Abstract:
A charge amplifier for use in radiation sensing includes an amplifier, at least one switch, and at least one capacitor. The switch selectively couples the input of the switch to one of at least two voltages. The capacitor is electrically coupled in series between the input of the amplifier and the input of the switch. The capacitor is electrically coupled to the input of the amplifier without a switch coupled therebetween. A method of measuring charge in radiation sensing includes selectively diverting charge from an input of an amplifier to an input of at least one capacitor by selectively coupling an output of the at least one capacitor to one of at least two voltages. The input of the at least one capacitor is operatively coupled to the input of the amplifier without a switch coupled therebetween. The method also includes calculating a total charge based on a sum of the amplified charge and the diverted charge.

Description:
This invention was made with Government support under contract number DE-AC02-98CH10886, awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to charge amplifiers for use in radiation sensors, and more particularly to such charge amplifiers that exhibit an increased dynamic range. 
     2. Description of the Prior Art 
     In radiation sensors, ionizing radiation generates free charges in an amount Q that is proportional to the energy of the ionizing radiation. Charge is typically quantified by using a low-noise charge amplifier, which performs an integration through a feedback capacitor C, thus converting the charge Q into a voltage V=Q/C. 
     In  FIG. 1 , a charge amplifier configuration  10  is shown, where  v n   2    represents a noise power from signal processing electronics following the charge amplifier; such as a filter, buffer, analog-to-digital converter, and the like. 
     In order to make the noise contribution from the processing electronics negligible compared to the signal voltage, which is determined by V=Q/C, a charge-to-voltage conversion gain 1/C is preferably maximized. This can be achieved by minimizing the value of C. Assuming a noiseless charge amplifier, the signal-to-noise ratio is given by 
               Q   /     (     C   ⁢         v   n   2     _         )       ,         
and the minimum detectable charge is given by
 
               Q   min     ≈     C   ⁢           v   n   2     _       .             
On the other hand, a saturation voltage of the amplifier, which is equal to the supply voltage V DD  in an ideal case, limits the measurable charge Q to a maximum value Q MAX =V DD C. An assumption is made that the virtual ground input of the amplifier is at 0V.
 
     The result is that a dynamic range Q MAX /Q MIN  of the configuration shown in  FIG. 1  is independent of C and given by 
               V   DD     /           v     n   ⁢             2     _       .           
Even if the dynamic range can be improved by filtering performed in the signal processing stage, the result is limited by technology and system constraints to typically no more than a few hundred, such as 100-300.
 
     Therefore, it would be advantageous if the dynamic range in charge amplifier configurations could be increased to well above a few hundred once technology and system constraints, such as signal processing and associated noise, are taken into consideration. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a charge amplifier for use in radiation sensing, which includes an amplifier including an input, at least one switch including an input, wherein the switch selectively couples the input of the switch to one of at least two voltages, and at least one capacitor electrically coupled in series between the input of the amplifier and the input of the switch, the capacitor is electrically coupled to the input of the operational amplifier without a switch coupled therebetween. 
     The amplifier may include an output and an operational amplifier, the input of the amplifier is operatively coupled to an anode of the operational amplifier, and the output of the amplifier is operatively coupled to a cathode of the operational amplifier. The charge amplifier may include a second capacitor electrically coupled in parallel with the amplifier. The charge amplifier may include a comparator including a first input, a second input, and an output, wherein the first input is operatively coupled to the output of the amplifier and the second input is operatively coupled to a threshold voltage, and a logic device operatively coupled to the output of the comparator. The logic device may control the switch in response to the output of the comparator. 
     The voltages may include a first voltage less than a second voltage, wherein the logic device controls the at least one switch to couple the input of the switch to the second voltage in response to the output of the amplifier being greater than the threshold voltage. The logic device may control the switch to couple the input of the switch to the first voltage in response to the output of the amplifier being less than the threshold voltage. The charge amplifier may be implemented using at least one of an Application Specific Integrated Circuit (ASIC), a microprocessor, a microcontroller, a programmable logic device, and/or a gate array 
     The present invention further relates to a method of measuring charge in radiation sensing, which includes selectively diverting charge from an input of an amplifier to an input of at least one capacitor by selectively coupling an output of the at least one capacitor to one of at least two voltages, wherein the input of the capacitor is operatively coupled to the input of the amplifier without a switch coupled therebetween, and calculating a total charge based on a sum of the amplified charge and the diverted charge. 
     The method may include comparing voltage at the output of the amplifier to a threshold voltage, and controlling the selective diversion of charge from the input of the amplifier to the input of the capacitor based on the comparison. The method may include controlling the selective diversion of charge from the input of the amplifier to the input of the capacitor by operatively coupling an output of the capacitor to the second voltage in response to the output of the amplifier being greater than the threshold voltage. The method may include controlling the selective diversion of charge from the input of the amplifier to the input of the at least one capacitor by operatively coupling an output of the capacitor to the first voltage in response to the output of the amplifier being less than the threshold voltage. The method may include implementing the charge amplifier using at least one of an Application Specific Integrated Circuit (ASIC), a microprocessor, a microcontroller, a programmable logic device, and/or a gate array. 
     The present invention still further relates to a computer-readable medium including instructions, wherein execution of the instructions by at least one computing device controls measurement of charge in radiation sensing by selectively diverting charge from an input of an amplifier to an input of at least one capacitor by selectively coupling an output of the capacitor to one of at least two voltages, wherein the input of the capacitor is operatively coupled to the input of the amplifier without a switch coupled therebetween, and calculating a total charge based on a sum of the amplified charge and the diverted charge. 
     Execution of the instructions by at least one computing device may control measurement of charge in radiation sensing by coupling a second capacitor electrically in parallel with the amplifier. Execution of the instructions by at least one computing device may control measurement of charge in radiation sensing by comparing voltage at the output of the amplifier to a threshold voltage, and controlling the selective diversion of charge from the input of the amplifier to the input of the capacitor based on the comparison. Execution of the instructions may control measurement of charge in radiation sensing by controlling the selective diversion of charge from the input of the amplifier to the input of the at least one capacitor by operatively coupling an output of the at least one capacitor to the second voltage in response to the output of the amplifier being greater than the threshold voltage. Execution of the instructions may control measurement of charge in radiation sensing by controlling the selective diversion of charge from the input of the amplifier to the input of the capacitor by operatively coupling an output of the capacitor to the first voltage in response to the output of the amplifier being less than the threshold voltage. 
     Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed as an illustration only and not as a definition of the limits of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a conventional charge amplifier configuration. 
         FIG. 2  is a block diagram of a charge amplifier in a multiple-gain configuration. 
         FIG. 3  is a block diagram of a charge amplifier in a charge-pump configuration. 
         FIG. 4  is a block diagram of a charge amplifier in a capacitive charge-pump configuration. 
         FIG. 5  is a block diagram of a charge amplifier configuration in accordance with a first embodiment of the present invention. 
         FIG. 6  is a block diagram of a charge amplifier configuration in accordance with a second embodiment of the present invention. 
         FIG. 7  is a flow chart of a method for determining charge using the charge amplifier configuration shown in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     To increase dynamic range in charge amplifiers, three methods are proposed: a multiple-gain method, a current charge-pump method, and a capacitive charge-pump method. 
     Multiple-Gain Method 
     This method includes progressively reducing the gain by adding in parallel to C one or more capacitors C j  through switches S j , as shown in  FIG. 2  and described in greater detail in V. Bonvicini, G. Orzan, G. Zampa, “CASIS10: A Prototype VLSI Front-End ASIC with Ultra-Large Dynamic Range and Integrated ADC for Silicon Calorimetry in Space Experiments,” Nucl. Instrum. Methods, A 572, pp. 340-344, 2007, which is incorporated herein by reference. If the output voltage V exceeds a threshold V th , a logic circuit  12  enables a first capacitor C 1  to reduce the gain from 1/C to 1/(C+C 1 ). If after enabling C 1 , V does not fall below the threshold, the logic circuit  12  enables connection of a second capacitor C 2 , reducing the gain to 1/(C+C 1 +C 2 ). This sequence continues for an arbitrary number of capacitors until V falls below the threshold. The closed switches preferably define the charge-to-voltage conversion gain to be applied to the resulting voltage V. The values of the capacitors C j  are preferably chosen to cover the dynamic range of interest. When the switch S j  is open, the terminal of the capacitor C j  can be connected either to the input or to ground, the latter is preferred to avoid charge injection from switches connected to the input. 
     The disadvantage of this method is that the reduction in gain limits the signal-to-noise ratio to that achievable with the configuration shown in  FIG. 1 . Accordingly, with one capacitor, the maximum signal-to-noise ratio is given by 
                 V   DD     ⁢     C   /     (     C   ⁢         v   n   2     _         )         =       V   DD           v   n   2     _               
Likewise, with N capacitors, the maximum dynamic range is identically given by
 
                 V   DD     ⁢     NC   /     (     NC   ⁢         v   n   2     _         )         =         V   DD           v   n   2     _         .           
Current Charge-Pump Method
 
     This method includes subtracting charge by enabling a controlled current source of value i p  for fixed time intervals Δt j , as shown in  FIG. 3  and described in greater detail in E. Kraft, P. Fisher, M. Karagounis, M. Koch, H. Krueger, I. Peric, N. Wermes, C. Herrmann, A. Nascetti, M. Overdick, and W. Ruetten, “Counting and Integrating Readout for Direct conversion X-ray Imaging: Concept, Realization and First Prototype Measurements,” IEEE Trans. Nucl. Sci., vol. 54, pp. 383-390, 2007, which is incorporated herein by reference. If the output voltage V exceeds a threshold V th , the logic circuit  14  generates a first control pulse having a duration Δt 1 , which subtracts a fixed charge q p1 =i p Δt 1 . If after the first control pulse V does not fall below the threshold, the logic circuit  14  generates a second control pulse of duration Δt 2 , which subtracts another fixed charge q p2 =i p Δt 2 . This sequence continues until V falls below the threshold. The number and duration of pulses defines the charge to be added to that measured from the resulting voltage V. 
     The disadvantage of this method is the length of time required for the entire integration to be performed, which is derived from the requirement concerning the accuracy of Δt. If q p =i p Δt is the subtracted charge and σ t  is the time jitter on Δt, the noise associated with the charge subtraction is given by σ q =σ t i p =σ t q p /Δt. The noise can then be reduced by increasing Δt, but the duration of the integration increases accordingly. 
     Capacitive Charge-Pump Method 
     This method includes subtracting amounts of charge by charging and discharging a capacitor C p , as shown in  FIG. 4  and described in further detail in G. Mazza, R. Cirio, M. Donetti, A. La Rose, A. Luparia, F. Marchetto, and C. Peroni, “A 64-Channel Wide Dynamic Range Charge Measurement ASIC for Strip and Pixel Ionization Detectors,” IEEE Trans. Nucl. Sci, vol. 52, pp. 847-853, 2005, which is incorporated herein by reference. The reference voltage V ref  is assumed to be equal to the amplifier input voltage. If the output voltage V exceeds a threshold V th , a logic circuit  16  generates a first cycle of control pulses that opens switches S 1  and S 2  and closes switches S 3  and S 4 , which subtracts a fixed charge q=V DD /C p . If after the first cycle of control pulses, V does not fall below the threshold, the logic circuit  16  generates a second cycle of control pulses, which subtracts another fixed charge q. This sequence continues for an arbitrary number of control pulses until V falls below the threshold. The number of control cycles defines the charge to be added to that measured from the resulting voltage V. 
     The disadvantage of this method is that it requires a switch connected to the input node of the charge amplifier. As a consequence, parasitic charge is injected during the switching activity. 
       FIG. 5  shows a preferred embodiment of a charge amplifier formed in accordance with the present invention. The charge amplifier includes an operational amplifier  20 , which is responsive to an applied charge source  22  and outputs a voltage to a signal processing circuit  24 . A capacitor  26  is electronically coupled in parallel with the operational amplifier  20 , that is, the capacitor  26  is connected across the anode and cathode of the operational amplifier  20 . Another capacitor  28  is electronically coupled in series between the anode of the operational amplifier  20  and a bank of switches S 1b    32  and S 1a    30  with common input. Yet another capacitor  34  is shown connected (similarly to capacitor  28 ) electrically in series between the anode of operational amplifier  20  and a bank of switches S 2a    36  and S 2b    38 . The logic circuit  18  preferably controls each of the switches S 1a    30 , S 1b    32 , S 2a    36 , and S 2b    38 . One input of a comparator  40  is preferably connected to the cathode of the operational amplifier  20 , and a remaining input of comparator  40  is connected to a threshold voltage V th . The output of comparator  40  is then provided to the logic circuit  18 . 
     The method in accordance with the present invention subtracts amounts of charge by using a number of additional capacitors C j  controlled through switches S ja  and S jb , as shown in  FIG. 5 . If the output voltage V exceeds a threshold voltage V th , the logic circuit  18 , by opening S 1a  and closing S 1b , routes the terminal of a first capacitor C 1  from a first fixed voltage V 1  (which is preferably ground, as shown in  FIG. 5 ) to a higher fixed voltage V 2  (preferably the supply V DD  shown in  FIG. 5 ), thereby subtracting a charge C 1 (V 2 −V 1 ) (C 1 V DD  in the case of  FIG. 5 ). If after the first subtraction V does not fall below the threshold, the logic circuit  18  preferably performs a second subtraction C 2 (V 2 −V 1 ) through a second capacitor C 2 . This sequence continues until V does fall below the threshold. The number of subtractions, along with the associated C j , defines the charge to be added to that measured from the resulting voltage V. The values of the capacitors C j  are chosen to cover the dynamic range of interest. 
     The configuration shown in  FIG. 5  is intended for measuring positive charges. A configuration for measuring negative charges can also be realized by inverting VDD and ground at each of the switches S Ja  and S Jb  in  FIG. 5 ). For example, switch S 1a    30  is connected to the higher fixed voltage V 2 , which is preferably V DD , and switch S 1b    32  is connected to the lower fixed voltage V 1 , which is preferably ground, as shown in  FIG. 6 . 
       FIG. 7  shows a flow chart of the method for use in accordance with a charge amplifier configuration shown in  FIG. 5 . Each of these switches, as designated by J=1 to N, where N is the total number of S a  switches and total number of S b  switches, is initiated by closing the S Ja  switches for J=1 to N and opening the S Jb  switches for J=1 to N in step  42 . J, an index variable, is then initialized to 0 in step  44  and the charge to be determined is inputted in step  46 . If V is greater than V th  in step  48 , then J is incremented by 1 instead of 50. VS a  switch corresponding to the current value of J is then opened, and the VS b  switch corresponding to the current value of J is closed in step  52  and the voltage V is then checked against the V th  in step  48 . 
     If the voltage V is not greater than V th  in step  48 , the measured charge is applied to the signal processing circuit in step  54 . The subtracted charged is then calculated in step  56  using the following equation: 
     
       
         
           
             
               ∑ 
               
                 i 
                 = 
                 1 
               
               j 
             
             ⁢ 
             
               
                 C 
                 i 
               
               ⁡ 
               
                 ( 
                 
                   
                     V 
                     2 
                   
                   - 
                   
                     V 
                     1 
                   
                 
                 ) 
               
             
           
         
       
     
     The subtracted charge calculated in step  56  is then added to the measured charge determined in step  54  to determine the total charge in step  58 . 
     As an example, if the total charge were Q=1.1 pC (picocoulombs), Cj=0.1pF, and V 2 −V 1 =2.5V, then four (4) subtractions would be required, each being of 0.25 coulombs, yielding a subtracted charge of 1.0 picocoulombs and a measured charge of 0.1 picocoulombs. 
     Thus, the method of the present invention advantageously increases the dynamic range of a charge amplifier without requiring accurate timing signals or switches connected to the input of the charge amplifier. The present invention also provides the advantage of a signal-to-noise ratio that is not limited to that achievable using the configuration shown in  FIG. 1 . In the case of  FIG. 5  and assuming identical Cj, for a given charge Q, the resulting voltage is given by V=Q/C−NV DD  where N is the number of subtractions. The maximum signal-to-noise ration or dynamic range is then effectively given by 
                 [       V   DD     +     NV   DD       ]     ⁢     C   /     (     C   ⁢         v   n   2     _         )         =         V   DD     ⁡     (     N   +   1     )             v   n   2     _               
which increases with N.
 
Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawing, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention.