FIELD OF THE INVENTION
 The present invention relates generally to IR focal plane array detectors. More specifically, the present invention is in the field of detector pixel signal readout and processing.
BACKGROUND OF THE INVENTION
 Photonic detector devices respond to received photons by creating an electric effect which can be quantified and hence provide information as regards the flux of the received photons. Focal plane arrays (FPAs) of detectors are used to obtain images of objects, whereby each detector provides a pixel in the image array. In the image, each pixel is provided with a unique address and a numeric value, which can further be used for manipulating the image for extracting information from the image.
 Thermal Infra red (IR) FPAs are used for obtaining images in the thermal IR range. The resolution of an imaging array in the IR is limited by two factors, namely noise equivalent temperature (NET) and spatial noise, of the analog chain: the focal plane processor (FPP), the external analog circuitry and the analog to digital conversion. An external interference, such as RF interference or signals induced by the digital system may also degrade the performance. The temporal noise of the FPP can be reduced by careful design to be negligible relative to the signal noise, providing low NET values. Yet, to overcome the spatial noise, an external process of non uniformity correction (NUC) must be carried out. A "two point correction" process is generally used, assuming the analog chain to be linear and having stable offset and gain. The final results depend upon the validity of these assumptions, and the spatial resolution becomes a main restriction, limiting the dynamic range and requiring frequent calibrations. The FPP is kept at a steady temperature, e.g. at about 77° K while the external circuitry is exposed to wide temperature variations, degrading the gain and offset stability. Also, the FPP is relatively shielded against external interference, which is not the case with the external elements. In order to reach such a low temperature and keep it steady, the sensor is disposed within a cooled dewar. The dewar has electric connections installed in the walls, in order to facilitate electrical connection with external circuits.
 Heat production is a matter of high importance in the design of FPPs. The efficiency of the cooling system being very low, the amount of power consumed by the circuits of the FPP at any time must also be limited. Sampling rates, sampling resolution, and output rates are factors which affect heat production. The use of components and combinations thereof having a low power consumption is favoured in the design of cooled FPPs. A trade-off between imaging parameters is a compromise often imposed for obtaining desirable image parameters. Accordingly, image frame rate at the expense of image resolution or image size are known in the art. Continuous attempts are made by various system designers to overcome the limitations described, striving to attain a high frame rate and a high resolution, with a reasonable power dissipation.
 The value of the electric effect associated with each detector of the FPA is converted into a digital format, before it can be subjected to digital image processing. However, there are a number of critical reasons favouring conversion to digital format at an early stage as possible of the signal corresponding to each pixel. U.S. Pat. No. 5,886,659 discloses an on-focal plane analog to digital conversion application. Benefits of having the analog-to-digital conversion in the focal plane are discussed in that publication, the contents of which are incorporated herewith by reference.
 Integrated converters have used several types of conversion schemes. Single-slope analog to digital conversion is a well-established technique. For example, U.S. Pat. No. 5,742,058 discloses an analog to digital conversion in the focal plane utilizing a single slope conversion cycle. The Dual-slope conversion provides better accuracy and stability (R. Van De Plassche, cited below). Yet, both techniques require quite long conversion times, being based on counters: the full-scale time of an n-bit single-slope converter is 2n*Tc, where Tc is the clock cycle time. The dual slope requires up to twice that time. The dual-ramp single-slope method, described by R. Van De Plassche in: INTEGRATED ANALOG TO DIGITAL AND DIGITAL TO ANALOG CONVERTERS, Kluwer Academic Press, pp. 274-289, 1994, the contents of which are incorporated herein by reference, promises both high resolution and reduced conversion time.
 It is on the background hitherto described, that the present invention was conceived. The invention described below provides a basis for increasing the rate of sampling digitally at the FPP level, without compromising the resolution of each pixel.
BRIEF DESCRIPTION OF THE DRAWINGS
 FIG. 1 is schematic description of the main structural features of an IR sensor system in which the invention is implemented;
 FIG. 2 is a flow chart describing schematically the process of the invention;
 FIG. 3A is a schematic description of the pixel charge converting circuit of the invention;
 FIG. 3B is a charge DAC implementation in accordance with an embodiment of the invention;
 FIG. 3C is another charge DAC implementation in accordance with an embodiment of the invention;
 FIG. 4 is an implementation of a pixel charge converting circuit of the invention adapted for an array of pixels;
 FIG. 5A is a detailed implementation of the circuit described in FIG. 4;
 FIG. 5B is a description of the main wave-shape forms in the circuit of FIG. 5A, implemented in the course of one conversion cycle;
 FIG. 6. is a schematic description of the interconnections existing between the main components of the imaging system, in accordance with the present invention.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
 A system of the invention is implemented in association with an integrated circuit device, a focal plane processor (FPP). The FPP is disposed within a cooled dewar, as described schematically in FIG. 1 to which reference is now made. A longitudinal cross section elevation in an IR sensor is shown, in which the FPP 14 of the invention is functional. Metal plate 16 is adjacent cold finger 18. The FPP is thermally connected while electrically isolated, with metal plate 16 through ceramic plate and spacers 20. To ceramic plate and spacers 20, which is electrically insulating, are connected wire bonds 22. FPP chip 14 supports detector array chip 24 through indium bumps 26, whereby for each detector in the detector array chip 24 there exists one bump connecting to the respective pixel in the FPP chip 14. Penetrating through the envelope of the dewar 26 are connecting pins 28. Wire bonds 22 connect pads (not shown) of FPP 14 to the appropriate connecting pins, having intermediate pads on the ceramic plate 20. The connecting pins provide electrical contact to external circuits 30.
 The process of the invention is summarized schematically in FIG. 2 to which reference is now made. In step 40, photon flux is captured on a detector pixel, causing a corresponding build-up of charge in the respective FPP pixel in step 42. In step 44 the charge in the FPP pixel is quantified digitally. In step 46 the results of the quantification are processed and in step 48 the processed data is transmitted to a programmable logic outside of the dewar for further processing. In step 50, digital numbers are calculated and pixels are remapped to restore the image.
FPP Pixel Charge Digital Conversion
 The charge accumulated in the pixel in the FPP of the invention is according to the present invention converted to a digital form by a circuit modified from Van De Plassche, cited above. The original circuit, known as a dual ramp single slope converter, converts input voltage into a digital number. The modification of the circuit to a charge converter circuit, and the function thereof are described in reference to FIG. 3A. The charge ADC 52 is composed of comparator 66, charge output digital-to-analog converter (DAC) 62 and of controller 64. The input port of ADC 52 is line 67, which connects between the output port of the DAC 62 and comparator 66. Qpx, the charge to be converted, is stored in pixel capacitor 60. DAC 62 produces a charge of opposite polarity, the value of which is controlled digitally by the controller 64. The digital control value (in bits) which eliminates Qpx (as indicated by the comparator 66 output) is the conversion output. Two exemplary implementations of a charge DAC of the invention are described in FIGS. 3B and 3C to which reference is now made. In FIG. 3B a DAC implementation is described in which the charge for elimination of the Qpx is provided by a set of several capacitors such as capacitor 68, used in a successive approximation mode. It is also possible to use only the smallest capacitor, switching it many times and counting the number of switching necessary to eliminate the converted charge. In FIG. 3C, the required charges are achieved by switching-on the relevant current sources, e.g. source 69, for a given time, producing charge packets accordingly.
 However, in practice, the pixels are arranged in the framework of a matrix of rows and columns which may be long such that certain implications are inevitable trying to adapt a solitary pixel ADC application to a pixel array application, such as a column of the array . Accordingly, the following two concerns must be dealt with. First, the long column line adds a parasitic capacitance, Ccl, the value of which is about one order of magnitude larger then the pixel capacitor Cpx. Second, during the conversion, a cross-talk exists between adjacent column lines, impairing the function of the matrix.
 In accordance with the present invention, an integrator, the input of which is a virtual ground, is added as a first stage of the comparator of the charge ADC circuit. This architectural feature helps to overcome the above mentioned problems as the column line in this configuration becomes connected to a virtual ground. Such a configuration is described in FIG. 4 to which reference is now made. Integrator 70, is disposed between the comparator 72 and the column line 74, The charge to be converted is stored in the integrator's capacitor 76, whereas the charge injected from DAC 78 into the virtual ground, eliminates this charge as described above.
 The use of an FPA of the invention is typically in IR imaging applications, in which accurate reference to the absolute temperature of a pixel is not required. Rather, in an IR imaging application, the accuracy of the differences among the different pixels is the matter of relevance. Accurate variability measurement and conversion can be supported by the "dual ramp single slope ADC" used in some embodiments of the present invention.
 The "dual ramp single slope ADC" employs a quantification cycle which implements two steps. In the first step MS (most significant) part of the numeric value is determined, and in a second step, LS (least significant) part of the numeric value of the charge is determined. The two parts of the digital values of the measured quantity must be combined to form a complete digital word, as will be explained later on. In the framework of a pixel array, the readout and quantification are performed in the context of columns and rows as has been discussed above, and will be further explained with reference to FIGS. 5A and 5B. In FIG. 5A is shown an ADC circuit in accordance with a preferred embodiment of the invention, the function of which is explained with reference to the conversion cycle and switching sequences described in FIG. 5B.
 At time t0, the quiescent output voltage of integrator 90, is forced by closing momentarily switch S1 91 to Vpch, (pre-charge voltage), which is also the reference voltage for comparator 92. Closing momentarily switch S3 94 at time t1, the charge of a pixel, accumulated in capacitor Cpxl 96, is transferred to integrator 90, lowering the output voltage accordingly. Closing switch S4 98 at time t2, a constant current source is established by the combination of a linear ramp voltage (ramp 148 of FIG. 5B), the capacitor C1 100 and the virtual ground of the integrator. Switch S5 102 closes simultaneously with switch S4 98. Counter 104, which has been reset earlier, starts counting the clock pulses while the integrator output goes up linearly. Reaching Vpch at time t3 the integrator activates the comparator. The period t3-t.sub.2 is a true indication of the signal to be measured. Yet, the output of the comparator is synchronized to the next clock, and only then it opens switch 98 and samples the counter reading (switch S5 102 remains closed until t4 in order to serve other ADCs). As a result, the output of the integrator continues to rise for a period Δt (in FIG. 5B), so that the counter reading provides only the MS count, and a value, proportional to Δt should be subtracted with higher resolution. This is done by resetting counter 104 and closing again switch S5 102 and switch S4 98 at time t5, which connects a second ramp (ramp 150 in FIG. 5B), the slope of which is positive and slower.
 The output of the integrator goes down and reaches Vpch at time t6, disabling the comparator. The counter reading is sampled at time t6, providing the LS count to be subtracted (with the proper weight ) from the digital number represented by the former MS bits. The ratio between the absolute values of the slopes of the two ramps determines the number and weight of LS bits. The value of Δt is nominally less then one clock time. Yet, in practice, due to delays in the comparator and the switching, it might reach as high as some clock times, the actual number of which depends also upon the clock frequency. Consequently, the period and the number of counter stages assigned to the second ramp is to be increased accordingly.
 Being used in IR imagers, an offset thus created is compensated for in the non-uniformity correction (NUC) procedure, provided it has a constant value. An advantage associated with the ADC implementation in accordance with the present invention, results from the comparator being activated at a constant input level by a signal of a constant slope, independent of the converted signal amplitude. Also, the focal plane is cooled to a very stable temperature. Therefore, the delays and the resulting offset are very stable. Another advantage obtained by using the invented ADC in focal plane arrays is the connection of the column line to a virtual ground. The column lines are long and parallel, resulting in a significant coupling capacitance between neighbors. During the period of transfer of charge from the pixel to the ADC, the column line potential is temporarily raised, only to revert to the original value by the end of that period. Therefore, finally no charge injection occurs between adjacent column lines.
 The combination a common linear voltage ramp (148 or 150 in FIG. 5B) driving the capacitor 100 (in FIG. 5A) into the virtual ground of the integrator, enables applying low level DC current sources of approximately the same amplitude to all the ADCs. Thus the use of current mirrors, which are not accurate enough (especially in the nano-ampere levels), is avoided. The ramp generator 106 (in FIG. 5A) is basically an integrator, fed at each step of the conversion cycle by a constant current source of appropriate amplitude and polarity.
 In order to further decrease the conversion time without loosing resolution, the "dual ramp single slope ADC" concept can be further extended into a multi ramp ADC, where additional conversion steps, having successively decreased weights, are employed. This may be desirable in the case of very high resolution converters.
Programmable Logic Applications in the Present Invention
 The FPP of the present invention is applicable to IR imagers in general. Yet, it is especially advantageous in cooled imagers, where the heat dissipated in the focal plane should be removed by cooling to a low and stable temperature. Therefore, architectural and functional considerations of the make-up of the integrated circuit have been made in order to decrease the power dissipation in the FPP integrated circuit. In accordance with one aspect of the invention, a programmable logic device, external to the dewar in which the integrated circuit of the sensor is disposed, performs several tasks. The first one relates to the digital functions of the ADCs which are partially performed outside of the dewar. These functions being performed in the uncooled environment, serve to decrease the amount of components and heat dissipation in the FPP, therefore potentially improving the overall performance of the imager. As explained above, the digital value of each pixel is obtained in a MS part and an LS part. The two parts, after initial processing in the FPP, are finally combined to form one digital word in the external programmable logic device. The second task relates to the remapping of the pixels. The data transmission from the FPP to the programmable logic is optimized to minimize focal plane power dissipation. Consequently, the bits of a digital word transmitted from the dewar include a mixture of data from two pixels. Such mixed data is remapped in the external logic device, to restore the image.
 The external programmable logic device also adds flexibility and optimizes the conversion cycle. Whereas the order of succession of the various conversion steps is fixed, the duration of each step is controlled externally. Thus, the duration of each step is set as required by the actual performance sequence, avoiding the need to reserve lengthy intervals of time for confidence margins. The data transfer associated with this aspect of the invention is described with reference to FIG. 6. The detector assembly 170, includes a focal plane module 172 and an external proximity board 174. The proximity board is a printed circuit board, placed close to the dewar pins, upon which the programmable logic such as FPGA (field programmable gate array) 176, a clock generator 178 and power supply regulation circuits 180 are mounted. In the focal plane module 172, the detector component 182 and the s FPP 184 are included. The interconnections between the proximity board 174 and the focal plane module 172 are divided into four groups: regulated FPP power supplies bus, slow communications link, FPP clock and timing link and the raw data bus.
 Through the conventional comparatively slow communication bus, various operation parameters of the FPP, which are not directly involved in the timing of the analog-to-digital conversion steps, are controlled, These include, inter alia, "integrate-then-read" (ITR) or "integrate-while-read" (IWR) modes of operation, working points, readout window size and location, etc. One parameter should be mentioned here, namely the resolution of the analog-to-digital conversion. The duration of a conversion cycle depends also on the resolution, increasing with the resolution. Some bits of the slow communication stream control the resolution, e.g. a 13 bit or a 15 bit resolution, enabling a trade-off between the resolution and the frame-rate.
 A faster link carries to the FPP the clock signal and the timing information of the conversion steps, mentioned earlier. In this fast channel, a stream of short pulses is sent on a wire from the external programmable logic to the FPP, where each pulse is recognized in the FPP as the start or end of a specific step. Programming the intervals between the pulses, the optimal conversion duration is determined. The raw data bus carries the partially processed data produced by the ADCs, from the FPP to be finally processed and remapped in the programmable logic device.
 A different aspect of the imaging array which relates to the external programmable logic, is its exploitation as a user interface. Thus, in some embodiments of the invention, the programmable logic device can be used also as an interface to the external system 190 in FIG. 6, simplifying the users' interaction with the FPP. While the FPP of the invention operates with the optimal clock and timing for the conversion, the user can approach the buffered output on the programmable logic implementing specific communications protocol and clock. The clock of the user system is independent of the clock of the detector assembly, and does not have to be synchronized with it. The dialog between the two systems can then be established by an handshaking implemented on the programmable logic device.
 As described so far, the system of the invention is most beneficial in cooled FPPs, typically implemented in thermal IR imaging. It is nevertheless contended that the same system can be used in any FPP systems, for other imaging purposes, such as visible, near infra-red and X-ray. In such systems, heat dissipation in the FPP may be less critical, but the benefits of the system are still evident.