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the basic principles of the present invention have been defined herein specifically to provide to a self-adjusting adaptive amplifier circuit that uniquely provides high charge-handling capacity for optimally coupling IR (or other high-current generating) detectors to multiplexing readouts in high-density staring FPAs. Any and all such modifications, equivalents and alternatives are intended to fall within the spirit and scope of the present invention. 
   The present invention provides a high-performance detector amplifier to optimally read out the signal from each pixel of an infrared focal plane array. According to the various embodiments of the present invention, one benefit is the simultaneous achievement of high sensitivity with video dynamic range of about 8 bits. A second benefit is an output signal that is rich with signal content rather than parasitic non-uniformities. The usable dynamic range of most high-performance thermal imagers is limited by the fixed pattern noise stemming from offset variations generated by the detector and amplifier. A third benefit is the flexibility to independently reprogram each pixel in order to pursue alternative algorithms for non-uniformity compensation, such as scene-based techniques for detecting fast-moving objects. 
   A first embodiment of the present invention is shown in FIG.  2 . The amplifier circuit  2  comprises a detector PD 1  that is directly coupled to the source of an input transistor Q 20 . The gate of Q 20  is adjusted to set the detector bias and the source of Q 20  is modulated by the photo-generated signal from PD 1 . Minority carriers are injected into an integration capacitor C int  where they are accumulated for subsequent readout. This integration capacitor can be reset by a transistor Q 21 B to an initial voltage V store  that may even be a large external voltage as taught by U.S. Pat. No. 6,064,431, noted above, or it may be generated on chip. Alternately, other variants can have the integration capacitor tied to a common level or ground. 
   To enhance the effective integration capacity, the integration capacitor C int  is connected to a self-adjusting current source comprising transistor Q 21  (preferably cascoded with transistor Q 21 A to prevent undesirable parasitic coupling from C int  to the gate of Q 21 ), a programming capacitor C program , a programming transistor Q 22  (preferably coupled through a small kTC-noise-lowering capacitor C lkTc ), and a trim capacitor C trim . The self-nulling current source provided by transistor Q 21  and Q 21 A produces a current sink that drains or “skims” a set level of current from the current being integrated on C int . The gate voltage of Q 21  is programmed by applying an enabling Φ auto  pulse to the gate of Q 22  to create a replicating current in Q 21 . As the current from Q 20  accumulates charge on C int , the voltage on C program  and C trim  (being fractionally coupled to the C int  voltage by C lkTC ) increases, raising the current through Q 21 , and reducing the rate at which the charge accumulates on C int . The gate voltage rises until the current through Q 21  equals the current entering through Q 20 , causing the charge build-up on C int , C program , and C trim  to cease. 
   To establish this near optimal bias condition on the gate of Q 21 , the programming gate voltage is self-adjusted by closing the programming switch. Closing this switch places kTC noise on the effective capacitance of the series of C lkTC  and C program +C trim , but the fraction of this noise that actually resides on the gate of Q 21  is reduced by the ratio [C lkTC /(C program +C trim )] 1/2  from what it would be in the absence of C lkTc . Also, the feedback of Q 21  minimizes kTC noise on its gate as it is turned on. 
   Fine-tuning of the voltage stored on each programming gate is provided by applying a trimming signal, V trim , to the C trim  capacitor. V trim  is adjusted to produce a slight residual current for integration on C int . The capacitive divider formed by the ratio of C trim  to C program  optimally reduces the sensitivity to V trim  and allows small changes to be made to the programming value using relatively large voltages. 
   The voltage on each C program  of the FPA can also be externally set via Q 23  by applying a voltage at V ext . The process of programming the entire FPA results in the application of a waveform to either V trim  or V ext  via, for example, a DAC with 4 to 12 bit resolution, depending on the desired voltage range. Note that selecting on a pixel-by-pixel basis whether or not to re-set the node to an external voltage, provides another approach to reducing the effects of kTC noise in the external programming mode. In this case, only those nodes are re-set which have outputs that, due to kTC noise, fall outsisde an acceptably uniform range. Clearly this process is cumbersome, but for some applications it can provide an extra degree of uniformity in the external programming mode. 
   For ease of reference, we define the Core Adaptive Skimming Circuit (CASC)  3  as the portion of the circuit  2  in  FIG. 2  consisting of C int , C program , C trim , C lkTC , Q 21 , Q 21 A, Q 21 B, Q 22  and Q 23  along with all bias leads, ground leads and interconnects, as indicated by the dashed region in the figure. The CASC  3  may be adpated to many circuit variants, such as those described in the related cases noted above. 
   A source follower transistor Q 24  may be used to buffer the signal and an access transistor Q 25  supplies the signal to the bus. This allows integration of small amounts of charge to be transmitted off chip without the gain reduction caused by the sharing of charge between the integration capacitor and the bus. Also, a common current source CS  1  may be connected to the bus and shared by all the pixels on the bus. 
   Once the skimming level is set, the value can be used for many frames to subtract the prescribed amount of charge. On the other hand, the circuit can be used for frame-to-frame subtraction by resetting the circuit at the beginning of each frame. 
   Each FPA is comprised of an array of such pixels. The maximum FPA output voltage swing will be limited by the range of charges integrated in the various pixels. The maximum total swing can be directly expressed as: 
         V   MAXOUT     =     5   ⁢       Δ   ⁢           ⁢     Q   int         C   int             
 
where 5ΔQ int  is the total range (for 99% of the pixels) of residual integrated charge after skimming. Further, 
         Δ   ⁢           ⁢     Q   int       =       t   int     ⁢   Δ   ⁢     {     I   (     1   -     ⅇ       -     V   skim         V   T           )     }           
 
where t int  is the integration time, I is the photodiode current, V skim  is the gate voltage on the skimming FET Q 21  (above threshold), V T  is the thermal voltage and Δ represents variation. The variation in the net integrated current simplifies to: 
               Δ   ⁢     {     I   (     1   -     ⅇ       -     V   skim         V   T           )     }       =       ⁢       {       Δ   ⁢           ⁢       I   (     1   -     ⅇ       -     V   skim         V   T           )     2       +       (     I   ⁢       Δ   ⁢           ⁢     V   skim         V   T         )     2       }       1   /   2                   =       ⁢       {       Δ   ⁢           ⁢       I   ⁡     (       V   skim       V   T       )       2       +       (     I   ⁢       Δ   ⁢           ⁢     V   skim         V   T         )     2       }       1   /   2                 
 
because the first term that would otherwise appear in braces is eliminated by the adaptive circuit. The variation in the skim voltage in the preceding expression is: 
         Δ   ⁢           ⁢     V   skim       =       [         kTC   ikTC         (       C   program     +     C   trim       )     2       +     2   ⁢   q   ⁢         It     skim   ,   int       ⁢     C   ikTC   2             C     skim   ,   int     2     ⁡     (       C   program     +     C   trim       )       2           ]       1   /   2           
 
The first term in the braces is the programming error due to kTC noise and may be further reduced by a feedback-enhanced technique known as tapered reset, as disclosed in U.S. Pat. No. 6,697,111 entitled “COMPACT LOW-NOISE ACTIVE PIXEL SENSOR WITH PROGRESSIVE ROW REST” filed Apr. 8, 1998, the disclosure of which is herein incorporated by reference. The second term is shot noise due to the charging currents in the reset procedure. The term t skim,int  is the time used to integrate the charge to establish the skim voltage on the skim transistor. The term C skim,int  is the amalgamated capacitor on which this charge is integrated and on which the skim voltage is established prior to isolating the skim FET. It is clear that it helps to make C skim,int  as large as possible to achieve the smallest error in the programming voltage, V skim . However, the kTC isolation capacitor C lkTC  also reduces this noise substantially. Proper setting of the intialo voltage conditions can allow It skim,int  to approach the maximum integrable charge on C int . If we assume this is 10 7  charges and we wish to integrate 10 9  charges (99%), we need to be sure that ΔV skim  is less than 0.2% of the thermal voltage, or about 14 μV at 80K. We can illustrate the significance of these terms by assuming reasonable capacitance values for a small (˜313 μm 2 ) unit cell. Taking C int =750 fF, C program 100 fF and C trim  and C lkTc  both=10 fF, the right hand term would give 87 μV—a bit large for the target skim fraction. The left hand term would give 30 μV. Optimizing the cell (within current design rules) by sizing C progam +C trim =C int  and making C lkTC  as small as possible in current design rules we would have C int =430 fF and C lkTC =5 fF with the result that the kTC noise is only 5.5 μV and the shot noise is 19.3 μV, very near the desired target.
 
   Since C skim,int  is the parallel combination of all the capacitances, it can be readily increased outside of the pixel by adding capacitance, C ext , to boost the basic value from C skim,int =C int +(C lkTC   −1 +(C program +C trim ) −1 ) −1 . The embodiment of the present invention shown in  FIG. 3  allows C ext  to be added through a vertical (or column) bus. This capacitance, C ext , is limited only by practical considerations. A C ext  of only approximately 1 pF is needed to allow 99% skim on 99% of the pixels to boost the effective charge handling capacity by 100 times. This large C skim,int  value somewhat slows the resetting procedure, but reduces the shot noise error on the reset in rough proportion to the square root of the slowing. The source follower (Q 24   FIG. 1 ) at the output of the amplifier  4  has been removed in a simplified illustration to better show the addition of the external capacitor C ext , but it can be inserted as before as long as there is a switch to connect the output line of Q 20  to the large external capacitor during charging. Such an optional change is familiar to those skilled in the art, and is often made depending on the size of the array and the concomitant charge-sharing that can occur between C int , the capacitance of the signal capacitance and the required video swing. 
   Since the best programming accuracy is achieved by using the largest possible C program , this capacitor is best provided by using a MOS capacitor in the substrate. Minimizing the drift of the programming voltage stored on such a capacitor requires that the capacitor by electrically and optically shielded from the switching transistors in the pixel. As a transistor is switched, a fraction of the hot carriers in the channel luminesce. Such photoluminescence injects light into the oxide and the silicon substrate. The upward light must be blocked by the overlying metal layers when the pixel is layed out. The light injected into the substrate must be blocked by surrounding n-well regions that are reverse-biased to deep depletion to collect the stray light. Otherwise, the programming capacitor will discharge and cause the skimming current to gradually drift from the optimum value. 
   The key aspects of the present self-adjusting adaptive input with minimal noise circuit are applicable to other types of circuits used for interfacing infrared detectors. For example,  FIG. 4  is a schematic circuit of an alternative embodiment for use with lower impedance detectors PD 2 , such as those fabricated in the HgCdTe ternary system for operation at cutoff wavelengths beyond 12 μm at operating temperatures above 60K. The negative feedback provided by amplifier  500  enhances the injection efficiency of detector current into the integration capacitor by decreasing the input impedance of source-modulated transistor Q 520 . The inverting amplifier can be of many types known to those of ordinary skill in the art, such as a basic CMOS inverter, a basic differential amplifier, or a folded cascode differential amplifier, among others. Regardless, the amplifier will also luminesce and must be optically shielded from the programming capacitor for proper operation of the skimming adjunct. 
   Those skilled in the art will appreciate that various adaptations and modifications of the just-described preferred embodiments can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.