# Patent

## Title
Minimized SAR-type column-wide ADC for image sensors

## Abstract

An improved analog-to-digital converter wherein a minimal amount of circuitry is provided for conversion of an analog signal to a series of digital bits. A comparator is provided for generating digital values to which the digital bits correspond. A digital-to-analog converter is provided for generating, via successive approximation, a feedback analog signal based on bits previously generated by the comparator. The analog-to-digital converter compares the feedback analog signal to the input analog signal, and based on the comparison generates a digital value that corresponds to a digital bit. In the method of the invention, an analog signal is converted to a digital value using the analog-to-digital converter, and a digital-to-analog converter comprising two capacitors and a reference selector generates, via successive approximation, a feedback analog signal that is applied to a comparator for comparison to the input analog signal being digitized.

## Background
The present invention relates generally to analog-to-digital converters in imager devices and to methods for using the same.
BACKGROUND OF THE INVENTION
Analog-to-digital converters (“ADCs”) convert an incoming analog signal to a corresponding series of digital bits. Image sensing devices (i.e., image sensors) generate analog signals that are typically converted by ADCs to digital bit values as part of image processing. Image sensors typically employ light detecting elements (e.g., photosensors) and are used in various applications. Such image sensors may be formed using a variety of fabrication techniques. Currently, two commonly fabricated image sensors are CMOS image sensors and charge coupled device (CCD) image sensors. Each sensor generally includes an array of pixels containing the photosensors. The image sensors typically use photosensors in the form of photogates, phototransistors or photodiodes.
When an image is focused on the image sensor array (also called an “imager array” or “pixel array”), light corresponding to the image is directed to the pixels usually through micro-lenses. Each micro-lens may be used to direct incoming light through a circuitry region of the corresponding pixel to the photosensor region, thereby increasing the amount of light reaching the photosensor. It is known in the art to use circuitry that includes storage regions (for example, floating diffusion regions) that collect pixel charge representing the light reaching the photosensors.
In a CMOS imager, the pixel array discharges this pixel charge as an analog voltage signal that is proportional to the light collected by the photosensor. It is known in the art to use row select transistors to select a particular row in the pixel array and cause the storage element (e.g., a floating diffusion region) for each pixel in the selected row to discharge its voltage for further processing by image sensor circuitry. In this way, the pixel array circuitry discharges the voltage stored in each column of the pixel array one row (i.e., the selected row) at a time. The discharged analog voltage signals may be used to replicate an image represented by the light incident on the photosensors of the array (i.e., the incident light). For example, the discharged analog voltage signals may be used to display a corresponding image on a monitor or to otherwise provide information about the image.
It is known in the art to convert analog voltage signals to digital signals using an ADC that uses a successive approximation (“SAR”) method, where digital bit values are successively approximated using the values of previously generated bits. An ADC typically includes: a sample and hold circuit for sampling and holding analog signals; a comparator for comparing an input analog signal to a feedback analog signal and for outputting a corresponding digital value; memory for storing digital bit values; and a feedback loop comprising a digital-to-analog converter (“DAC”) for generating the feedback analog signal. In operation, the DAC receives a digital value (e.g., a digital bit previously generated by the ADC) and outputs a corresponding analog signal that is applied to one of the comparator inputs and used to generate the next successive digital bit value of the digital bit series.
Error signals may, arise in the analog voltage signal applied to the comparator of an ADC employed in an image sensor. For example, such errors may arise because of floating diffusion reset noise embedded in the incoming analog voltage signal. In image sensors, the floating diffusion region of a pixel acts as a capacitor that stores charge corresponding to the light incident on the pixel photosensors. Each time pixel charge is read out of the floating diffusion region, the floating diffusion region must be set to a reference level (the “floating diffusion reset level”) in order to accept the next incoming charge. Thus, when the floating diffusion region of the pixel array discharges, the resulting output (e.g., Vsig) includes not only the charge representing the incident light, but may also include noise in the floating diffusion reset level. Such noise is undesirable.
For a comparator in the form of an operational amplifier, error signals also may arise due to offset values that are inherent to the operational amplifier and therefore embedded in the operational amplifier output. Such offset values are undesirable.
Successive approximation type ADCs that require one storage capacitor for storing the value of each bit of an N-bit series (i.e., N capacitors) are known in the art. For example, if ten bits are to be converted, the ADC circuitry includes ten capacitors. Such configurations require significant die area to accommodate the numerous circuit elements. Moreover, for high resolution ADCs, the size of the capacitors significantly increases from least significant bit (“LSB”) to most significant bit (“MSB”) in order to compensate for charge leakage and thermal noise on the MSB capacitor. For example, SARs with charge-sharing DACs are known where each of the N capacitors are sized 20, 21, 22, 24 . . . 2N relative to one another. Such SARs rely on capacitor matching and a large ratio from the largest capacitance to the smallest.
Other DACs having a limited number of capacitors (e.g., two) are known in the art. However, the comparator of these SAR ADCs typically process signals MSB first, LSB last, while serial DACs typically process bits LSB first, MSB last. In order to use such a DAC in an ADC, the comparator of the ADC must completely convert the analog input to digital values before the LSB is generated and DAC processing can begin. This is undesirable.
Furthermore, these known SAR ADCs fail to compensate for offset errors prior to conversion of the pixel cell output (the analog signals) used to generate the digital bits. Therefore these known SAR ADCs fail to adequately account for offset errors from either the comparator or the pixel cells which is undesirable.
DACs that process MSB first, LSB last are also known, but these DACs typically require integrator circuitry prior to transferring the analog output to a comparator. Such integrator circuitry requires additional electronic components (e.g., a separate operational amplifier and a capacitor network and takes up additional die space), and is therefore undesirable.
Other DACs that process MSB first, LSB last are limited to use in monotonic DACs (i.e., the DACs' output either increases (for a digital input of 1) or stays the same (for an input of 0)). Thus, the DAC output is not sufficiently sensitive for high resolution applications such as high speed image sensor applications.
Accordingly, there is a desire and need for an image sensor having a successive approximation type analog-to-digital converter comprising circuitry that includes a minimal number of circuit components that take up minimal die area.
There is also a need for an improved method of processing signals through an image sensor using a successive approximation type analog-to-digital conversion method having a serial digital-to-analog converter that is capable of processing bits MSB first, LSB last.
There is further a need for an improved method of correcting for error signals in a successive approximation type analog-to-digital converter.
Further there is a need for an improved method of converting analog signals to digital signals using a successive approximation type analog-to-digital converter comprising a digital to analog conversion method for high resolution applications.
BRIEF SUMMARY OF THE INVENTION
The present invention provides an improved analog-to-digital converter and ADC conversion method for image sensors requiring one amplifier, three capacitors and a buffer per column for converting analog signals by successive approximation. The analog-to-digital converter includes a digital-to-analog converter and digital-to-analog conversion method requiring only previously converted bits of the digital series.
The above and other features and advantages are achieved in various exemplary embodiments of the invention by providing an analog-to-digital converter utilizing a comparator, memory, and a digital-to-analog converter. Successive approximation is used in the present invention to enable the digital-to-analog converter to process signals MSB first, LSB last.



BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other advantages and features of the invention will become more apparent from the detailed description of exemplary embodiments provided below with reference to the accompanying drawings in which:
FIG. 1A is a block diagram of a conventional image sensor;
FIG. 1B is a schematic of a pixel cell in an image sensor that generates analog signals;
FIG. 2 is a schematic of an exemplary embodiment of an image sensor having an analog-to-digital converter;
FIG. 3A is a schematic of an exemplary embodiment of the memory of the image sensor of FIG. 2;
FIG. 3B is a schematic of an alternative exemplary embodiment of the memory of the image sensor of FIG. 2;
FIG. 4 is a schematic of an exemplary embodiment of the image sensor of FIG. 2 during an exemplary correlated double sampling method having a pixel array output signal with floating diffusion reset voltage applied to a digital-to-analog converter;
FIG. 5 is a schematic of an exemplary embodiment of the image sensor of FIG. 2 during an exemplary correlated double sampling method having a pixel array output signal and an operational amplifier output signal each sampled and held on an sample and hold capacitor;
FIG. 6 is a schematic of an exemplary embodiment of the image sensor of FIG. 2 during an exemplary method of generating a bit in an N-bit series having a first reference voltage applied to a digital-to-analog converter;
FIG. 7 is a schematic of an exemplary embodiment of the image sensor of FIG. 2 during an exemplary method of generating a bit in an N-bit series having a second reference voltage applied to a digital-to-analog converter;
FIG. 8 is a schematic of an exemplary embodiment of the image sensor of FIG. 2 during an exemplary method of generating a bit in an N-bit series having a digital-to-analog converter charge sharing voltage generated;
FIG. 9 is a schematic of an exemplary embodiment of the image sensor of FIG. 2 during an exemplary method of generating a bit in an N-bit series having a previously generated bit applied to a digital-to-analog converter;
FIG. 10 is a schematic of an exemplary embodiment of the image sensor of FIG. 2 during an exemplary method of generating a bit in an N-bit series having a digital-to-analog converter output voltage generated;
FIG. 11A is a flowchart of top level states of an exemplary embodiment of correlated double sampling and successive approximation during the exemplary method of FIGS. 4–10.
FIG. 11B is a flowchart of top level states of an exemplary embodiment of successive approximation during the exemplary method of FIGS. 6–10.
FIG. 11C is a flowchart of an exemplary embodiment of using address pointers during the exemplary method of FIGS. 6–10;
FIG. 12 is a chart of example bit values generated using the exemplary method of FIGS. 6–10; and
FIG. 13 is a diagram of a processor system incorporating an image sensor comprising an analog-to-digital converter constructed in accordance with an exemplary embodiment of the invention.



DETAILED DESCRIPTION OF THE INVENTION
In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and which illustrate specific embodiments of the present invention. These embodiments are described in sufficient detail to enable those of ordinary skill in the art to make and use the invention. It is also understood that structural, logical, or procedural changes may be made to the specific embodiments disclosed without departing from the spirit and scope of the present invention. The following detailed description is not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.
FIG. 1A illustrates a block diagram for a CMOS imager 10. The imager 10 includes a pixel array 11. The pixel array 11 comprises a plurality of pixels arranged in a predetermined number of columns and rows. The pixels of each row in array 11 are all turned on at the same time by a row select line and the pixels of each column are selectively output by a column select line. A plurality of row lines (not shown) and column lines are provided for the entire array 11.
The row lines are selectively activated by the row driver 13 in response to row address decoder 15 and the column select lines are selectively activated in response to column address decoder 19. Through the use of address decoders 15 and 19, a row and column address is provided for each pixel. The CMOS imager 10 is operated by the control circuit 21, which controls address decoders 15, 19 for selecting the appropriate row and column lines for pixel readout, and row driver circuitry 13 which apply driving voltage to the drive transistors of the selected row lines. Although a column driver may be used, it is not required for a column parallel architecture because the column decoder 19 may be used to drive the selected column as illustrated in FIG. 1. A column parallel analog-to-digital converter (“ADC”) 44 is used to convert analog signals received from a given pixel in a corresponding column to a digital value which may be stored in memory 50. For a given imager with Y rows and X columns of pixels, there will be X ADC's 44 or a ratio thereof. As one row of pixels is selected, the X ADC's 44 convert the pixels signals from that row simultaneously to digital codes that are stored in one N-bit memory array per column 50.
The pixel signal, Vsig, output from the pixel array is an analog voltage. Pixels also generate a floating diffusion reset signal, Vrst, that is used to adjust the pixel signal Vsig by subtracting Vsig from Vrst (i.e., Vrst−Vsig). This adjusted pixel signal, Vrst−Vsig, is the signal that is digitized by an analog-to-digital converter (ADC) in the column parallel array of ADC's with one ADC 44 per column.
FIG. 1B shows a pixel cell 12 used in a CMOS imager such as the imager 10 illustrated in FIG. 1A. The pixel cell 12 includes a photosensor 16, floating diffusion node 22, transfer transistor 20, reset transistor 38, source follower transistor 28 and row select transistor 30. The photosensor 16 is shown as a photodiode, but other forms of photosensors may be used (for example a photogate, etc.). The photosensor 16 is connected to the floating diffusion node 22 by the transfer transistor 20 when the transfer transistor 20 is activated by a control signal TX. The reset transistor 38 is connected between the floating diffusion node 22 and an array pixel supply voltage Vaa-Pix. A reset control signal RESET is used to activate the reset transistor 38, which resets the floating diffusion node 22 to set it to Vrst as is known in the art.
The source follower transistor 28 has its gate connected to the floating diffusion node 22 and is connected between pixel supply voltage Vaa-Pix and the row select transistor 30. The source follower transistor 28 converts the stored charge at the floating diffusion node 22 into an electrical output voltage signal. The row select transistor 30 is controllable by a row select signal SELECT for selectively connecting the source follower transistor 28 and its output voltage signal to a column line 23 of a pixel array. It should be appreciated that the illustrated pixel 12 is an example of the type of pixel that may be used with the invention and that the invention is not limited to the type or configuration of the pixel 12.
In operation, the column voltages Vrst and Vsig for the pixel 12 are each respectively discharged to an analog-to-digital converter which generates the adjusted pixel voltage signal (Vrst−Vsig) and digitizes these voltages.
An exemplary embodiment of one ADC 44 in the array of ADC's is shown in FIG. 2. ADC 44 may be used to convert the analog signals, received from pixel cells 12, to digital signals. As shown in FIG. 2, the ADC 44 includes an operational amplifier 46, output buffer 48, memory 50, sample and hold circuit 52, and DAC 54. As shown in FIG. 2, ADC 44 is coupled to an array of pixel cells 12 (for clarity purposes, only one cell 12 having a photosensor 16, transfer gate 20, reset gate 38, row select transistor 30 and source follower transistor 28 is shown).
As further shown in FIG. 2, sample and hold circuit 52 may comprise a sample and hold capacitor 56 coupled at one plate 56b via switch SW6 to pixel cell 12, and via switch SW5 to a voltage source 58 that generates a low reference voltage VrefL. Sample and hold capacitor 56 is coupled at its other plate 56a to a first input line 60 of operational amplifier 46, and (via switch SW3) to an amplifier output line 47. Sample and hold capacitor 56 may be used to apply an analog voltage signal, Vin, to the first input line 60 for conversion to digital bits.
A second input line 62 of operational amplifier 46 is coupled via switch SW1 to pixel cell 12. Second input line 62 is also coupled to the feedback output line 55 of the DAC 54, such that the DAC output value may be applied as an input to operational amplifier 46.
Preferably, a switch SW4 couples amplifier output line 47 to the output buffer 48 (preferably an inverter). As further shown in FIG. 2, an output line 75 couples operational amplifier 46 (and preferably output buffer 48) to the memory 50. When SW4 is in the open position, it decouples the operational amplifier 46 output from output buffer 48 (as well as from memory 50) and, when operational amplifier 46 is in analog mode, this open SW4 configuration prevents high operating current in inverter 48.
According to the exemplary embodiment of FIG. 2, memory 50 is also coupled to DAC 54. DAC 54 comprises an a reference selector 64, a first capacitor 66 and a second capacitor 68. Preferably capacitors 66, 68 are matched capacitors. Switch SW8 couples DAC reference selector 64 to first capacitor 66. Preferably, an input of reference selector 64 is coupled to a node 77 for receiving command signals from a reset control block 57. Switches SW8 and SW7 couple DAC reference selector 64 and first capacitor 66 to second capacitor 68 and feedback output line 55.
DAC 54 of the exemplary embodiment outputs an analog voltage on line 55 that is determined by the value of the digital bit value that is input to the DAC reference selector 64. The DAC 54 output voltage is applied via feedback output line 55 to second input line 62 of operational amplifier 46. During a compare mode of operational amplifier 46, the DAC output voltage is compared to the analog voltage, Vin, that is applied at first input line 60. Vin corresponds the analog voltage from pixel cells 12. As a result of comparing the voltages at its first input line 60 and second input line 62, operational amplifier 46 generates a digital value, and a corresponding value is stored in memory 50 as biti of an N-bit digital series.
The ADC 44 of FIG. 2 may be fabricated according to conventional semiconductor fabrication processes (e.g., mask-based/lithography or other fabrication processes known in the art). The ADC 44 may be fabricated on a chip comprising the circuitry for generating the analog voltages that are supplied to ADC 44, such as for example on an imager chip.
FIG. 3a illustrates an exemplary embodiment of a memory 50a that can be used as memory 50 (FIG. 2) for storing digital bit values generated by the ADC 44 of FIG. 2. Memory 50a comprises a RAM cell 72 including three tri-state inverters, 74, 76, 78. First tri-state inverter 74 receives a digital bit value from ADC 44 via amplifier output line 75. First tri-state inverter 74 is also coupled to a write command signal line 82. Upon receiving a write command via write command signal line 82, first tri-state inverter 74 inverts the digital bit value received from ADC 44 and outputs it to store node 80. Store node 80 is coupled to the inputs of the second tri-state inverter 76 and the third tri-state inverter 78.
Second tri-state inverter 76 is also coupled to a read command signal line 84. Upon receiving a read command on line 84, second tri-state inverter 76 inverts the digital bit value stored at store node 80, and outputs the inverted value to digital input line 77, which is the input line of DAC reference selector 64 (FIG. 2). In this way, the digital bit value received from ADC 44 by first tri-state inverter 74 is input to DAC 54.
Third tri-state inverter 78 is coupled to a column select command line 86. Upon receiving a column select signal on line 86, third tri-state inverter 78 inverts the digital bit value stored at store node 80 and drives this inverted value onto the bit line bus BL. In this way, a series of bit values may be read out of column memory 50 (shown in FIG. 1A) successively during line readout.
FIG. 3b illustrates a memory 50b that can be used as memory 50 (FIG. 2). As shown in FIG. 3b, a fourth tri-state inverter 88 is coupled between store node 80 and third tri-state inverter 78. Fourth tri-state inverter 88 is also coupled to a transfer control line 90. Between bit line readout operations (e.g., between column select commands), a line transfer control signal may be used to cause fourth tri-state inverter 88 to invert the digital bit value stored at store node 80 and transfer the value to a second store node 81 in response to a signal from transfer control line 90. In this way, a value used in the current ADC conversion for a given pixel row can be stored at store node 80, while a value corresponding to the ADC conversion of the preceding pixel row can be simultaneously stored at second store node 81 for readout on bit line bus BL. Because memory 50b uses three tri-state invertors (i.e., tri-state inverters 74, 88, and 78) to buffer a bit value received on line 75, the bit values readout to bit bus line BL are inverted from those received via line 75.
FIG. 11A illustrates top-level states of an exemplary method of Correlated Double Sampling and successive approximation performed using an exemplary image sensor having one ADC 44 of FIG. 2 per column. As shown in FIG. 11A (at steps A–C), upon power up of the system (e.g., the image sensor), an ADC for successive approximation may remain in the idle state with operational amplifier 46 powered down until beginning acquisition of analog signals from a row of pixel cells 12. As shown in FIG. 11A at step D (and further described below with reference to FIG. 4), when beginning the acquisition of analog signals, Vrst is sampled and buffered on to the left side of sample and hold capacitor 56. Next, as shown in FIG. 11A at step E (and further described below with reference to FIG. 5), Vsig is sampled onto the right side of sample and hold capacitor 56. Next, as further shown in FIG. 11A at step F (and further described below with reference to FIGS. 6–10), the ADC conversion to digital values occurs.
An exemplary Correlated Double Sampling (“CDS”) method using the ADC 44 of FIG. 2, is now described with reference to FIGS. 4–6 and 11A. A Correlated Double Sampling (“CDS”) method may be used to reduce the effect of noise in the analog voltage signal representing the light incident on the pixel cell caused by the floating diffusion region. CDS determines an adjusted pixel voltage signal which is the difference between a floating diffusion reset level (Vrst) and the analog voltage signal (Vsig) from the pixel cell. FIG. 4 illustrates a Floating Diffusion Sampling (“FDS”) state of the exemplary CDS method. During the FDS state, reset gate 38 drives the floating diffusion reset voltage, Vrst, on to the floating diffusion region 22, which is output through source follower transistor 28 through switch SW1 to DAC 54. Switch SW7 of DAC 54 is in the closed position while switch SW8 is in the open position. This switch configuration enables first capacitor 66 and second capacitor 68 to sample and hold Vrst such that the output of DAC 54 is equal to Vrst, (i.e., 1/2[Vrst+Vrst]). Vrst is applied as the DAC output value to second input line 62 of operational amplifier 46. Switch SW6, which couples the output of pixel cell 12 to operational amplifier 46 (through sample and hold capacitor 56) is in the open position, thereby preventing the pixel cell output voltage (e.g., Vsig) from being coupled through sample and hold capacitor 56 to the first input line 60 of operational amplifier 46. Switch SW5 is also in the open position, preventing the low reference voltage value VrefL from being applied to sample and hold capacitor 56. Operational amplifier 46 therefore outputs a signal equal to the DAC output value (i.e., equal to the floating diffusion reset value, Vrst) plus an inherent operational amplifier offset value, Vampoff. For example, using the example voltage values shown in FIG. 12 where Vrst=2V, and Vampoff=0.05V, then operational amplifier 46 outputs a voltage of 2.05V.
During the FDS state, switch SW3 is in the closed position and the operational amplifier 46 output signal (Vrst+Vampoff) is driven through switch SW3 onto plate 56a of sample and hold capacitor 56. When SW3 is closed, operational amplifier 46 is in unity gain configuration. In this way, Vrst is sampled and buffered onto sample and hold capacitor 56 as illustrated in FIG. 11A at step D.
A “Signal Sampling” state of the exemplary CDS method is illustrated in FIG. 5. During the Signal Sampling state, switch SW 1 is opened, thereby preventing pixel cell 12 from driving an analog voltage value onto either DAC 54 or second input line 62. Pixel cell 12 discharges the analog signal value, Vsig. Switch SW6 is closed and couples the output line 23 to plate 56b of the sample and hold capacitor 56, thereby applying Vsig to plate 56b. For example purposes, as shown in FIG. 12, Vsig=1V. Vsig is applied to plate 56b of sample and hold capacitor 56 (as also illustrated in FIG. 11A at step E). Further, using the example voltages described with reference to the Floating Diffusion Sampling (FDS) state, plate 56a holds Vrst+Vampoff=2.05V. The voltage across sample and hold capacitor 56 is therefore:

Vs/h=[(Vrst+Vampoff)−Vsig]=  (1)

2.05V−1V=1.05V  (2)

A “Voltage Adjustment” state of the method is illustrated in FIG. 6. During the Voltage Adjustment state, switch SW5 is in the closed position, thereby coupling plate 56b of sample and hold capacitor 56 to VrefL. VrefL is then strapped to the voltage across sample and hold capacitor 56 (i.e., Vs/h), causing both voltages to be driven on to first input line 60 of operational amplifier 46. Strapping VrefL to Vs/h ensures that the voltage signal being digitized is greater than VrefL and that the digital output generated by operational amplifier 54 will be near zero at black (i.e., for Vrst=Vsig). The voltage input, Vin, at input line 60 is:

Vin=[Vs/h+VrefL].  (3)

For example purposes, as shown at FIG. 12, VS/h=1.05V and VrefL=0.8V. Thus, the voltage at operational amplifier input line 60 becomes Vin=[Vs/h+VrefL]=1.05+0.8=1.85V. An ideal comparison voltage is Vrst−Vsig+VrefL=1.8V, but the actual compare voltage has an additional voltage equal to the amplifier offset error Vampoff (e.g., 0.05 V). This makes it possible to remove amplifier offset errors from operational amplifier 46 which would otherwise create fixed pattern column noise in the imager.

An exemplary top level flow for ADC conversion is shown in FIG. 11B for converting an analog signal to N bits ranging from N−1 (the MSB) to 0 (the LSB). After entering the ADC conversion process (step A in FIG. 11B), a pointer (i.e., FindPtr) may be used to designate the bit to be determined by the ADC. As shown at step B, the process begins with the pointer set to N−1 (i.e., the most significant bit). Next, as shown at step C, a DAC guess generation is performed for generating an analog value that is compared to Vin for generation of the bit that is designated by the pointer (e.g., bitN-1 for FindPtr=N−1). The DAC guess generation is described in more detail below with reference to FIGS. 6–10, 11C and 12. Next, as shown in step D, the bit value resulting from the DAC guess generation is written to a memory location having an address corresponding to the pointer (i.e., corresponding to FindPtr). Next, the pointer value is decremented (as shown in step E), and if the decremented pointer value is at or above a minimum value (i.e., if FindPtr >=0) as shown in step F, then steps C–E are repeated. Thus, for example, after decrementing FindPtr from N−1 to N−2, the DAC guess generation process is performed to generate an analog value that is compared to Vin. Based on this comparison, a bit value is generated (step C) and written to memory 50 (step D) as the second most significant bit (i.e., bitN-2) of the N-bit series. FindPtr is decremented from N−2 to N−3 (step E), which is greater than the minimum value of 0 (for FindPtr corresponding to N−1 through 0 (step F)), and steps C–E are repeated to determine bitN-3. When the pointer value is not at or above the minimum value, this indicates that all N-bits have been generated (e.g., bit values corresponding to FindPtr=N−1 through 0 have been generated), then as shown in steps F and G, the ADC conversion process is complete.
An exemplary embodiment of a method of using ADC 44 of FIG. 2 to reset DAC 54 and to generate the MSB of an N-bit digital series corresponding to the voltage applied at the first input line 60, Vin, is now described with reference to FIGS. 6–8.
In addition to the Voltage Adjustment state, a First Capacitor Reset state is illustrated in FIG. 6. During the First Capacitor Reset state, switch SW4 is in the open position, and DAC 54 goes through reset mode as follows. A control signal is applied to reference selector 64. In response to the control signal, reference selector 64 outputs a reference voltage. For example, reference selector 64 may be configured to output VrefH in response to a first control signal, and output VrefL in response to a second control signal. For example, as shown in FIG. 6, a reset control block 57 may be used to force control signals in the form of logic low and high control values onto node 77 thereby causing reference selector 64 to select VrefH in response to logic high or VrefL in response to logic low. (Alternatively, VrefH could be selected in response to logic low and VrefL in response to logic high). Using the example voltages in FIG. 12, VrefH=2.5V or VrefL=0.8V. Depending on the control signal, reference selector 64 outputs either a high reference voltage VrefH or VrefL. For example purposes, reference selector 64 outputs VrefH=0.8V during the First Capacitor Reset state. During this state, as shown in FIG. 6, both switch SW7 and switch SW8 of DAC 54 are in the closed position, thereby coupling reference selector 64 to both first and second capacitors 66, 68. Thus, during the First Capacitor Reset state, the output of reference selector 64 is driven on to first capacitor 66 and second capacitor 68.
During a “Second Capacitor Reset” state (shown in FIG. 7), reference selector 64 outputs a second reference voltage value. If, for example, reference selector 64 outputs VrefH in response to a first control signal (e.g., a logic high at node 77), then reference selector 64 will output VrefL in response to a second control signal (e.g., a logic low at node 77) during the Second Capacitor Reset state. As shown FIG. 7, during the Second Capacitor Reset state, switch SW7 is in the open position, while switch SW8 remains closed. Thus, the output of reference selector 64 is applied to only first capacitor 66.
During a “Charge Sharing” state, as shown in FIG. 8, switch SW7 is in the closed position and switch SW8 is in the open position. This switch configuration generates a DAC reset voltage, DACreset, at the output of DAC 54 that is equal to the charge shared voltage of the first capacitor 66 (VDACcap1) and second capacitor 68 (VDACcap2):

1⁄2[VDACcap1+VDACcap2]=1⁄2[VrefL+VrefH]  (4)

As shown in FIG. 12, using the example voltage values of VreL=0.8V and VrefH=2.5V:

DACreset=1⁄2[0.8V+2.5V]=1.65V  (5)

An “MSB Bit Generation” state is now described with further reference to FIG. 8. The “MSB Bit Generation” state may be used to generate the bits of an N-bit digital series. During this state, DACout is input to operational amplifier 46 via second input line 62. In this case, DACout is DACreset (i.e., 1⁄2[VrefL+VrefH]).
During the MSB Bit Generation state, switch SW4 is in the closed position, thereby coupling the output of operational amplifier 46 to output buffer 48. Operational amplifier 46 is in compare mode and compares Vin on first input line 60 to DACout (i.e., DACreset) on second input line 62. Based on this comparison, operational amplifier 46 outputs an amplifier output 47 that is inverted by output buffer 48 to an inverted digital output value.
Operational amplifier 46 may be configured such that if the voltage Vin at first input line 60 is greater than the voltage at second input line 62, then operational amplifier 46 outputs a digital low signal, which is inverted to a digital high signal by output buffer 48, (i.e., biti=high); and if the voltage Vin at first input line 60 is not greater than the voltage at second input line 62, then operational amplifier outputs a digital high signal and output buffer 48 outputs biti=low.
As previously described, Vin includes the ideal comparison voltage of 1.8V adjusted by the offset error of 0.05V. This adjustment compensates for offset errors that operational amplifier 46 will apply to the DACout voltage. In this way, DACout is effectively compared to the ideal comparison voltage. Using the example voltage values shown in FIG. 12, DACout=DACreset=1.65V and Vin=1.85V. Thus, for generation of the MSB (i.e., bitN-1), Vin (1.85V) is greater than DACout (1.65V)—in other words, the ideal comparison voltage (1.8V) plus the amplifier offset (0.05V) that appears at input line 60 is greater than DACout (1.65V) that appears at input line 62 plus the amplifier offset voltage (0.05) that will be inherently applied to DACout by operational amplifier 46. Because Vin is greater than DACout, operational amplifier 46 outputs a low value, which output buffer 48 inverts to high. A digital value corresponding to this high digital value from output buffer 48 is stored in memory 50 at MEM9 as corresponding to the MSB. If, for example, exemplary memory 50a (shown in FIG. 3a) were used, the bit stored at store node 80 is a low (because first tri-state inverter 74 inverts the digital signal incoming via line 75), and the bit output to bit line bus BL is high. Note that other operational amplifier configurations are also suitable—for example, the operational amplifier 46 could be configured to output a digital high if Vin>DACout, and a digital low if Vin<DACout.
An exemplary method of generating a next successive bit, biti, in an N-bit series using the ADC 44 of FIG. 2 is now described with reference to FIGS. 6–10. For i=N−2, the MSB (bitN-1) is used to generate the next successive bit in the N-bit series, biti=N-2. According to this exemplary method, a bit corresponding to the MSB (e.g., the MSB bit) has been previously generated and stored in memory 50. For example, the MSB bit may be generated and stored in memory 50 according the method previously described with reference to FIGS. 6–8. Next, first capacitor 66 and second capacitor 68 are reset according to the First Capacitor Reset, Second Capacitor Reset, and Charge Sharing states described above with reference to FIGS. 6–8 such that the voltage at the output of DAC 54 is DACreset=1⁄2(VrefL+VrefH).
As shown in FIG. 9, during a “Prior-Bit Feedback” state, a digital value corresponding to the previously generated bit (biti+1) is read from memory 50 and applied as an input to reference selector 64. For i=N−2, the previously generated bit is bit(N-2)+1. Thus, a digital value corresponding to MSB, bitN-1, is applied to reference selector 64. Reference selector 64 is configured such that if the biti+1 is high, reference selector 64 outputs VrefH, and if biti+1 is low, reference selector 64 outputs VrefL. It should be noted that other configurations are suitable, for example, reference selector 64 may be configured to output VrefL if biti+1 is high, and VrefH if biti+1 is low. As shown in FIG. 9, switch SW7 is in the open position and switch SW8 is in the closed position, such that reference selector 64 and first capacitor 66 are coupled together, but neither is coupled to second capacitor 68 or feedback output line 55. In this way, the reference selector 64 drives either VrefH (if biti+1=high) or VrefL (if biti+1=low) on to first capacitor 66. For example purposes, i=N−2 (the second most significant bit) and biti+1 is bitN−, the MSB. Using the example MSB value (high), as shown in FIG. 12, reference selector 64 outputs VrefH=2.5V which is applied to first capacitor 66.
During a “Feedback Charge Share” state (shown in FIG. 10), switch SW8 is in the open position and switch SW7 is in the closed position. This causes first and second capacitors 66, 68 to charge share such that the DAC output on line 55 is equal to one half the combined sum of the voltages on the first capacitor 66 and second capacitor 68. The voltage on first capacitor 66 is the voltage output by reference selector 64, VDACcapx. The voltage on second capacitor 68 is the previously generated DAC output voltage. The DAC output is not used by the operational amplifier 46 to compare to Vin for generation of biti until the successive approximation process is complete for x=i+1 through x=N−1. Thus, until the successive approximation process is complete for x=i+1 through x=N−1, the DAC output values are generated as temporary values. Thus, the DAC output is:

DACtempout(N-1)-x=1⁄2[VDACcap2+VDACcap1]=1⁄2[DACtempout(N-1)-x-1)+(VDACapx)]  (6)

Where DACtempout(N-1)-x is the temporary DAC output being generated, DACtempout(N-1)-(x-1) is the voltage on second DAC capacitor 68 (i.e., the previously generated temporary output voltage) and VDACcapx is the voltage on the first capacitor 66 (i.e., the reference selector 64 output (VrefL or VrefH)).

During successive approximation, each of the previously generated bits (e.g., bitsi+i through the MSB, bitN-1) is successively applied to DAC 54 to generate a temporary DAC output starting with the immediately previously generated bit i+1, and ending with the MSB; thus x in equation (6) identifies the current bit that is being read out of memory and applied to the DAC reference selector to generate VDACcapx and DACtempout(N-1)-x for the successive approximation. Thus, temporary DAC outputs are generated until the successive approximation for a given biti ends after inputting the MSB to DAC 54.
According to the exemplary method, DACtempout0 signifies that the MSB has been applied to the DAC reference selector and successive approximation using all previously generated bits has occurred. Thus, the Prior-Bit Feedback and Feedback Charge share states are repeated for x=i+1 through x=N−1, at which time DACtempout0 is generated.
Although SW4 is in the closed position after DACtempout0 is generated, preferably, switch SW4 is in the open position during generation of the temporary DAC output values that are not compared by operational amplifier 46 to Vin in order to generate a digital bit of the N-bit series. Alternative configurations also may be used to prevent a given temporary DAC output from being used to generate a digital bit in the N-bit series. For example, a control signal may be used to prevent operational amplifier 46 from generating a bit of the N-bit series, or where exemplary memories 50a (FIG. 3a) and 50b (FIG. 3b) are used, a control signal may be applied to write line 82 to control when a digital bit is written to store node 80.
To generate biti of an N-bit series, equation (6) is determined for x=i+1, i+2, . . . N−1, where i+1 is the bit number of the immediately previously generated bit, and N−1 is the bit number of the MSB. For example, in an N-bit series where N=10 (i.e., a 10 bit series) having bits 9 through 0, the MSB is bit N−1=9. In such a series, the 3rd most significant bit would be considered biti=N-3=7, and in order to generate this bit, a DACouti value for i=7 would be calculated by calculating equation (6) for x values of x=8 (i.e. i+1), and x=9 (i.e., i+2), where x=9=N−1. Alternatively, if the second most significant bit, biti=N-2=8, is being generated, then equation (6) would be calculated for just x=i+1=9 in order to determine DACouti=8. Thus, when generating biti=N−2 (i.e., the second most significant bit), the first bit applied is bitx=i+1=(N−2)+1=N-1 (i.e., the MSB). For x=N−1, equation (6) reduces to:

DACtempout(N-1)-(N-1)=1⁄2[DACtempoutN-1-((N-1)-1)+(VDACcapN-1)]=  (7)

DACtempout0=1⁄2[DACtempout1+(VDACcapN-1)]  (8)

Here, the previously generated DAC output voltage was the DACreset that was used to reset the DAC after generation of the MSB bit. Thus:

DACtempoutN-1−(x−1)=DACtempout1=DACreset  (9)

VDACcpx=N-1 is the voltage applied by reference selector 64 to first capacitor 66 based on the value of bitx=N-1 (i.e., the MSB). Using the previously described example values (shown in FIG. 12), DACreset=1.65V, and the MSB is (high), reference selector 66 outputs VrefH=2.5V. Thus, equation (8) reduces to:

DACtempout0=1⁄2[DACreset+VrefH]=1⁄2[1.65V+2.5V]=2.075V  (10)

Generation of DACtempout N-1-(x-1)=DACtempout0 signifies that successive approximation through bitx=N-1 (i.e., the MSB has occurred).

A Successive Bit Generation state is shown in FIG. 10. During this state, DACtempout0=DACouti=2 is applied to second input line 62 of operational amplifier 46 and compared to Vin for generation of bitN-2. Operational amplifier 46 is in the compare mode, and compares DACouti=2 (e.g., 2.075V using the example voltage value calculated by equation (10) for i=N−2) to the incoming analog voltage signal, Vin (e.g., 1.85Vusing the example voltage value). After being applied to operational amplifier 46, DACouti will be adjusted up by the inherent amplifier offset (0.05V). Further Vin, as applied to input line 60, is the ideal comparison voltage of 1.8V plus the offset of 0.05V. Thus, operational amplifier 46 effectively compares the ideal comparison voltage to DACouti. For i=N−2, the resultant output from operational amplifier 46 is a digital bit value that is inverted by buffer 48 and stored in memory 50 as bitN-2 of the N-bit series. For example purposes, during compare mode, operational amplifier 46 is configured to output a digital high if the voltage at the first input line 60 (e.g., Vin=1.85V) is not greater than the voltage at the second input line 62, (e.g., DACout1=2.075V); and output buffer 48 inverts this signal to a digital low and, as shown in FIG. 12, a digital value corresponding to the digital low is stored in memory 50 corresponding to the second most significant bit, bitN-2.
The next successive bit, biti=N-3, of the N-bit series is generated as follows. DAC 54 is reset to DACreset=1⁄2[VrefL+VrefH] according to the First Capacitor Reset, Second Capacitor Reset and Charge Sharing states described above with reference to FIGS. 6–8 and equation (4). Using the example voltage values described above (and shown in FIG. 12), DACreset=1⁄2[0.8V+25V]=1.65V. A DAC output is generated for x=i+1 as follows. A digital value corresponding to previously generated and stored bitx=i+1 is read out from memory 50, and applied to reference selector 64 according to the Prior-Bit Feedback state previously described with reference to FIG. 9. During the Prior-Bit Feedback state, the first bit read from memory 50 to reference selector 64 for successive approximation of biti=N-3, is the immediately previously generated bit, biti+1. Thus, a digital value corresponding to bit(N-3)+1 (i.e., bitN-2) is applied to reference selector 64. Switch SW8 is in the closed position and switch SW7 is in the open position, and based on the value of bitN-2, reference selector 64 outputs either VrefH or VrefL to first capacitor 66. Next, switch SW8 is in the open position, and switch SW7 is in the closed position, and a Feedback Charge Share state results in charge sharing between first and second capacitors 66, 68, resulting in a voltage value at the DAC output, according to Equation (6) for x=N−2:

DACtempout(N-1)-(N-2)=1⁄2[DACtempoutN-1-((N-2)-1)+(VDACcap(N-2))]=  (11)

DACtempout1=1⁄2[DACtempout2+(VDACcap(N-2))].  (12)

Where VDACcapN-2 is the voltage output by DAC reference selector 64 (VrefH or VrefL) corresponding to bitx=N-2. DACtempout2 is the previously generated DAC output, DACreset. Because the DACtempout0 has not been generated, the successive approximation process continues, and the DACtempout1 value at the output of DAC 54 is not compared to Vin for generation of the digital bit value biti=N-3. 

Next a DAC output, DACtempout(N-1)-X is generated for x=i+2. Because biti=N-3 is being generated, i=N−3, and x=i+2=(N−3+2)=N−1. Switch SW8 is in the closed position and switch SW7 is in the open position (so that reference selector 64 is coupled to only first capacitor 66) as shown in FIG. 9. A digital value corresponding to biti−2 is input to reference selector 64 as described above with reference to FIG. 9. Depending on the value of biti−2 reference selector 64 outputs either VrefH or VrefL, and applies the outputted reference voltage to first capacitor 66. Next, switch SW8 is opened and switch SW7 is closed as described with reference to FIG. 10, such that a DAC output signal is generated, according to Equation (6) for x=N−1:

DACtempout(N-1)-(N-1)=1⁄2[DACtempout(N-1)-((N-1)-1)+VDACcapN-1]=  (13)

DACtempout0=1⁄2[DACtempout1+VDACcapN-1]  (14)

Where DACtempout0 is DACouti=N-3, and DACtempout1 is the immediately previously generated DAC output value (i.e., the DAC output value generated during the successive approximation for x=i+1), and VDACcapN−1 is the voltage output by reference selector 64 (VrefH or VrefL) corresponding to bitN−1 (i.e., corresponding to the MSB). Generation of DACtempout0 signifies that the Prior Bit Feedback and Feedback Charge Share states have been repeated for x=i+1 through x=N−1 and successive approximation in the feedback loop is complete for biti.

This new DACtempout0=DACouti=N−3 value is input to operational amplifier 46 and compared to Vin to generate bitN-3 according to the Successive Bit Generation state previously described with reference to FIG. 10.
Using the example voltage values previously defined, MSB bitx=N-1 was previously generated as a digital high, and the second most significant bit, bitx=N-2 was previously generated as a digital low. Biti=N-3 can then be determined as follows.
The DAC output value is reset to DACreset=1⁄2[VrefL+VrefH]=1.65V according to the First Capacitor Reset, Second Capacitor Reset, and Charge Sharing states previously described above with reference to FIGS. 6–8. Next, according to the Prior-Bit Feedback state described above with reference to FIG. 9, switch SW8 is in the closed position and switch SW7 is in the open position. Successive approximation is applied for x=i+1. Because biti=N-3 is being generated, x=(N−3)+1=N−2. Thus, the first bit output from memory 50 and applied to reference selector 64 corresponds to bitx=N-2. Using the example values, bitN-2 was previously generated as a digital low, thus reference selector 64 outputs VrefL to first capacitor 66. Next, according to the Feedback Charge Share state (described above with reference to FIG. 10) switch SW7 is in the closed position, and switch SW8 is in the open position, causing DAC capacitors 66, 68 to charge share the voltage values of VDACcapi−1=VrefL=0.8V (shown in FIG. 12 for bit i=N−3=7 at x=i+1) and the previously generated DAC output value, DACtempout(N-1)-(x-1). For x=i+1, the previously generated DAC output is DACtempout(N-1)-x-1)=DACreset (e.g., 1.65V) because according to the exemplary method, DAC 54 is reset prior to generating each digital bit. Thus, the DAC output voltage is DACreset at the start of successive approximation for given biti (i.e., the first time a previously generated bit (i.e., bitx=i+1) is read into DAC 54). This results in a temporary DAC output (as shown in FIG. 12) being generated according to Equation (6) for x=N−2:

DACtempout(N-1)-(N-2)=1⁄2[DACreset+VDACcapN-2]  (15)

DACtempout1=1⁄2[1.65V+0.8V]=1.225V  (16)

Next, DACtempout(N-1)-x is determined for x=i+2. According to the Prior Bit Feedback state, switch SW7 is opened and switch SW8 is closed, and a digital value corresponding to bitx=i+2 is output from memory 50 and input to reference selector 64. Since i=N−3, and x=i+2, therefore x=[(N−3)+2]=N−1, and a value corresponding to the MSB, bitx=N-1, is applied to reference selector 64. Using the example values, bitx=N-1 was previously calculated as a digital high. Therefore, reference selector 64 outputs VrefH which is held at first capacitor 66 as VDACcapN-1 (shown in FIG. 12 for bit i=7 at x=i+2=9). Next, according to the Feedback Charge Share state, switch SW7 is closed, and switch SW8 is opened, resulting in charge sharing between first and second capacitors 66, 68 such that a DAC output value (shown in FIG. 12) is generated according to Equation (6) for x=N−1:

DACtempout(N-1)-(N-1)=1⁄2[DACtempout(N-1)-((N-1)-1)+VDACcapN-1]=  (17)

DACtempout0=1⁄2[DACtempout1+VDACCapN-1]=  (18)

DACtempout0=1⁄2[1.225V+2.5V]=1.8625V  (19)

DACtempout0 signifies that successive approximation using bitx=i+1 through bitx=N-1 is complete, and DACouti(i.e., DACtempout0) has been generated. According to the Successive Bit Generation state DACouti for i=N−3, is applied to the second input line 62 of operational amplifier 46 for generation of biti=N-3.
Operational amplifier 46 compares DACouti=N-3 to Vin, and outputs a digital value. In the present example, DACouti (1.8625V) is greater than Vin (1.85V); therefore operational amplifier 46 outputs a digital high. Output buffer 48 inverts the digital high to a digital low, and a digital value corresponding to the digital low is stored in memory 50 as corresponding to bitN-3 of the N-bit digital series.
In the same manner as previously described for biti=N-3 with reference to FIGS. 6–10, a successive biti in the N-bit digital series may be generated by resetting DAC 54 according to the First Capacitor Reset, Second Capacitor Reset, and Charge Sharing states described with reference to FIGS. 6–8. Successive DAC output values (DACtempout(N-1)-X) may be generated according to the Prior Bit Feedback and Feedback Charge Share states and according to equation (18) (i.e., DACtempout(N-1)-x=1⁄2[DACtempoutN-1-(x-1)+(VDACcapx)]) for x=i+1, i+2, . . . N−1. Generation of DACtempout0 signifies that bitsx=i+1 through bitx=N-1 have been applied to DAC reference selector 64 for generating successive DACtempout(N-1)-x values, and thus DACtemput0 is equal to DACouti. According to the Successive Bit Generation state described with reference to FIG. 10, DACouti is applied to second input line 62 of operational amplifier 46, which compares DACouti to the signal being digitized, Vin, and outputs a digital value based on this comparison.
FIG. 11C illustrates an exemplary pointer flow for generating DACouti. As shown in FIG. 11C step A, DAC 54 is reset. For example, DAC 54 may be reset by resetting first capacitor 66 and second capacitor 68 according to the First Capacitor Reset, Second Capacitor Reset and Charge Sharing states described above with reference to FIGS. 6–8 such that the output voltage of DAC 54 is 1⁄2(VrefH+VrefL). As shown in FIG. 11C, FINDPTR identifies the biti; that is being approximated. This bit corresponds to points to a bit address i, in which a digital value corresponding to the bit currently being generated by ADC 44 (i.e., biti), will be stored. If, as shown in step B, FINDPTR is pointing to address N−1, which corresponds to bitN-1, the MSB is being generated and the process exits to Step J as additional successive approximation steps are not needed. If, as shown in FIG. 11C step B, FINDPTR is not pointing to address N−1 (which indicates the MSB has already been generated), then the process continues through steps C through I to perform successive approximate to generate a DAC output value (e.g., DACouti) to be compared to Vin for generation of biti.
At Step C, a pointer, READPTR is set to FINDPTR+1. This causes READPTR to point to address i+1, which contains a digital value corresponding to biti+1. At step D, biti+1 is read from memory. A digital value corresponding to biti+1 is applied to reference selector 64 (as previously described with reference to the Prior-Bit Feedback state and FIG. 9). At step E a reference value is output based on the value of biti+1. For example, as previously described with reference to the Prior-Bit Feedback state and FIG. 9, the reference selector 64 may be configured such that if the digital value corresponding to biti+1 is logic high (e.g., logic 1), then first capacitor 66 is set to VrefH, and if not, then first capacitor 66 is set to VrefL. Thus, if at Step E, the digital value corresponding to biti+1 is logical high, C2 is set to VrefH (step F); otherwise C2 is set to VrefL (step F).
At step G, first capacitor 66 and second capacitor 68 charge share such that the output of DAC 54 is one half the sum of the voltages on first capacitor 66 and second capacitor 68. The voltage on first capacitor 66 is the voltage output by reference selector 64, which, as previously described with reference to the Feedback Charge Share state and FIG. 10, is VDACcapx. The voltage on second capacitor 68 is the previous DAC output voltage value (e.g., DACtempoutN-1-(x-1) as previously described with reference to FIG. 10) and thus, the charge shared voltage is the voltage of Equation (6): DACtempout(N-1)-x1⁄2[DACtempoutN-1-(x-1)+(VDACcapx)].
At step H, READPTR is incremented so that it now points to biti+2, and if, at step I, the READPTR is less than N, the process loops back to step D. Steps D–I are repeated until READPTR is no longer less than N (which signifies that the digital value corresponding to the MSB has been read in and applied to DAC 64 through successive approximation), and the process exits to Step J.
FIG. 13 shows system 600, a typical processor based system modified to include an imager device 500 comprising an ADC of the present invention. Examples of processor based systems, which may employ the imager device 500, include, without limitation, computer systems, camera systems, scanners, machine vision systems, vehicle navigation systems, video telephones, surveillance systems, auto focus systems, star tracker systems, motion detection systems, image stabilization systems, and others.
System 600 includes a central processing unit (CPU) 602 that communicates with various devices over a bus 620. Some of the devices connected to the bus 620 provide communication into and out of the system 600, illustratively including an input/output (I/O) device 606 and imager device 500. Other devices connected to the bus 620 provide memory, illustratively including a random access memory (RAM) 604, hard drive 612, and one or more peripheral memory devices such as a floppy disk drive 614 and compact disk (CD) drive 616. The imager device 500 may be combined with a processor, such as a CPU, digital signal processor, or microprocessor, in a single integrated circuit. The imager device 500 may be a CCD imager or CMOS imager constructed in accordance with any of the illustrated embodiments.
The processes and devices described above illustrate preferred methods and typical devices of many that could be used and produced. The above description and drawings illustrate embodiments, which achieve the objects, features, and advantages of the present invention. However, it is not intended that the present invention be strictly limited to the above-described and illustrated embodiments. Any modification, though presently unforeseeable, of the present invention that comes within the spirit and scope of the following claims should be considered part of the present invention.

## Claims

1. An analog-to-digital converter comprising:
an operational amplifier for generating digital bits in an N-bit digital bit series in response to a first analog input signal; wherein said operational amplifier comprises:
a first input line for receiving the first analog input signal,
a second input line for receiving an analog feedback circuit signal, and
an amplifier output line for outputting a digital amplifier output signal;
a memory for receiving and storing a digital bit corresponding to said digital amplifier output signal; and
a digital-to-analog converter for generating said analog feedback circuit signal in response to receiving a digital bit signal applied to said digital-to-analog converter comprising:
a first capacitor and a second capacitor connected in parallel,
a reference selector for receiving said digital bit signal from said memory, and
a feedback output line coupled to said second input line for applying said analog feedback circuit signal to said second input line.

2. The analog-to-digital converter of claim 1, further comprising a sample and hold circuit, wherein said sample and hold circuit receives a second analog input signal and, in response to said second analog input signal, applies said first analog input signal to said first input line of said operational amplifier.

3. The analog-to-digital converter of claim 2 further comprising a voltage source coupled to said sample and hold circuit,
wherein said sample and hold circuit further comprises a sample and hold capacitor, and
wherein said voltage source drives a reference voltage on to said sample and hold capacitor.

4. The analog-to-digital converter of claim 1 further comprising an output buffer circuit comprising:
an output buffer;
a buffer input line coupled to the amplifier output line; and
a buffer output line coupled to said memory, wherein said output buffer is coupled between said operational amplifier and said memory and outputs a digital bit signal corresponding to the digital amplifier output signal from said operational amplifier.

5. The analog-to-digital converter of claim 4 wherein said output buffer is an inverter.

6. The analog-to-digital converter of claim 1, wherein said memory comprises a random access memory cell.

7. The analog-to-digital converter of claim 1 wherein said first and second capacitors are matched capacitors.

8. The analog-to-digital converter of claim 1 wherein said digital-to-analog converter further comprises:
a first switch for coupling said reference selector to said first capacitor; and
a second switch for coupling said second capacitor and said feedback output line to said first capacitor and said reference selector.

9. An analog-to-digital converter comprising:
an operational amplifier for generating digital bits in an N-bit digital bit series in response to a first analog input signal, said operational amplifier comprising:
a first input line for receiving said first analog input signal;
a second input line for receiving a second analog input signal; and
an amplifier output line for outputting digital amplifier output signals;
a memory for receiving and storing digital bit values corresponding to said digital amplifier output signals, said memory comprising:
a random access memory cell,
a first tri-state inverter for storing a first digital bit value on to a first storage node,
a second tri-state inverter for receiving said first digital bit value from said first storage node, and outputting a second digital bit value on to a feedback input line, and
a third tri-state inverter for receiving said first digital bit value from said first storage node, and outputting a third digital bit value on to a bit line for readout of a digital bit value of said N-bit digital bit series; and
a digital-to-analog converter connected to said feedback input line, wherein said digital-to-analog converter receives said second digital bit value from said memory via said feedback input line and, in response to said second digital bit value, generates an analog output value that is applied to said second input line of said operational amplifier.

10. The analog-to-digital converter of claim 9, wherein said second tri state inverter outputs said second digital bit value on to said feedback input line in response to a command signal.

11. The analog-to-digital converter of claim 9, wherein said third tri state inverter writes said third digital bit value on to said bit line in response to a command signal.

12. The analog-to-digital converter of claim 9, further comprising
a fourth tri-state inverter coupled between said first storage node and said third tri-state inverter, and
a second storage node coupled between said fourth tri-state inverter and said third tri-state inverter,
wherein said fourth tri-state inverter receives said first digital bit value from said first storage node and in response to a command signal, said fourth tri-state inverter stores a second digital bit value onto said second storage node, and wherein said third tri-state inverter receives said second digital bit value from said second storage node in response to a command signal.

13. The analog-to-digital converter of claim 12, wherein said fourth tri-state inverter applies a fourth digital bit value to said third tri-state inverter in between command signals.

14. An analog-to-digital conversion method for generating a digital bit, biti, where i<N−1, in an N-bit digital bit series having bits N−1 through 0, said method comprising:
generating an analog reset voltage, wherein said analog reset voltage is generated by charge sharing a first capacitor and a second capacitor of a digital-to-analog converter;
holding said analog reset voltage at the output of said digital-to-analog converter;
applying to the input of said digital-to-analog converter a feedback digital value corresponding to a stored bite, wherein x=i+1 and bitx is the immediately previous bit in the N-bit digital series;
generating an analog reference voltage based on the value of said feedback digital bitx, and applying said analog reference voltage to said first capacitor;
charge sharing said first capacitor and said second capacitor to generate a temporary digital-to-analog converter output value corresponding to feedback digital bitx;
incrementing x, and repeating said steps of applying a feedback digital value, generating and applying said analog reference voltage, and charge sharing said first capacitor and said second capacitor, for each of said incremented x values through x=N−1 to generate a feedback circuit output value;
applying the feedback circuit output value to said operational amplifier; and
comparing a first analog input signal value to said feedback circuit output value and outputting an amplifier digital output value from said operational amplifier in response to said comparing.

15. The analog-to-digital conversion method of claim 14, further comprising the step of storing in a memory bit which corresponds to said amplifier digital output value.

16. The analog-to-digital conversion method of claim 14, wherein, said digital-to-analog converter further comprises a reference selector for generating and applying said analog reference voltage.

17. The analog-to-digital conversion method of claim 16, wherein said digital-to-analog converter further comprises a first switch for coupling said reference selector to said first capacitor; and a second switch for coupling said second capacitor and a feedback output line to said first capacitor and to said reference selector, and wherein said first switch is closed and said second switch is closed during said step of generating and applying said analog reference voltage, and wherein said first switch is open, and said second switch is closed during said step of charge sharing.

18. An analog-to-digital conversion system comprising:
a processor;
an analog-to-digital converter coupled to said processor, said analog-to-digital converter comprising:
an operational amplifier for generating digital bits in an N-bit digital bit series in response to a first analog input signal, Vin, wherein said operational amplifier comprises:
a first input line for receiving a first analog input signal,
a second input line for receiving a second analog input signal, and
an amplifier output line for outputting a digital output value;
a memory for storing a digital bit corresponding to said digital output value;
a digital-to-analog converter comprising:
a reference selector coupled to said memory, wherein said reference selector receives a digital signal corresponding to said digital bit from said memory and generates a first analog voltage signal in response to said digital signal;
a first capacitor and a second capacitor connected in parallel, wherein said first capacitor and second capacitor are coupled to said reference selector and receive said first analog voltage signal, and in response to said first analog voltage signal, said first capacitor and second capacitor generate said second analog input signal;
a feedback output line coupling said first capacitor and second capacitor to said operational amplifier, wherein said second analog input signal is applied to said second input line via said feedback output line.

19. An imager sensor comprising:
a pixel array;
an analog-to-digital converter comprising,
an operational amplifier for generating digital bits in an N-bit digital bit series in response to a first analog input signal; wherein said operational amplifier comprises:
a first input line for receiving the first analog input signal,
a second input line for receiving an analog feedback circuit signal, and
an amplifier output line for outputting a digital amplifier output signal;
a memory for receiving and storing a digital bit corresponding to said digital amplifier output signal; and
a digital-to-analog converter for generating said analog feedback circuit signal in response to receiving a digital bit signal applied to said digital-to-analog converter comprising:
a first capacitor and a second capacitor connected in parallel,
a reference selector for receiving said digital bit signal from said memory, and
a feedback output line coupled to said second input line for applying said analog feedback circuit signal to said second input line.

20. The image sensor of claim 19 wherein the pixel cell and analog-to-digital converter are on a single integrated circuit chip.

21. A method of fabricating an imager comprising:
fabricating on an integrated circuit chip a pixel array comprising pixel cells and readout circuitry;
fabricating on the integrated circuit chip an analog-to-digital converter comprising an operational amplifier, sample and hold circuit, memory, and digital-to-analog converter; and
fabricating connections for transmitting analog signals from the pixel array to the analog-to-digital converter,
wherein said operational amplifier comprises a first input line for receiving the first analog input signal, a second input line for receiving an analog feedback circuit signal, and an amplifier output line for outputting a digital amplifier output signal,
wherein said digital-to-analog converter generates said analog feedback circuit signal in response to receiving a digital bit signal applied to said digital-to-analog converter, said digital-to-analog converter comprising a first capacitor and a second capacitor connected in parallel, a reference selector for receiving said digital bit signal from said memory, and a feedback output line coupled to said second input line for applying said analog feedback circuit signal to said second input line.