Abstract:
Data signal amplification and processing circuitry with multiple signal gains for increasing dynamic signal range. An incoming data signal is processed in accordance with multiple signal gains. The resultant signals have multiple signal values which are compared to predetermined lower and higher thresholds. The desired output signal is: the maximum sample signal value when the maximum value fails to traverse a predetermined lower threshold; the minimum sample signal value when an adjacent larger one of the plurality of sample signal values traverses a predetermined higher threshold; a smaller of mutually adjacent ones of a pair of the plurality of sample signal values when a larger one of the pair of sample signal values traverses the predetermined higher threshold and the smaller value fails to traverse the predetermined lower threshold; and a combination of the smaller and larger ones of the pair of sample signal values when the larger value traverses the predetermined lower threshold and fails to traverse the predetermined higher threshold.

Description:
[0001]     This is a continuation of U.S. patent application Ser. No. 10/685,787, filed on Oct. 15, 2005, and entitled “Data Signal Amplifier and Processor With Multiple Signal Gains For Increased Dynamic Signal Range”. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to analog amplifiers for pre-amplifying low level charge-based signals, and in particular, to amplifier and processor circuits with analog pre-amplifier circuits and analog-to-digital (ADC) conversion circuits.  
         [0004]     2. Description of the Related Art  
         [0005]     High dynamic signal range is a key parameter for many types of circuits. This is particularly true in the area of flat panel x-ray imaging systems. As is well known in the art, such systems use a detector cassette containing a scintillation layer that absorbs and converts impinging x-ray photons to visible light photons for detection by photosensitive elements that are also within the detector array. As is further well known, such a detector array contains a two dimensional array of microscopic squares referred to as picture elements, or “pixels”. Each pixel includes an addressable photosensitive element, such as a photodiode and switching transistor combination. From such circuitry individual pixel data signals, generally in the form of charge based signals, are provided for amplification and further processing. (Further discussion of this type of imaging system can be found in U.S. Pat. No. 5,970,115, entitled “Multiple Mode Digital X-Ray Imaging System”, the disclosure of which is incorporated herein by reference.)  
         [0006]     Present methods of reading out image pixel information from such flat panel detector arrays involves linear conversion of the electrical charge of each pixel to a voltage. Such voltage is then processed linearly and converted to a digital value using a conventional analog-to-digital converter (ADC). Both processes have inherent limitations, however. The process of linear charge-to-voltage conversion and amplification will have limitations due to noise, linearity issues and limited signal saturation levels. The digital conversion process has a limitation on dynamic range, which presently is typically at 14 bits. (Converters capable of more than 14-bit conversion at speeds required for flat panel imaging arrays have limited, if any, availability as commercial products.)  
         [0007]     Improvement of the dynamic range of the linear conversion process has been described in U.S. Pat. No. 6,486,808, entitled “Data Signal Amplifier With Automatically Controllable Dynamic Signal Range” (the disclosure of which is incorporated herein by reference). Such a technique involves automatic switching of the charge-to-voltage gain factor to a lower value when pixels with high signal levels are encountered. Pixel values corresponding to these dynamically reduced gain values are identified (e.g., flagged with an additional bit) such that downstream image processing circuitry can detect such pixel values and correct for the reduced gain factor. This allows signal values large enough to otherwise cause saturation of the charge-to-voltage converter, the signal processing path or the ADC to be “compressed”, converted to appropriate digital data and subsequently expanded, or “decompressed”, thereby extending the dynamic range of signals for conversion.  
         [0008]     Compensation or improvement of the digital conversion portion of the processing is disclosed in U.S. Pat. No. 5,760,723, entitled “Delta-Sigma Analog-To-Digital Converter Including Charge Coupled With Devices” (the disclosure of which is incorporated herein by reference).  
         [0009]     Extending the dynamic range of the charge-to-voltage conversion, particularly those used in present flat panel imaging arrays, is problematic. Significant limitations exist due to electronic noise and non-ideal and non-linear behavior of electronic components. As for the digital conversion, increasing bit depth of an ADC results in significant additional system cost and power dissipation. Further, ADCs capable of more than 14-bit conversion at the requisite speeds are not readily available as commercial products.  
         [0010]     While the dynamic gain switching technique of U.S. Pat. No. 6,486,808 succeeds in extending the dynamic range without increasing physical complexity, cost or power dissipation, correction of the compressed pixel values requires precise information concerning the transfer functions of the signal processing electronics in both standard and reduced gain modes. The binary nature of the gain switching decision along with imperfect restoration of low gain pixel values can produce contour artifacts in image regions where the pixel values cross over the gain switching threshold. Moreover, implementation of this technique requires more complex circuitry which is generally less flexible in terms of being adapted to revised application requirements.  
         [0011]     Regarding the digital conversion process, extending the dynamic range at extremely low signal levels is very difficult, and generally requires development and use of significantly more complex circuits which are directed to specific product implementations or system requirements and not readily adaptable to revised application requirements. Further, practical implementation is often precluded due to constraints on power dissipation and available space.  
       SUMMARY OF THE INVENTION  
       [0012]     In accordance with the presently claimed invention, data signal amplification and processing circuitry uses multiple signal gains for increasing dynamic signal range. An incoming data signal is processed in accordance with multiple signal gains. The resultant signals have multiple signal values which are compared to predetermined lower and higher thresholds. The desired output signal is: the maximum sample signal value when the maximum value fails to traverse a predetermined lower threshold; the minimum sample signal value when an adjacent larger one of the plurality of sample signal values traverses a predetermined higher threshold; a smaller of mutually adjacent ones of a pair of the plurality of sample signal values when a larger one of the pair of sample signal values traverses the predetermined higher threshold and the smaller value fails to traverse the predetermined lower threshold; and a combination of the smaller and larger ones of the pair of sample signal values when the larger value traverses the predetermined lower threshold and fails to traverse the predetermined higher threshold.  
         [0013]     In accordance with one embodiment of the presently claimed invention, data signal amplifier circuitry with multiple signal gains includes sampling amplifier circuitry and processing circuitry. The sampling amplifier circuitry is responsive to at least one control signal and an input signal having an input signal value by providing at least one sample signal having a plurality of sample signal values, including maximum and minimum sample signal values, and corresponding to the input signal value. The processing circuitry is coupled to the sampling amplifier circuitry and responsive to the at least one sample signal and one or more processing control signals by processing the at least one sample signal to provide a processed signal having a value corresponding to the input signal value. The processed signal value represents one of a plurality of values which includes: the maximum sample signal value; the minimum sample signal value; a first one of the plurality of sample signal values; and a combination of the first one and a second one of the plurality of sample signal values.  
         [0014]     In accordance with another embodiment of the presently claimed invention, data signal amplification and processing circuitry with multiple signal gains includes sampling amplifier means and processor means. The sampling amplifier means is for receiving at least one control signal and an input signal having an input signal value and in response thereto providing at least one sample signal having a plurality of sample signal values, including maximum and minimum sample signal values, and corresponding to the input signal value. The processor means is for receiving the at least one sample signal and one or more processing control signals and in response thereto processing the at least one sample signal and providing a processed signal having a value corresponding to the input signal value. The processed signal value represents one of a plurality of values which includes: the maximum sample signal value; the minimum sample signal value; a first one of the plurality of sample signal values; and a combination of the first one and a second one of the plurality of sample signal values.  
         [0015]     In accordance with another embodiment of the presently claimed invention, a method of data signal amplification and processing with multiple signal gains includes: 
        receiving at least one control signal;     receiving an input signal having an input signal value;     generating, in response to the at least one control signal and the input signal, at least one sample signal having a plurality of sample signal values, including maximum and minimum sample signal values, and corresponding to the input signal value; and     receiving one or more processing control signals and in response thereto processing the at least one sample signal and generating a processed signal having a value corresponding to the input signal value;     wherein the processed signal value represents one of a plurality of values which includes 
            the maximum sample signal value,     the minimum sample signal value,     a first one of the plurality of sample signal values, and     a combination of the first one and a second one of the plurality of sample signal values.    
               
 
         [0025]     In accordance with another embodiment of the presently claimed invention, data signal amplifier circuitry with multiple signal gains includes sampling amplifier circuitry and processing circuitry. The sampling amplifier circuitry, with a plurality of signal gains, including at least maximum and minimum signal gains, is responsive to at least one control signal and an input signal having an input signal value by providing at least one sample signal having a plurality of sample signal values related to the plurality of signal gains and corresponding to the input signal value. The processing circuitry is coupled to the sampling amplifier circuitry and responsive to the at least one sample signal and one or more processing control signals by processing the at least one sample signal to provide a processed signal having a value corresponding to the input signal value. The processed signal value represents one of a plurality of values which includes: a first value corresponding to the minimum signal gain; a second value corresponding to the maximum signal gain; a third value corresponding to a first one of the plurality of signal gains; and a fourth value corresponding to a combination of the first one and a second one of the plurality of signal gains.  
         [0026]     In accordance with another embodiment of the presently claimed invention, data signal amplification and processing circuitry with multiple signal gains includes sampling amplifier means and processor means. The sampling amplifier means, with a plurality of signal gains, including at least maximum and minimum signal gains, is for receiving at least one control signal and an input signal having an input signal value and in response thereto providing at least one sample signal having a plurality of sample signal values related to the plurality of signal gains and corresponding to the input signal value. The processor means is for receiving the at least one sample signal and one or more processing control signals and in response thereto processing the at least one sample signal and providing a processed signal having a value corresponding to the input signal value. The processed signal value represents one of a plurality of values which includes: a first value corresponding to the minimum signal gain; a second value corresponding to the maximum signal gain; a third value corresponding to a first one of the plurality of signal gains; and a fourth value corresponding to a combination of the first one and a second one of the plurality of signal gains.  
         [0027]     In accordance with another embodiment of the presently claimed invention, a method of data signal amplification and processing with multiple signal gains includes: 
        receiving at least one control signal;     receiving an input signal having an input signal value;     generating, in conformance with a plurality of signal gains, including at least maximum and minimum signal gains, and in response to the at least one control signal and the input signal, at least one sample signal having a plurality of sample signal values related to the plurality of signal gains and corresponding to the input signal value; and     receiving one or more processing control signals and in response thereto processing the at least one sample signal and generating a processed signal having a value corresponding to the input signal value;     wherein the processed signal value represents one of a plurality of values which includes 
            a first value corresponding to the minimum signal gain,     a second value corresponding to the maximum signal gain,     a third value corresponding to a first one of the plurality of signal gains, and     a fourth value corresponding to a combination of the first one and a second one of the plurality of signal gains.    
               
 
         [0037]     In accordance with another embodiment of the presently claimed invention, a method of data signal amplification and processing with multiple signal gains includes: 
        sampling an input signal having an input signal value to provide at least one sample signal having a plurality of sample signal values, including maximum and minimum sample signal values, and corresponding to the input signal value; and     processing the at least one sample signal to provide a processed signal having a value corresponding to the input signal value;     wherein the processed signal value represents one of a plurality of values which includes 
            the maximum sample signal value,     the minimum sample signal value,     a first one of the plurality of sample signal values, and     a combination of the first one and a second one of the plurality of sample signal values.    
               
 
         [0045]     In accordance with another embodiment of the presently claimed invention, a method of data signal amplification and processing with multiple signal gains includes: 
        sampling, in conformance with a plurality of signal gains, including at least maximum and minimum signal gains, an input signal having an input signal value to provide at least one sample signal having a plurality of sample signal values related to the plurality of signal gains and corresponding to the input signal value; and     processing the at least one sample signal to provide a processed signal having a value corresponding to the input signal value;     wherein the processed signal value represents one of a plurality of values which includes 
            a first value corresponding to the minimum signal gain,     a second value corresponding to the maximum signal gain,     a third value corresponding to a first one of the plurality of signal gains, and     a fourth value corresponding to a combination of the first one and a second one of the plurality of signal gains.    
               
 
         [0053]     In accordance with another embodiment of the presently claimed invention, data signal processing circuitry for data signals with multiple signal gains includes sampling circuitry and processing circuitry. The sampling circuitry is responsive to at least one control signal and at least one input signal having a plurality of input signal values by providing at least one sample signal having a plurality of sample signal values, including maximum and minimum sample signal values, and corresponding to the plurality of input signal values. The processing circuitry is coupled to the sampling circuitry and responsive to the at least one sample signal and one or more processing control signals by processing the at least one sample signal to provide a processed signal having a value corresponding to one or more of the plurality of input signal values. The processed signal value represents one of a plurality of values which includes: the maximum sample signal value; the minimum sample signal value; a first one of the plurality of sample signal values; and a combination of the first one and a second one of the plurality of sample signal values.  
         [0054]     In accordance with another embodiment of the presently claimed invention, data signal processing circuitry for data signals with multiple signal gains includes sampling means and processing means. The sampling means is for receiving at least one control signal and at least one input signal having a plurality of input signal values and in response thereto providing at least one sample signal having a plurality of sample signal values, including maximum and minimum sample signal values, and corresponding to the plurality of input signal values. The processing means is for receiving the at least one sample signal and one or more processing control signals and in response thereto processing the at least one sample signal to provide a processed signal having a value corresponding to one or more of the plurality of input signal values. The processed signal value represents one of a plurality of values which includes: the maximum sample signal value; the minimum sample signal value; a first one of the plurality of sample signal values; and a combination of the first one and a second one of the plurality of sample signal values.  
         [0055]     In accordance with another embodiment of the presently claimed invention, a method of data signal processing for data signals with multiple signal gains includes: 
        receiving at least one control signal;     receiving at least one input signal having a plurality of input signal values;     generating, in response to the at least one control signal and the at least one input signal, at least one sample signal having a plurality of sample signal values, including maximum and minimum sample signal values, and corresponding to the plurality of input signal values; and     receiving one or more processing control signals and in response thereto processing the at least one sample signal and generating a processed signal having a value corresponding to one or more of the plurality of input signal values;     wherein the processed signal value represents one of a plurality of values which includes 
            the maximum sample signal value,     the minimum sample signal value,     a first one of the plurality of sample signal values, and     a combination of the first one and a second one of the plurality of sample signal values.    
               
 
         [0065]     In accordance with another embodiment of the presently claimed invention, a method of data signal processing for data signals with multiple signal gains includes: 
        sampling at least one input signal having a plurality of input signal values to provide at least one sample signal having a plurality of sample signal values, including maximum and minimum sample signal values, and corresponding to the plurality of input signal values; and     processing the at least one sample signal to provide a processed signal having a value corresponding to one or more of the plurality of input signal values;     wherein the processed signal value represents one of a plurality of values which includes 
            the maximum sample signal value,     the minimum sample signal value,     a first one of the plurality of sample signal values, and     a combination of the first one and a second one of the plurality of sample signal values.    
               
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0073]      FIG. 1  is a schematic diagram of data signal amplification and processing with multiple signal gains for increasing dynamic signal range in accordance with one nt of the presently claimed invention.  
         [0074]      FIG. 2  is a signal timing diagram of timing and control signals for the circuit of  FIG. 1 .  
         [0075]      FIG. 3  is a functional block diagram of one example of processing circuitry or use with the circuit of  FIG. 1 .  
         [0076]      FIG. 4  is a graph of measured digital value versus analog input value for the  FIG. 1 .  
         [0077]      FIG. 5  is a graph of calculated digital value versus analog input value in ce with the presently claimed invention.  
         [0078]      FIG. 5A  is a graph depicting processing of more than two signal samples in ce with the presently claimed invention.  
         [0079]      FIG. 6  is a schematic diagram of the linear amplifier portion of a circuit in ce with an alternative embodiment of the presently claimed invention.  
         [0080]      FIG. 7  is a schematic diagram of the linear amplifier portion of a circuit in ce with another alternative embodiment of the presently claimed invention.  
         [0081]      FIG. 8  is a schematic and signal timing diagram of the input circuitry for a circuit in accordance with another alternative embodiment of the presently claimed invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0082]     The following detailed description is of example embodiments of the presently claimed invention with references to the accompanying drawings. Such description is intended to be illustrative and not limiting with respect to the scope of the present invention. Such embodiments are described in sufficient detail to enable one of ordinary skill in the art to practice the subject invention, and it will be understood that other embodiments may be practiced with some variations without departing from the spirit or scope of the subject invention.  
         [0083]     Throughout the present disclosure, absent a clear indication to the contrary from the context, it will be understood that individual circuit elements as described may be singular or plural in number. For example, the terms “circuit” and “circuitry” may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together (e.g., as one or more integrated circuit chips) to provide the described function. Additionally, the term “signal” may refer to one or more currents, one or more voltages, or a data signal. Within the drawings, like or related elements will have like or related alpha, numeric or alphanumeric designators. Further, while the present invention has been discussed in the context of implementations using discrete electronic circuitry (preferably in the form of one or more integrated circuit chips), the functions of any part of such circuitry may alternatively be implemented using one or more appropriately programmed processors, depending upon the signal frequencies or data rates to be processed.  
         [0084]     As will be discussed in more detail below, the presently claimed invention uses charge-to-voltage conversion in a pre-amplification stage with multiple selectable conversion factors, or gains, to achieve increased dynamic range in data signal acquisition. The charge of each pixel signal is read and converted to a voltage in a high gain mode of the pre-amplification stage, and then read and converted again in a lower gain mode. Both voltages are converted to corresponding digital values by common ADC circuitry, following which downstream signal processing circuitry rescales and combines, as appropriate, these digital values to produce a single pixel signal value.  
         [0085]     Referring to  FIG. 1 , data signal amplification and processing circuitry  10  with multiple signal gains for increasing dynamic signal range in accordance with one embodiment of the presently claimed invention includes a linear amplifier stage  12 , a sample and hold stage  14  and an ADC stage  16 , all interconnected substantially as shown. The linear amplifier stage  12  includes a differential amplifier X 1  and feedback capacitance circuitry including capacitors C 1  and C 2 , and switches S 1  and S 2 . In an integrated circuit environment, as is well known in the art, such switches S 1 , S 2  are generally designed as pass transistors or transmission gates (dual pass transistors connected in parallel). Additionally, it will be understood that the primary feedback capacitor C 1 , instead of being a fixed capacitance, can be a variable capacitance (e.g., varactor) controlled by an additional gain control signal (not shown). Switch S 1  is a reset switch which when closed, with switch S 2  also closed, resets the circuit by discharging both capacitors C 1 , C 2 .  
         [0086]     Input circuitry in the form of capacitors C 3  and C 4  are represented to identify any finite stray capacitance (C 3 ) and capacitance C 4  of the subject pixel for which electrical charge is being converted to a voltage.  
         [0087]     The sample and hold circuitry  14  includes a serially coupled switch S 3 , a resistor R 1  and a shunt capacitor C 5 . The ADC circuitry  16  includes an ADC U 1  which converts the analog voltage across capacitor C 5  to a digital signal  11 , e.g., 14-bits wide.  
         [0088]     Referring to  FIG. 2 , timing and operation of the circuitry of  FIG. 1  can be better understood. Initially, the system is placed in a low gain mode by having signal  9   l  active (e.g., asserted) so that switch S 2  is closed. The reset signal  9   r  is activated (e.g., asserted), thereby closing switch S 1  and resetting the system by discharging the feedback capacitors C 1 , C 2 . Both switches S 1 , S 2  are then opened and the system is operating in a high gain mode. Pixel signal  9   p  becomes active, thereby closing switch S 4  (e.g., a thin film transistor (TFT)) and causing signal charge accumulated on the pixel capacitance C 4  to be shared with the feedback capacitor C 1  of the amplifier stage  12 . The resulting voltage signal at the output  13  of the amplifier X 1  is sampled by activating sample signal  9   s , thereby closing switch S 3  and causing shunt capacitor C 5  to charge to the same voltage. Following that, the convert signal  9   c  is activated, thereby causing the voltage across capacitor C 5  to be converted to its digital equivalent as a multi-bit digital signal  11 .  
         [0089]     As is typical, particularly in a flat panel imaging system readout sequence, a zero sample is acquired first, whereby the reset voltage at the output  13  of the amplifier  12 , following reset (with the feedback capacitors C 1 , C 2  discharged), is sampled and stored across the shunt sample and hold capacitor C 5 , following which the convert signal  9   c  causes such voltage (approximately, though not necessarily exactly, equal to zero) to be converted to its digital equivalent as a “zero” digital signal  11 . As is well known in the art, as part of a correlated double sampling (CDS) process, such zero signal sample can be subtracted from later signal values acquired during high and low gain circuit operation (discussed in more detail below).  
         [0090]     Next, the low gain control signal  9   l  is activated, thereby closing switch S 2  and placing capacitor C 2  in parallel with capacitor C 1 , and causing the amplifier stage  12  to operate in its low gain mode. The electrical charge initially transferred from the pixel capacitance C 4  and stored on the high gain capacitor C 1  is now shared between both feedback capacitors C 1 , C 2 , according to the ratio of their respective capacitance values. This results in a lower voltage across these capacitors together C 1 , C 2 , thereby also reducing the voltage at the output  13  of the amplifier stage  12 . This lower voltage is sampled by the sample and hold circuit  14 , following which the sampled voltage across shunt capacitor C 5  is converted by the ADC U 1  to its digital signal  11  counterpart.  
         [0091]     Referring to  FIG. 3 , subsequent processing of the digitized sample signal  11  uses the above-discussed three signal values, “zero” signal sample, high gain mode signal sample, and low gain mode signal sample, to construct a final pixel signal  29 . As discussed in more detail below, the low gain pixel values are transformed to a set of equivalent high gain values. This is done by multiplication of the low gain pixel value by a transformation factor larger than unity. This can be considered as a form of decompression of the low gain pixel values to their equivalent high gain values. For those values where the actual high gain data are compromised due to electronic signal saturation, such digital values would be produced which are much larger than those which the actual high gain mode of operation is capable of producing.  
         [0092]     Meanwhile, pixel data values acquired in the high gain mode are tested against a threshold value T that, a priori, is known to avoid saturation of the electronic components in the signal path during the high gain mode of operation. For those pixel data values where the high gain values are clearly below such threshold value T, the high gain values are used as the final pixel values. For those values where the high gain pixel values are clearly above such threshold value T, the transformed low gain values are used as the final pixel data values. For those values where the high gain pixel values are within a predetermined window about such threshold value T, e.g., between a lower threshold value Th_low and a higher threshold Th_high (where Th_low&lt;T&lt;Th_high), combinations (e.g., linear) of the high gain values and transformed low gain values are used to construct a combination, or composite, pixel data value.  
         [0093]     Such comparison and ultimate use of one of three different possible pixel data values avoid image artifacts that may develop in the recombination process which might otherwise occur due to imperfect transformation of low gain values to equivalent high gain values.  
         [0094]     As one possible example of performing this operation, the digitized pixel data  11  is multiplexed with a multiplexor  22  controlled by a select signal  21   s  for selective storage in a memory  24 . The “zero” data  23   z  is stored in a “zero” data section  24   z , the high gain mode data  23   h  is stored in a high gain mode section  24   h  and the low gain mode data  23   l  is stored in a low gain mode data section  24   l . Some form of subtraction circuitry  26   h ,  26   l  is used to subtract the stored “zero” data  25   z  from the stored high  25   h  and low  25   l  gain mode data to produce (in accordance with conventional CDS techniques, as discussed above) corrected high  27   h  and low  27   l  gain mode pixel data.  
         [0095]     These high  27   h  and low  27   l  gain mode pixel data are provided to a data comparison stage  30  and data combining stage  32 , as well as to an output multiplexor  28 . These data  27   h ,  27   l  are compared to the lower  21   l  and higher  21   h  threshold values, as discussed above (and in further detail below) to produce a multiplexor control signal  31  for the output multiplexor  28 . These data  27   h ,  27   l  are also combined in the combining stage  32  as a function of the lower  21   l  and higher  21   h  threshold data values (discussed in more detail below) to produce a combined data signal  33 . In accordance with the control signal  31 , one of these data signals  27   h ,  27   l ,  33  is then selected as the final pixel data signal  29 .  
         [0096]     Referring to  FIGS. 4 and 5 , operation of the circuitry of  FIGS. 1 and 3  can be better understood. The graphs in these figures represent an example of an operating scenario where a 14-bit ADC converts the incoming pixel signal values such that the low gain mode of operation uses a gain which is ¼ of the gain used in the high gain mode of operation. Accordingly, reconstruction of the low gain values to their respective equivalent high gain values would require a simple multiplication by a factor (e.g., “gain ratio”) of four. As seen in the figures, the recombination operates in three regions. In Region  1 , the final pixel value will equal the pixel value acquired during the high gain mode of operation. In Region  3 , the final pixel value will equal the pixel value acquired during the low gain mode of operation multiplied by the inverse (4) of the gain reduction factor (¼). In Region  2 , i.e., the overlap region defined by the upper Th_high and lower Th_low boundaries, the final pixel value is calculated according to the following formula (in which linear signal measurements are assumed): 
 
 D out=Gain_Ratio* D _low*(( D high− Th _low)/( Th _high− Th _low))+ D _high*(( Th _high− D high)/( Th _high−Th_low)) 
 
 where: 
 
         [0097]     Dout=final pixel value  
         [0098]     D_low=digital value acquired in low gain mode  
         [0099]     D_high=digital value acquired in high gain mode  
         [0100]     Gain_Ratio=ratio of high value reading to low value reading (=4 in present example)  
         [0101]     Th_low=threshold window lower limit  
         [0102]     Th_high=threshold window, upper limit  
         [0103]     Based upon the foregoing, a number of benefits of the presently claimed invention should be evident. At low pixel data signal values, data acquired at the higher resolution characteristic of the high gain mode of operation is used. Where the incoming pixel data signal values approach or exceed those corresponding to the saturation level of the signal processing path associated with the high gain mode of operation, data acquired at the low resolution characteristic of the low gain mode of operation is used, thereby avoiding image saturation. This is particularly advantageous in the context of x-ray image pixel data in that this technique according to the presently claimed invention complements the logarithmic nature of the x-ray image data typically acquired with flat panel imaging arrays. In those image regions with signal levels near the gain crossover point, the system, e.g., image processing computer, has the option of using an intelligent combination of the two values (due to high and low gain modes of operation) per pixel, thereby avoiding image artifacts that may otherwise result due to abrupt changes in pixel data signal gain.  
         [0104]     In conformance with the foregoing discussion, it should be evident to one of ordinary skill in the art that adding additional capacitor and switch pairs and correspondingly increasing the acquisition cycle to three or more signal samples can increase the dynamic range of the circuitry even further by providing multiple selectable signal gain factors. Additionally, it will be understood that the combination algorithm, while discussed as being a linear interpolation of the low and high gain pixel values, can be more complex, e.g., quadratic or cubic spline interpolation methods, to account for effects from nonlinear signal measurements as well as other causes of inaccurate signal measurements. Additionally, for purposes of simplification, the “zero” data need not necessarily be used as otherwise suggested in the discussion above.  
         [0105]     Referring to  FIG. 5A , in accordance with the presently claimed invention, it will be understood that, as noted above, increasing the acquisition cycle to three or more signal samples, can potentially increase the dynamic range of the circuitry further by providing multiple selectable signal gain factors. In this example, the acquisition cycle uses four signal samples, thereby providing four sample signal values S 1 , S 2 , S 3 , S 4 . In conformance with the discussion above, these values S 1 , S 2 , S 3 , S 4  have successive values that decrease in accordance with the charge sharing characteristics of the circuitry of  FIG. 1 . It will be understood for this example that the circuitry of  FIG. 1  would be modified in accordance with well-known circuit techniques to provide for four signal gains by adding additional feedback capacitors with associated switches (not shown), and the timing diagram of  FIG. 2  would include additional signal sampling pulses  9   s  (not shown).  
         [0106]     In conformance with the discussion above, when determining which one or more of the signal samples S 1 , S 2 , S 3 , S 4  are to be used in determining the value for the final pixel signal  29  ( FIG. 3 ), one or more of such signal sample values S 1 , S 2 , S 3 , S 4  are compared to the criteria established by the higher Th_high and lower Th_low thresholds. It should be understood that references to the thresholds as being “higher” and “lower” are merely used for convenience to indicate that one threshold is “higher” than the other in the sense that such “higher” threshold extends further than the “lower” threshold from the point of reference used. For example, the point of reference would normally be considered to have a null (e.g., zero) value and the “higher” threshold would have a magnitude greater than that of the “lower” threshold, with it being immaterial whether the thresholds themselves were positive or negative in polarity.  
         [0107]     The first signal sample S 1  is compared to the thresholds  21   h ,  21   l  to determine whether such signal sample S 1  has a value in region  1 ,  2  or  3 . If this signal sample S 1 , which has the maximum signal sample value, is in region  1 , i.e., is less than the lower threshold, this signal sample S 1  provides the basis for the final pixel signal  29 . If this signal sample S 1  is in region  3 , its value is not used in generating the final pixel signal  29 . If this signal sample S 1  lies in region  2 , then it is selectively combined with the value of the next signal sample S 2 , in conformance with the discussion above. This comparison process is repeated for the remaining signal samples S 2 , S 3 , S 4  in succession. With respect to the final signal sample S 4 , in the event that preceding signal sample S 3  has a value in region  3 , then this last signal sample S 4 , being the minimum signal sample, is used for generating the final pixel signal  29 .  
         [0108]     Referring to  FIG. 6 , in accordance with an alternative embodiment of the presently claimed invention, the input amplifier stage  12   a  can be implemented such that the charge-to-voltage conversion stage has a single signal gain associated with it. Accordingly, a single feedback capacitance C 1  is used (along with a reset switch S 1 , as discussed above). Variable gain for the stage  12   a  as a whole is provided by another amplifier X 2  having a variable signal gain (many types of which are well known to and readily implemented by one of ordinary skill in the art). The voltage signal  13   a  is processed by this second amplifier X 2  using multiple gain settings for the amplifier X 2  according to a gain control signal  9   g  (e.g., similar to signal  9   l ). For example, with reference to the timing diagram of  FIG. 2 , the voltage signal  13   a  can be processed by the second amplifier X 2  using a higher gain setting, with the resulting output signal  13   b  sampled by the sample and hold circuitry  14 . Subsequently, the gain of the second amplifier X 2 , in accordance with the gain control signal  9   g , can be reduced for amplifying the input voltage signal  13   a  to produce a lower-valued voltage signal  13   b  for sampling by the sample and hold circuitry  14 .  
         [0109]     Referring to  FIG. 7 , another alternative embodiment of the presently claimed invention also has a modified charge-to-voltage conversion circuit  12   b . In this embodiment, a single feedback capacitance C 1  is used and an additional resistance r 2  is placed in series with the reset switch S 1 . In this embodiment, the gain of the amplifier stage  12   b  is controlled, i.e., varied, by activating the reset switch S 1  for a short time interval (i.e., less time than that needed to fully discharge the capacitance C 1 ) in between the first and second signal sample pulses of the sample control signal  9   s  ( FIG. 2 ). Accordingly, in the event that the pixel charge acquired prior to the first signal sample pulse causes saturation within the signal path, such interim activation of the reset switch S 1 , and the resulting discharging of a portion of the charge across capacitance C 1  through resistor R 2 , will reduce the voltage signal  13   c  enough to avoid saturation of the signal path during the second signal sample pulse of the sample control signal  9   s . (Depending upon the desired signal gain, signal sampling rate and dynamic range of the potential pixel charge signal, values for the feedback capacitance C 1  and resistor r 2  can be readily determined by one of ordinary skill in the art.)  
         [0110]     Referring to  FIG. 8 , another alternative embodiment of the presently claimed invention provides for variable gain within the signal path by controlling the amount of electrical charge, i.e., the input signal, acquired from the pixel capacitance C 4  prior to the signal sampling pulses of the sampling control signal  9   s . For example, the initial pulse P 1  of the pixel sampling control signal  9   p  will have a short pulse duration (i.e., TFT switch s 4  is activated, or turned on, for a short time interval), thereby causing only a portion of the pixel charge from capacitance C 4  to be shared with capacitance C 1  in the amplifier  12   c . Accordingly, the initial signal sampling interval would form the low gain mode of operation. The subsequent pixel sampling signal  9   p  pulse P 2  would be of sufficient duration so as to allow for fuller sharing of the pixel charge, thereby causing the mode of operation during the second signal sampling interval to be the high gain mode of operation.  
         [0111]     Various other modifications and alternations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and the spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. It is intended that the following claims define the scope of the present invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.