Abstract:
A gain characteristic correctable dynamic range enhancement system (DRES) receives input signals from an imager device connected to a correlated double sampling (CDS) circuit for receiving the video signal from the CCD imaging device. The dynamic range enhancement system includes a variable gain amplifier (VGA), and a limited bit-width analog-to-digital converter (ADC) which digitizes the analog signal received from the VGA. The output of the ADC is provided to an initial bit range position of a wider bit-width shifter connected to the output of the ADC. The DRES system correctably extends the dynamic range of the imager device, subject to offsets providing linearity corrections at predetermined trip points, subject to determined offset values, to ensure that there are no discontinuities in the system transfer function.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to patent application Ser. Nos. 09/283,779, 09/283,098, 09/282,524, 09/282,515, and 09/283,112, respectively entitled “Dynamic Range Extender Apparatus, System, and Method for Digital Receiver System” having inventors Sandra Marie Johnson and Nadi Rafik Itani and issued as U.S. Pat. No. 6,252,536 on Jun. 26, 2001 (Cirrus Logic Docket No. 0930-CIC); “Phase Locked Loop Circuits, Systems, and Methods” having inventors Douglas R. Holberg and Sandra Marie Johnson (Cirrus Logic Docket No. 0893-CIC); “Preview Mode Low Resolution Output System and Method” having inventors Douglas R. Holberg, Sandra Marie Johnson, and Nadi Rafik Itani (Cirrus Logic Docket No. 0944-CIC); “Amplifier System with Reducable Power” having as inventor, Nadi Rafik Itani (Cirrus Logic Docket No. 0965-CA); “CCD Imager Analog Processor Systems and Methods” having inventors Douglas R. Holberg, Sandra Marie Johnson, Nadi Rafik Itani, and Argos R. Cue (Cirrus Logic Docket No. 0929-CIC). Each of the above applications is filed on even date herewith. Additionally, this application is related to patent application Ser. No. 08/856,468, entitled “System and Method for Enhancing Dynamic Range in Images” invented by S. Khalid Azim and issued as U.S. Pat. No. 6,137,533 on Oct. 24, 2000 (Cirrus Logic No. 0720-CIC). Each of these applications is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to analog-to-digital converter circuitry and more particularly to successive approximation calibration apparatus, systems, and methods for dynamic range extension in analog-to-digital converter circuitry for camera and imaging systems. 
     2. Description of Related Art 
     In recent years, solutions to difficult mixed signal problems related to dynamic range control in camera and imager devices and systems have been attempted. Particularly, it has been desired to develop low cost and low power approaches to improving the dynamic range of digital images. Further, new solutions to image data acquisition and processing have been attempted to result in visible improvements in image quality. Digital camera image quality improvement are sought for video as well as still image camera systems and imaging systems which use charge-coupled device (CCD) imagers, CMOS imagers, and other kinds of imagers. 
     It is known that the number of bits required for analog-to-digital conversion of CCD data depends upon the noise floor of a CCD, based upon photon shot noise, dark-current noise, and thermal noise from a CCD output amplifier. A system to capture the CCD output requires a quantization noise level lower than the noise floor. The maximum output of the CCD and the noise floor of the CCD can be used to determine the maximum number of bits required for an analog-to-digital converter to have its quantization noise level below the noise level of the CCD. For a particular CCD, the noise voltage level is estimated at about 150 μVrms. The maximum CCD output voltage is about 800 mV. Based upon these conditions, a 12-bit analog-to-digital converter is useful based upon dynamic range requirements. Unfortunately, a 12-bit converter is costly in terms of power and area. 
     It is further desirable to achieve enhanced image quality with images having improved detail in both dark and light image regions, while avoiding the penalties of high power consumption and large silicon area usage. 
     U.S. Pat. No. 4,647,975, entitled “Exposure Control System for an Electronic Imaging Camera Having Increased Dynamic Range” describes an electronic imaging system with an expanded dynamic exposure range implemented in two exposure intervals. 
     It is additionally desirable to maintain output linearity and monotonicity during dynamic range extension for analog-to-digital converter circuitry, so that continuity is maintained between segments of the operational characteristic of the converter circuitry irrespective of proximity to trip points. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a dynamic range expandable imaging system which has a correlated double sampling system, a variable gain amplifier circuit connected to said correlated double sampling system, an analog-to digital converter connected to said variable gain amplifier circuit, and a shifter containing a predetermined number of bits greater than the digital output width of said analog-to-digital converter is calibrated according to a successive approximation technique to ensure output linearity and monotonicity during dynamic range extension for analog-to-digital converter circuitry, so that continuity is maintained between segments of the operational characteristic of the converter circuitry irrespective of proximity to trip points. A shifter is connected to said analog-to-digital converter for receiving the output bit set of the analog-to-digital converter into predetermined locations in the shifter. Input test signals are injected from a predetermined input circuit for sampling by a correlated double sampling system, above and below a first trip point in VGA input values at which VGA gain shifts have been determined, and the difference in analog-to-digital converter output corresponding to said first trip point is determined. Further, input test signals from a predetermined input circuit are provided for sampling by a correlated double sampling system, above and below a next trip point in VGA input values at which VGA gain shifts have been determined and the difference in analog-to-digital converter output corresponding to said next trip point is determined as a calibration value. According to the present invention, a dynamic range enhancement system (DRES) is provided for an imager device which includes a correlated double sampling (CDS) circuit for receiving the video signal from the CCD imaging device, a variable gain amplifier (VGA)subject to automatic gain control, an analog-to-digital converter (ADC) which digitizes the analog signal received from the VGA, an offset mechanism which adjusts the digital output of the ADC to ensure trip-point monotonicity and linearity, and a shifter for adjusting the bit-width of the digital signal to compensate for a change in the amplification provided by the variable gain amplifier. According to the present invention, dynamic range enhancement is achieved in a signal processing system for an imager device subject to offset correction at predetermined trip-points in the ADC characteristic. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a controllable dynamic range extension signal processing circuit including a correlated double sampling (CDS) circuit for receiving the video signal from the CCD imaging device, a variable gain amplifier (VGA)subject to automatic gain control, and an analog-to-digital converter (ADC) which digitizes the analog signal received from the VGA, according to the present invention; 
     FIG. 2 is a graph of a variable gain control function subject to dynamic range extension according to one embodiment of the present invention; 
     FIG. 3 is a graph of DOUT as a function of VGA input, according to one embodiment of the present invention; 
     FIG. 4 is a circuit diagram of a 2-bit analog-to-digital converter system including first, second, and third comparators set to successively increasing thresholds, for effecting dynamic range extension, according to one embodiment of the present invention; 
     FIG. 5 is a block diagram of a logic circuit according to one embodiment of the present invention, for generating values of A, C, B_Z, and A_Z, for connecting switch settings of a variable gain amplifier (VGA) as shown in FIG. 7; 
     FIG. 6 is a block diagram of a correlated double sampling (CDS) circuit for receiving the video signal from the CCD imaging device, according to the present invention; 
     FIG. 7 is a block diagram of a variable gain amplifier (VGA) subject to automatic gain control, according to the present invention; 
     FIG. 8A is a block diagram of a circuit system in a controllable dynamic range extension signal processing (DRX) circuit including a correlated double sampling (CDS) circuit for receiving the video signal from the CCD imaging device, including a calibration register, in accordance with an embodiment of the present invention; 
     FIG. 8B is a block diagram of an offset storage bit register system for a controllable dynamic range extension signal processing circuit, in accordance with the present invention; 
     FIG. 8C is a block diagram of a calibration reference selection circuit for a controllable dynamic range extension signal processing circuit, in accordance with the present invention; 
     FIG. 8D is a graph of the output of an analog-to-digital converter for selected gain settings of high and low gain, according to one embodiment of the present invention; 
     FIG. 8E is a diagram of selected portions of a controllable dynamic range extension signal processing (DRX) circuit according to the present invention, for processing signals received from a selected imaging device; 
     FIG. 9A is a flow chart of a offset value determination method according to the present invention showing the successive determination of respective offset values, OFFSET 1 , OFFSET 2 , and OFFSET 3 ; and 
     FIG. 9B is a flow chart of a method according to the present invention for the determination of a single one of the offset values, OFFSET 1 , OFFSET 2 , and OFFSET 3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, there is shown a block diagram of a controllable dynamic range extension signal processing (DRX) circuit  2  for processing signals received from a selected imaging device  3 . The DRX circuit  2  includes a correlated double sampling (CDS) circuit  4  which receives the video signals from the imaging device  3  which may be a charge coupled device (CCD), for example. The DRX circuit  2  further includes a variable gain amplifier (VGA)  5  subject to automatic gain control according to the present invention, as will be discussed in detail below. The VGA  5  has input and output connections which respectively receive an input analog signal and produce an output amplified analog signal in accordance with the gain setting which is current for the VGA  5 . The DRX circuit  2  further includes an analog-to-digital converter (ADC)  6  which digitizes the analog signal received from the VGA  5 . According to one embodiment of the present invention, the ADC produces a 10-bit digital output representative of the analog signal received from the VGA  5 . The DRX circuit  2  further includes a shifter  7  for controllably  1 shifting the digital bits in a group to increase or decrease the magnitude of the digital output. Such an increase or decrease is accomplished according to one embodiment of the present invention by applying successive factors of two (2) or one-half (½), for example. The DRX circuit  2  further includes logic circuitry  8  for processing a received PIX_GAIN value to produce a control signal governing whether to shift the bit contents of shifter  7  and, if so, in what direction and to what extent. The DRX circuit  2  further includes a multiplexer  9  for applying a selected, predetermined offset value, e.g., OFFSET 1 , OFFSET 2 , or OFFSET 3 , under direction of an output signal from logic circuitry  8 . These offset values are generated with calibration system  20  as will be discussed in greater detail below, and they are stored according to one embodiment in respective offset registers  21 - 23 . The DRX circuit  2  further includes a summer  10  connected to multiplexer  9  and to ADC  6 . The summer  10  receives a selected digital offset value from multiplexer  9  as directed by logic circuitry  8  for summation with the digital output received from ADC  6 . The DRX circuit  2  further includes a 2-bit analog-to-digital converter  11  according to one embodiment of the present invention, to establish a PIX_GAIN value. The PIX_GAIN value is an input to logic circuitry  8 , and the output of logic circuitry  8  is provided to VGA  5  as a control signal to determine the gain setting of VGA  5 . As noted above, an offset value is provided to summer  10  from multiplexer  9 . Further, the shift status and the amount to be exercised by shifter  7  is determined for producing an extended dynamic range output signal DOUT, for example to a precision level of 13 bits for example. The 2-bit ADC  11  within DRX circuit  2  in turn includes first, second, and third comparators, respectively  14 - 12 , set to successively increasing doubled thresholds for example, for effecting dynamic range extension, according to one embodiment of the present invention. CDS  4  receives a signal from an imaging device  3  which may be a charge coupled device (CCD). The CDS  4  is connected at its output to VGA  5  and 2-bit ADC  11 . The output of 2-bit ADC  11  is connected through logic circuitry  8  to VGA  5  to control the level of VGA amplification, and to logic circuitry  8  to provide it with a pixel gain value, PIX_GAIN, for selection of an offset value with multiplexer  9 . The output of VGA  5  is connected to the input of ADC  6 , and the outputs of ADC  6  and multiplexer  9  are connected to input sides of adder  10 . The output of adder  10  is connected to shifter  7  to enable shifting operation in accordance with the output of logic circuitry  8 . The output of shifter  7  is DOUT. The DRX circuit  2  further includes a calibration system  20  and first through third offset registers respectively  21 - 23 , according to one embodiment of the present invention. The details of calibration system  20  and its operation are described below in connection with FIGS. 8A-8C. 
     Referring now to FIG. 2, there is shown a graph of the output of ADC  6  for selected gain settings of x 1 -x 8 , according to one embodiment of the present invention. The ADC_OUTPUT, i.e., the output of the analog-to-digital converter  6 , can range from zero to full-scale (i.e., from zero to 1023) while VGA_INPUT values range from zero to about 0.2 at a gain setting of x 8 . Alternatively, the output of the analog-to-digital converter  6 , can range from zero to full-scale (i.e., from zero to 1023) when the VGA_INPUT values range from about zero to about 0.4 at a final gain setting of x 4 . In another case, the output of the analog-to-digital converter  6 , can range from zero to full-scale (i.e., from zero to 1023), while the VGA_INPUT ranges from zero to about 0.8 at a final gain setting of x 2 . In even another case, the output of the analog-to-digital converter  6 , can range from zero to full-scale (i.e., from zero to 1023), while the VGA_INPUT ranges from about zero to about 1.6 at a final gain setting of the VGA  5  of x 1 . In operation according to the present invention, the highest possible gain setting is selected for a particular VGA input signal range. When a trip point is reached at which the VGA input corresponds to an out-of-range ADC output value, e.g., greater than 1023, the VGA gain is reduced to a next lower level, which is one half of the immediately prior gain. The trip points lie at regular intervals spaced from each other, for example at VGA input values which are double the value of the next lower value trip point. As the VGA input increases in value beyond a particular trip point, the gain of the VGA  5  is cut in half, resulting in an approximately halved ADC  6  output level. For example, when the ADC output reaches approximately 1023 according to one embodiment, the output level of the ADC  6  abruptly drops to one half of 1023, i.e., approximately to 512, as the gain of the VGA  5  is suddenly cut in half. The trip points illustrated graphically in FIG. 2 are implemented according to one embodiment of the present invention with the comparators  12 - 14  in 2-bit ADC  11 . As shown in FIG. 1, the indicated comparators  14 - 12  are provided with successively doubled threshold values which correspond to the respective trip points expressed in FIG.  2 . The analog output voltage level from CDS  4  is such that it exceeds particular ones of the negative input settings provided to the respective comparators  12 - 14 . Consequently, a selected different output signal from the particular associated comparator is provided to VGA  5  and to logic circuitry  8 , to indicate the fact of exceeding. The comparators are intentionally biased slightly below the ideal trip point. In this way, the gain is guaranteed to switch before the ADC reaches the full scale level of 1023. This intentional offset is needed, since non-idealities in the analog circuitry can produce offsets which could cause the ADC to saturate at 1023 for a portion of the transfer function before the gain is changed in the VGA, thus producing some flat regions in the transfer function. The trip point uncertainty is shown as a dotted region in FIG.  2  and the intentional offset biasing is shown in FIG. 4 as offset A, B, C. The logic circuitry  8  provides a compensatory signal to shifter  7  to cause a doubling shift in the shifter  7  whenever a trip point is reached which halves the VGA and corresponding ADC output levels. According to one embodiment of the present invention, the ADC output is adjusted at summer  10  to ensure continuity in the shifter output as trip points are crossed with the resulting adjustment of VGA gain levels and shifter bit settings. These adjustments are to correct for the potential offset error caused by the fact that the analog gain changes in the VGA can not match the gain changes or shifts in the shifter section. Thus, these offset adjustments are needed when switching between various VGA settings. 
     Referring now to FIG. 3, there is shown a graph of the output of shifter  7  (DOUT) as a function of VGA input, with DOUT ranging from zero to 8191, according to one embodiment of the present invention. To express the DOUT range corresponding to a VGA_INPUT range from zero to about 0.2, output bits  9 - 0  are employed. To express the DOUT range corresponding to a VGA_INPUT range from 0.2 to about 0.4, output bits  10 - 1  are employed. To express the DOUT range corresponding to a VGA_INPUT range from 0.4 to about 0.8, output bits  11 - 2  are employed. To express the DOUT range corresponding to a VGA_INPUT range from 0.8 to about 1.6, output bits  12 - 3  are employed. As can be seen, the curve of DOUT is smooth, monotonic, and continuous, even at transitions associated with trip points 0.2, 0.4, and 0.8. The point 1.6 marks the end-of-range for VGA input values, and does not represent a trip point according to this embodiment of the present invention. According to another embodiment of the present invention, in which a 3-bit ADC or an n-bit ADC is used in lieu of 2-bit ADC  11 , additional thresholds are established within the scope and meaning of the present invention. Such thresholds amount to additional trip points, and require additional comparators connected in series to supplement the configuration of the ADC  6  embodiment expressed in FIG.  1 . 
     Referring now to FIG. 4, there is shown a circuit diagram of a 2-bit analog-to-digital converter (ADC) system  11  according to the present invention. The ADC system  11  includes first, second, and third comparators, respectively  12 ,  13 , and  14 . These comparators  14 - 12  are set to successively increasing thresholds, for detecting the need for dynamic range extension and for producing signals used to set the level of amplification applied by VGA  5 , according to one embodiment of the present invention. Each of comparators  12 - 14  has a positive and a negative input. According to the indicated embodiment, the positive inputs of respective comparators  12 - 14  are connected to the output of CDS  4 . The 2-bit analog-to-digital converter system  11  further includes series connected resistors respectively  41 - 44 , having respective connection nodes there between. In particular, resistor  44  is connected to resistor  43  at a first connection node on one side of resistor  44 , as well as to a selected reference voltage, Vref, at the remaining side of resistor  44 . Resistor  43  is connected to resistor  42  at a second connecting node, and resistor  42  is connected to resistor  41  at a third connecting node. The respective first, second, and third connecting nodes provide voltage settings for respective comparators  14 - 12 , at which the comparators express trip points at which VGA gain levels are switched and shifter action is required to compensate for the VGA gain level switching that has been accomplished. According to one embodiment of the present invention, each of resistors  43 - 44  is fabricated to be substantially equal to the other one of the resistors  43 ,  44 . The resistance of resistor  42  is further twice the resistance of resistor  43 , and the resistance of resistor  41  is twice the resistance of resistor  42 , according to one embodiment of the present invention. Resistors  41 - 44  are thus configured as a voltage divider circuit. Resistor  41  is connected to a voltage level of 1.6 volts plus Vref, according to one embodiment of the present invention, causing the voltage level at the node between resistors  41  and  42  to be 0.8 volts plus Vref, subject to an intentional voltage offset OFFSET C away from the predetermined design value. Further, the voltage level between resistors  42  and  43  is 0.4 volts plus Vref subject to an intentional voltage offset OFFSET B. Further, the voltage level between resistors  43  and  44  is 0.2 volts plus Vref subject to an intentional voltage offset OFFSET A away from the predetermined design value. These intentional offsets bias the comparators slightly below the ideal trip point in order to prevent the 10-bit ADC from becoming saturated before the trip point is reached, and to prevent a flat region in the transfer function. 
     Referring now to FIG. 5, there is shown a block diagram of a logic circuit  8  according to one embodiment of the present invention, for generating the signals A, C, B_Z, and A_Z, for connecting switch settings of a variable gain amplifier (VGA)  5  as shown in FIG.  7 . Logic circuit  8  particularly includes first, second, and third flip-flops respectively  51 ,  52 , and  53 ; and first, second, third, and fourth OR gates  55 ,  56 ,  57 , and  58 . The output pixel gain values from respective comparators  12 - 14  of 2-bit ADC  11  are provided as respective pixel gain setting values PIX-GAIN_C, PIX-GAIN_B, and PIX-GAIN_A to corresponding flip-flops  51 - 53 . When flip-flops  51 - 53  are clocked by a clock signal from clock source φ 1 (bar), the respective flip-flop output values are provided to multiplexer  9  and shifter  7  and calibration system  20 . The outputs of flip-flops  52  and  53  are additionally provided to respective OR gates  57  and  58  to produce respective logical outputs B_Z and A_Z, which are provided as control outputs in conjunction with the outputs of OR gates  55  and  56  to VGA  5 . The outputs of OR gates  55  and  56  are provided with clock φ 2  pulses. 
     Referring now to FIG. 6, there is shown a block diagram of a correlated double sampling (CDS) circuit  4  for receiving a signal from an imaging device  3  such as a charge coupled device (CCD), as used in connection with one embodiment of the present invention including features and elements permitting calibration of offset registers  21 - 23  as discussed herein. CDS  4  particularly includes first and second switches  58  and  59  respectively, which are opened and closed according to separate clock phases, φ 1  and φ 2 , for applying respective voltage levels Vref+0.8 and Vref at the respective indicated clock times to conduct respective switching operations with first and second switches  58  and  59 . Switches  58  and  59  are connected to an offset capacitor  107  (Coff), permitting alternate application of Vref+0.8 and Vref voltage levels to Coff with respective clock signals φ 1  and φ 2 , CDS  4  further includes third and fourth switches  61  and  62  respectively, which are opened and closed according to respective clock signals φ 2  and φ 1 , for applying respective voltage levels Vref+0.2 and Vref at the respective indicated clock times to conduct respective switching operations with third and fourth switches  61  and  62 . CDS circuit  4  additionally includes a variable black level setting capacitor  63 , and an op-amp  64  which has a negative and a positive input node. The positive input node of op-amp  64  is set to Vref. The negative input node of op-amp  64  is a common node for various components of CDS circuit  4 . CDS circuit  4  additionally includes a switch  65  which opens and closes according to clock phase φ 1  and is connected from the negative input node of op-amp  64  to its output connection. CDS circuit  4  additionally includes a capacitor C 1 , i.e., capacitor  66 , which is connected from the negative input node of op-amp  64  to its output connection. CDS circuit  4  additionally includes an analog input pad  67  which is connected to imaging device  3  for receiving input analog signals of selected kinds, such as for example input video signals. CDS circuit  4  additionally includes a capacitor Cl, i.e., capacitor  68 , which is connectable in series with analog input pad  67  and is connectable to the negative input node of comparator  64 . CDS circuit  4  further includes first through third calibration switches  69 - 71 , which respectively connect capacitor  68  to VS, Vref, and to analog input pad  67  at respective signal pulse times CAL &amp; φ 1 , CAL &amp; φ 2 , and {overscore (CAL)}. CDS circuit  4  enables calibration of DRX circuit  2  according to the present invention. CDS circuit  4  further assists in establishing the amounts of offset values provided to multiplexer  9  for selective application to summation node  10 . This ensures continuity and monotonicity over trip points at which coordinated gain settings and bit shifts are undertaken. It further results in an extended dynamic range from an abbreviated bit length ADC  6 . An example of the construction of calibration input circuit  60  according to an embodiment of the present invention is set forth in FIG.  8 A. 
     Referring now to FIG. 7, there is shown a variable gain amplifier (VGA)  5  subject to automatic gain control in accordance with feedback from the output of 2-bit ADC  11  by controlling the opening and closing of particular switches, according to the present invention. In particular, VGA  5  includes first and second op-amps  72  and  73  respectively. VGA  5  further includes first and second capacitors  74  and  75 , i.e., capacitors C 2  and C 3 . The first capacitor  74  is connected to CDS  4  (see FIG. 1) and to the negative input connection of op-amp  72 . The positive connection of op-amp  72  is connected to Vref, as is the positive input connection of op-amp  73 . The second capacitor  75  is connected to the output connection of op-amp  72  and the negative input connection of op-amp  73 . Op-amp  72  is adjustable to gain settings of x 1 , x 2 , and x( 2  and  2 / 3 ). Op-amp  73  is settable to gain settings of x 1 , x 2 , and x 3 . VGA  5  further includes capacitors  76 ,  77 , and  78 ; switches  79 - 81 ; capacitors  85 - 87 ; and switches  88 - 90 . Capacitor  76  and switch  80  are connected in series. Capacitor  77  is connected in series with switch  81 . Switch  79 , the series combination of capacitor  76  and switch  80 , the series combination of capacitor  77  and switch  81 , and capacitor  78  are connected in parallel between the negative input node of op-amp  72  and its output node. Switch  79  opens and closes as a function of clock phase φ 2 . Switch  80  opens and closes with the logical value of logical signal C at the output of logic circuitry  8 . Switch  81  opens and closes with the logical value of logical signal A at the output of logic circuitry  8 . The value of capacitor  76  according to one embodiment of the present invention is ½ of the capacitance of capacitor  74 . The value of capacitor  77  according to one embodiment of the present invention is ⅛ of the capacitance of capacitor  74 . The value of capacitor  78  according to one embodiment of the present invention is ⅜ of the capacitance of capacitor  74 . Capacitor  85  and switch  89  are connected in series. Capacitor  86  is connected in series with switch  90 . Switch  88 , the series combination of capacitor  85  and switch  89 , the series combination of capacitor  86  and switch  90 , and capacitor  87  are connected in parallel between the negative input node of op-amp  73  and its output node. Switch  88  opens and closes as a function of clock phase φ 1 . Switch  89  opens and closes with the logical value of logical signal B_Z at the output of logic circuitry  8 . Switch  90  opens and closes with the logical value of logical signal A_Z at the output of logic circuitry  8 . The value of capacitor  85  according to one embodiment of the present invention is ½ of the capacitance of capacitor  75 . The value of capacitor  77  according to one embodiment of the present invention is ⅙ of the capacitance of capacitor  75 . The value of capacitor  87  according to one embodiment of the present invention is ⅓ of the capacitance of capacitor  75 . 
     Referring now to FIG. 8A, there is shown a block diagram of circuit system  91  for a portion of a controllable dynamic range extension signal processing (DRX) circuit, which includes a correlated double sampling (CDS) circuit  4  for receiving the video signal from the CCD imaging device  3  for transmittal to a variable gain amplifier (VGA) (not shown) and an analog-to-digital converter (ADC) (not shown) for digitizing the analog signal received from the VGA. The circuit system  91  further includes a 2-bit ADC  11 , and calibration system  20 , in accordance an embodiment of the present invention. The 2-bit ADC  11  includes first, second, and third op-amps, respectively  14 - 12 , set to successively increasing thresholds, for effecting dynamic range extension, according to one embodiment of the present invention. CDS  4  includes first and second switches  61  and  62  respectively, which are opened and closed according to separate clock phases, φ 1  and φ 2 , for applying respective voltage levels V ref  and V ref +0.2 at the respective indicated clock times. CDS circuit  4  additionally includes a variable black level setting capacitor  63 , and an op-amp  64  which has a negative and a positive input node. The positive input node of op-amp  64  is a common node for various components of CDS circuit  4 . CDS circuit  4  additionally includes a switch  65  which opens and closes according to clock phase φ 1  and is connected between the negative input node and the output node of op-amp  64 . CDS circuit  4  additionally includes a capacitor Cl, i.e., capacitor  66 , which is connected between the negative input node and the output node of op-amp  64 . CDS circuit  4  additionally includes an analog input pad  67  which is connected to imaging device  3  through an emitter-follower and AC coupling capacitor for receiving input analog signals of selected kinds, such as for example input video signals. CDS circuit  4  additionally includes a capacitor C 1 , i.e., capacitor  68 , which is connected in series with switch  71  and is connected to a common node at the negative input node of op-amp  64 . CDS circuit  4  is connected at the common node to calibration circuitry for calibrating the DRX circuit  2  according to the present invention. This is done to establish the values of offset values provided to multiplexer  9  for selective application to summation node  10 . As a result, it is ensured that continuity and monotonicity are established over selected trip points. At these trip points, coordinated gain settings and bit shifts are undertaken in order to obtain an extended dynamic range from an abbreviated bit length ADC  6 . The circuit system further includes first, second, third and fourth switches  58 - 62 ; and an offset capacitor  107  connected to capacitor  68 . Capacitor  63  is connected to third and fourth switches  61 - 62 . First switch  58  connects capacitor  107  to V ref +0.8 at φ 1  phase determined times, and to V ref  at φ 2  phase determined times. Switches  69 - 70  are connected to capacitor  68 . As noted above, CDS circuit  4  thus assists in establishing offset values for application to summation node  10  to ensure continuity and monotonicity over selected trip points. Calibration system  20  is connected to 2-bit ADC circuit  11  and to CDS  5  according to one embodiment of the present invention, with an offset code control line to variable capacitor  63 . Calibration system  20  includes a multiplexer  101  connected to the output of 2-bit ADC  11  for receiving each of the three output lines of the comparators  12 - 14  of the ADC  11 . Calibration system  20  further includes an averaging circuit  102  for averaging over a selected number, e.g.,  16  samples. Calibration system  20  further includes a multiple bit register system  103  for storing bits from a most significant to least significant bit, to provide an offset code value to control the capacitance of variable capacitor  63 . Bit register system  103  includes a plurality of bit circuits  111 - 119  connected in parallel and providing correction bits of ascending significance in a register system according to the present invention. 
     Referring now to FIG. 8B, there is shown a block diagram of an offset storage bit register  200  serving as an example of one of bit circuits  111 - 119 , for a controllable dynamic range extension signal processing (DRX) circuit  2 , in accordance with the present invention. The DRX circuit  2  includes first, second, and third series connected multiplexers respectively  201 - 203  and a flip-flop  204  connected to multiplexer  203  at the output thereof. The signal CAL controls the opening and closing of switches  69 - 71  (FIG. 6) during calibration operation. In particular, to calibrate the respective offset registers  21 - 23  (shown in FIG. 1) with corresponding offset values having according to a preferred embodiment nine bits, the value of each individual bit is determined separately, beginning with the most significant bit. To begin, a start signal activates multiplexer  203  to set flip-flop  204  to zero. Initially, the output of multiplexer  202  is a logical “one” value which is used to preset the value of the bit X flip-flop  204 , which is the x-th component of register  103  as shown in FIG.  8 A. The first, second, and third multiplexers  201 - 203  are series connected each to produce a single bit, and the last multiplexer  203  in the series is connected to flip-flop  204  for providing an output signal offset X. The offset signal is fed back to the black level capacitor  63  to control the offset added to the output of the CDS circuit  4  that is fed to the 2-bit ADC  11 . Thus, to calibrate the respective offset registers  21 - 23 , the value of each individual bit is determined separately, beginning with the most significant bit. The flip-flop  204  is set to zero, and then, the output of multiplexer  203  is allowed through to flip-flop  204 . The output of multiplexer  202  is initially a logical “one” value. This value is used to preset the value of the bit x flip-flop  204 , which is the x-th component of register  103 . To determine a particular offset value, nine significant bits are tested to establish each offset value. For each significant bit, the output of multiplexer  101  is repeatedly observed, and the keep status of a test bit is established by averaging the results of the multiplexer observations. Accordingly, particular ones of register latch elements  111 - 119  are successively determined. The offset code is used to establish a particular setting of variable capacitor  63  (e.g., the black code capacitor). The variable capacitor  63  provides an offset value which is subject to a shift provided by offset capacitor  107 , to determine whether the test value processed is to be kept or rejected. By successively checking from most to least significant test values, offset values are determined for each transition. 
     Referring now to FIG. 8C, there is shown a block diagram of a calibration reference selection circuit  300  for a controllable dynamic range extension signal processing (DRX) circuit  2 , in accordance with the present invention. In particular, input circuit  300  includes a multiplexer  301 . The multiplexer  301  is controlled by a calibration level signal cal_lvl which is used to apply a two-bit calibration level selection code to select one of input voltage levels, Vref+0.8, Vref+0.4, or Vref+0.2, for application at the output of multiplexer  301 . The selected output value from multiplexer  301  is applied to provide an output signal VS for use in DRX circuit  2  according to one embodiment of the present invention. In FIG. 6, the signal VS is applied to capacitor  68  when calibration signal CAL &amp; φ 1  closes switch  69 . Then, when the calibration signal CAL &amp; φ 1  has so applied VS, the reference voltage signal Vref is applied according to clock signal φ 2 . These calibration reference voltages are used in combination with an offset level to find the exact trip point levels of comparators  12 - 14 . 
     Referring now to FIG. 8D, there is shown a graph of the output of ADC  6  for selected gain settings of high and low gain, according to one embodiment of the present invention. As is evident, during high gain operation, the level of the VGA input remains below an indicated trip point. After the level of VGA input increases beyond the trip point, operation continues in a low gain mode. 
     Referring now to FIG. 8E, there is shown a diagram of selected portions of a controllable dynamic range extension signal processing (DRX) circuit for processing signals received from a selected imaging device. The DRX circuit portions shown include a multiplexer  301  and a variable gain amplifier (VGA)  5  subject to automatic gain control according to the present invention. The VGA  5  has input and output connections which respectively receive an input analog signal and produce an output amplified analog signal in accordance with the gain setting which is current for the VGA  5 . The DRX circuit further includes an analog-to-digital converter (ADC)  6  which digitizes the analog signal received from the VGA  5 . The DRX circuit further includes respective circuit blocks  401  and  402  respectively for averaging the values of a set of eight high gain pixels, and for averaging the values of a set of eight low gain pixels. The DRX circuit further includes a multiplication block  403 , and a summation block  404  to produce a desired offset code according to the present invention. In operation, the multiplexer  301  is controlled by a calibration level signal cal_lvl which is used to apply a two-bit calibration level selection code to select one of input voltage levels, Vref+0.8, Vref+0.4, or Vref+0.2, for application at the output of multiplexer  301 . An offset value is provided from black capacitor  63  based upon the particular offset code applied. The combined multiplexer and offset values are provided to amplifier  64  which in turn feeds the VGA  5  subject to a predetermined gain-override value, which according to the present invention is set for  8  sessions of average testing to a high gain value for high gain averaging  401 , and for  8  sessions to a halved low gain value for low gain averaging  402  subject to level multiplication by doubler circuit  403 . The unitary and halved average values are provided to a summation node  404  which produces the offset code according to the present invention. 
     Referring now to FIG. 9A, there is shown a flow chart of an offset value determination method  899  according to the present invention showing the successive determination of respective offset values, OFFSET 1 , OFFSET 2 , and OFFSET 3 . In particular, the offset value determination method  899  starts  900  and then determines  901  the value of OFFSET 1 , as will be discussed below. Next, the value of OFFSET 2  is determined  902 . Finally, the value of OFFSET 3  is determined  903 , followed by completion and stopping  904  of the offset determination method  899 . 
     Referring now to FIG. 9B, there is shown a flow chart of a method  949  accomplished after power-up of the system shown in FIG. 1, according to the present invention for the determination of a single one of the offset values, OFFSET 1 , OFFSET 2 , and OFFSET 3 , expressly set forth in FIG.  1 . In particular, the determination of a selected one of the offset values starts  950  with determination of whether to keep a most significant one of the bits of the particular offset value, by setting  951  predetermined initial values of certain variables including M=9 and ACC=0. ACC is an accumulator value which accumulates an index permitting assessment of a bit keep determination by averaging a predetermined plurality of keep tests as to a particular selected test bit. According to one embodiment of the present invention, an averaged keep test as to a particular significant test bit includes the average of 16 tests. Accordingly, an averaging test counter variable “C” is set  952  equal to 16. Next, the value of C is reduced  953  by a single number. Then, a determination is made as to a single one of the 16 tests, to determine whether the tested bit is to be kept in view of the output value of multiplexer  101 . The multiplexer  101  produces a keep or not keep value in response to an OFFSET CODE containing the test bit, setting the variable capacitance of the black capacitor  63 . This results in op-amp  64  providing an input to 2-bit ADC  11 . Next, a determination is made  955  as to the logical value of the keep indication provided by multiplexer  101 . If the keep indication is affirmative, the accumulator variable ACC is upward incremented  956  by a single unit amount. If 16 averaging evolutions have not yet been completed  957 , operation continues with decrementation  953  of the counter C. Once the averaging evolutions have been completed, a check is undertaken  958  to determine whether the accumulated value of the accumulator variable ACC is greater than eight (8). If ACC is greater than 8, the keep flag is set  959  for the particular test bit, based upon averaging. Next, the accumulator variable is reset to zero, and a new test bit of next lesser significance is selected  960 , for initiation  952  of the 16 averages cycle just discussed. If all significant bits have already been tested  961  to produce an averaged multi-bit code offset value, the operation according to the method of the invention stops  962 .