Abstract:
Apparatuses and methods for providing offset compensation include a primary amplifier which includes a first output, a second output, a first load input, and a second load input, a first feedback loop connected to the primary amplifier and which includes a first switch located between the first output of the primary amplifier and the first load input, and a first sampling capacitor coupled to the first switch between the first switch and the first load input and a second feedback loop connected to the primary amplifier and which includes a second switch located between the second output of the primary amplifier and the second load input, and a second sampling capacitor coupled to the second switch between the second switch and the second load input.

Description:
FIELD OF THE INVENTION 
     Disclosed embodiments relate generally to the field of providing offset compensation for an operational amplifier to minimize the offset voltage in the readout chain of an imaging device. 
     BACKGROUND OF THE INVENTION 
       FIG. 1  illustrates an imaging device  100  having a CMOS pixel array  140 . Row lines of the array  140  are selectively activated by a row driver  145  in response to row address decoder  155 . A column driver  160  and column address decoder  170  are also included in the imaging device  100 . The imaging device  100  is operated by the timing and control circuit  150 , which controls the address decoders  155 ,  170 . The control circuit  150  also controls the row and column driver circuitry  145 ,  160 . 
     The following devices form the readout chain of the imaging sensor  100 . A sample and hold circuit  161  associated with the column driver  160  samples and holds a pixel reset signal Vrst and a pixel image signal Vsig for each selected pixel of the array  140 . A differential signal (Vrst−Vsig) is produced by a differential amplifier  162  for each pixel and is digitized by analog-to-digital converter  175  (ADC). The analog-to-digital converter  175  supplies the digitized pixel signals to an image processor  180 , which forms and may output a digital image. 
     The output of an imaging device  100  is very sensitive to operational amplifier (“opamp”) offset voltage. The offset voltage is the voltage required across the input terminals of the operational amplifier to drive the output voltage to zero. In an ideal operation amplifier, there would be no offset voltage required. However, offset voltage is required in real-world operational amplifiers because of internal imperfections. Some industry techniques have been developed to reduce the offset voltage together with noise (which has similar characteristics as offset voltage at low frequencies). These techniques, described below, include chopper stabilization, auto-zero, and correlated double sampling (CDS). In the chopper stabilization technique, the input signal is modulated with a high frequency carrier before amplification and is then demodulated after amplification to obtain an amplified version of the input signal. The offset voltage is modulated only once in the path and is filtered out with a subsequent low-pass filter. The main drawbacks of chopper stabilization are that it is bandwidth limited to half of the chopper frequency to avoid signal aliasing, and that it requires filtering to remove the large ripple voltages generated by chopping. 
       FIG. 2  is a schematic diagram of a conventional auto-zero amplifier  200 . Amplifier  200  includes a primary amplifier A B , an auxiliary amplifier A A , two auto-zero phase switches  201 ,  202 , controlled by an auto-zero phase signal Φ A , for use during an auto-zero phase, and two amplification phase switches  203 ,  204 , controlled by an amplification phase signal Φ B , for use during an amplification phase. Amplifier  200  further includes biasing capacitors V OSA , C M1 , C M2 . The primary amplifier A B  includes a non-inverting input  210 , an inverting input  220 , an offset nulling port B B , and an output  250 . The auxiliary amplifier A A  includes a non-inverting input  230 , an inverting input  240 , an offset nulling port −B A , and an output  260 . 
     As shown in  FIG. 2 , during the auto-zero phase, the auxiliary amplifier A A  is configured in unity gain feedback (i.e., the output tracks the input without amplification) between the output port  260  and the offset nulling port −B A , while inputs of the auxiliary amplifier A A  are shorted by closing the auto-zero phase switches  201 ,  202  and opening the amplification phase switches  203 ,  204 . The biasing condition of the nulling port −B A  is sampled in the auto-zero phase and held throughout the subsequent amplification phase, when the amplification phase switches  203 ,  204  are closed and the auto-zero phase switches  201 ,  202  are open. This results in an almost offset voltage-free auxiliary amplifier A A  in the amplification phase. 
     In the amplification phase, the signal path V OA  has significantly more gain (primary amplifier A B  gain times auxiliary amplifier A A  gain) than the offset path V NA  (primary amplifier A B  gain). Thus, the equivalent input offset voltage of the amplifier  200  is greatly reduced by the gain ratio of the two paths V OA , V NA . Drawbacks of this conventional design are that it requires two amplifiers A A , A B  and it often has a larger layout area than other amplifier designs. 
       FIG. 3  is a schematic diagram of a conventional amplifier  300  with offset compensation for auto-zeroing. Amplifier  300  includes a distributed amplifier  310 , an offset compensation block  320 , and a two-stage amplifier  330 . The distributed amplifier includes first and second transistors m 1 , m 2 , first and second resistance circuits R 1 , R 2 , a first biasing transistor Bias, an input terminal IN, a reference voltage input REF, and a switch  311  controlled by an auto-zero signal AZ for connecting the input terminal IN to the reference voltage input REF. The offset compensation block  320  includes third and fourth transistors m 3 , m 4 , a second biasing transistor B, two capacitors C 1 , C 2 , and two switches  321 ,  322  controlled by the auto-zero signal AZ. Two-stage amplifier  330  includes first and second amplifiers A 1 , A 2 , where the second amplifier A 2  outputs to two switches  331 ,  332 , each controlled by an inverse auto-zero signal/AZ. The first amplifier A 1  inputs signals from the distributed amplifier  310  via a pair of capacitors  327 ,  328 , each connected to a respective switch  325 ,  326 , respectively connected to the first and second transistors m 1 , m 2 . Switches  331 ,  332  receive signals from the offset compensation block  320  via switches  321 ,  322 . 
     This configuration implements offset cancellation using the same principle as the auto-zero phase of the auxiliary amplifier A A  presented in  FIG. 2 . A drawback of this design is that the phase margin of the signal path will be jeopardized due to the additional offset compensation block  320  before the traditional two-stage amplifier  330 . In addition, more noise may be introduced due to additional CMOS elements in the offset compensation block. 
     Correlated double sampling (CDS) is another approach to reduce the offset voltage and noise. Correlated double sampling typically involves two steps of sampling. The first step samples the offset voltage and stores the sampled offset voltage in a capacitor. The second step samples both the offset voltage and the signal and performs a difference operation with the offset voltage sampled in the first step. The offset voltage is thus ideally cancelled out by correlated double sampling. 
       FIG. 4  is a schematic diagram of a conventional circuit  400  for implementing correlated double sampling. Circuit  400  includes an ideal operational amplifier  410 , having two inputs  411 ,  412  and an output  413 . An offset voltage source Vos is connected in series with input  411 . Circuit  400  further includes three switches  415 ,  420 ,  425 . Switches  420 ,  425  are controlled by a first phase signal phi 1 , and switch  415  is controlled by a second phase signal phi 2 . 
     In a first phase, the first phase signal phi 1  is asserted, closing switches  420  and  425 , and the operational amplifier  410  is configured in unity gain feedback, such that the output tracks the input without amplification. An offset voltage is sampled and stored in an offset capacitor Cos. In a second phase, the second phase signal phi 2  is asserted, closing switch  415 , and the voltage stored in offset capacitor Cos is used to cancel the offset voltage of the operational amplifier  410 . This implementation has a disadvantage in that the operational amplifier  410  is required to be unity-gain stable, which is usually difficult and unnecessary in certain cases. In addition, due to the input parasitic capacitance of the operational amplifier, the feedback factor is usually degraded undesirably. 
       FIG. 5  is a circuit diagram of a conventional differential telescopic operational amplifier  500 . Amplifier  500  includes first through tenth transistors  505 ,  510 ,  515 ,  520 ,  525 ,  530 ,  535 ,  540 ,  545 ,  550 . First through fourth transistors  505 ,  510 ,  515 ,  520  are p-type transistors. Fifth through tenth transistors  525 ,  530 ,  535 ,  540 ,  545 ,  550  are n-type transistors. Transistors  505 ,  515 ,  525 ,  535 ,  545  are connected in series, and transistors  510 ,  520 ,  530 ,  540 ,  550  are connected in series. 
     Transistors  505 ,  510  have their gates connected to each other and to a first line  555 , which is connected to a first bias output BIASP. Transistors  515 ,  520  have their gates connected to each other and to a second line  560 , which is connected to biasing circuitry (not shown). Transistors  525 ,  530  have their gates connected to each other and to a third line  565 , which is connected to biasing circuitry (not shown). A first signal output OUTN is located between transistors  515 ,  525 . A second signal output OUTP is located between transistors  520 ,  530 . Transistors  535 ,  540  receive first and second signal inputs INP, INN at their respective gates. A third line  570  is connected between transistors  535 ,  545 , and between transistors  540 ,  550 , and is connected to biasing circuitry (not shown). A fourth line  575  is connected to biasing circuitry (not shown), and to a gate of the ninth transistor  545 , and to a second bias output BIASN. A common mode feedback input CMFB is connected to a gate of the tenth transistor  550  for receiving a common mode feedback based on the outputs OUTP, OUTN. Amplifier  500  has a disadvantage in that the biases of the first and second transistors  505 ,  510  are fixed and reliant on each other. 
     Accordingly, there is a need and desire for a method and apparatus for providing offset compensation for an operational amplifier to minimize the offset voltage in a readout chain of an imaging device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a block diagram of a conventional imaging device. 
         FIG. 2  is a schematic diagram of a conventional auto-zero amplifier. 
         FIG. 3  is a schematic diagram of a conventional amplifier with offset compensation for auto-zeroing. 
         FIG. 4  is a schematic diagram of a conventional circuit for implementing correlated double sampling. 
         FIG. 5  is a circuit diagram of a conventional differential telescopic operational amplifier. 
         FIG. 6  is a circuit diagram of a differential telescopic operational amplifier constructed in accordance with an embodiment described herein. 
         FIG. 7  is a schematic diagram of a circuit for offset cancellation constructed in accordance with an embodiment described herein. 
         FIG. 8  is a schematic diagram of a circuit for offset cancellation constructed in accordance with an embodiment described herein. 
         FIG. 9  is a schematic diagram of a circuit for offset cancellation constructed in accordance with an embodiment described herein. 
         FIG. 10  is a schematic diagram of a circuit for offset cancellation constructed in accordance with an embodiment described herein. 
         FIG. 11  is a graph of charts of two simulation test results. 
         FIG. 12  is an embodiment of a camera system that can be used an imaging device constructed in accordance with an embodiment described herein. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof and show by way of illustration specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice them, and it is to be understood that other embodiments may be utilized, and that structural, logical, processing, and electrical changes may be made. The progression of processing steps described is an example; however, the sequence of steps is not limited to that set forth herein and may be changed as is known in the art, with the exception of steps necessarily occurring in a certain order. 
     Accordingly, the following detailed description is not to be taken in a limiting sense, and the described embodiments are defined only by the appended claims. 
       FIG. 6  is a circuit diagram of a differential telescopic operational amplifier  600  constructed in accordance with an embodiment described herein. The operational amplifier  600  is similar to the operational amplifier  500  of  FIG. 5 , except that the first and second transistors  505 ,  510  of  FIG. 5  are replaced with two load transistors M_loadp, M_loadn having gates respectively dynamically biased by load lines ofsp, ofsn, and the  FIG. 5  first line  555  and first bias output BIASP of amplifier  500  have been eliminated. 
       FIG. 7  is a schematic diagram of a circuit  700 , constructed in accordance with an embodiment described herein, for offset cancellation using amplifier  600  and dynamically biasing the operational amplifier  600 . Circuit  700  includes the amplifier  600  of  FIG. 6 , and first through fourth switches  701 - 704  controlled by an auto-zero signal AZ. A method of operating the circuit  700  is described below. Referring to  FIGS. 6 and 7 , in an auto-zero phase, the differential outputs OUTP, OUTN of the operational amplifier  600  are connected to gates of the two load transistors M_loadp, M_loadn via the load lines ofsp and ofsn, while the inputs INP, INN of the operational amplifier  600  are shorted to the input common mode voltage. Switches  703 ,  704  are included in respective feedback loops  705 ,  706  which connect the load transistors&#39; inputs ofsp, ofsn to the differential outputs OUTP, OUTN. 
     Differential offset voltages appearing at the outputs OUTP, OUTN of the operational amplifier  600  are sampled in two offset capacitors Cofsp, Cofsn connected to the gates of the two load transistors M_loadp, M_loadn during the auto-zero phase and are held throughout a subsequent amplifying phase. After the auto-zero phase, the input offset voltages can be expressed as OUTP/A and OUTN/A, where A is the DC gain of the operational amplifier  600 . This value is typically very small, thus the operational amplifier  600  can be treated as an almost offset-free (i.e., ideal) operational amplifier. Moreover, low frequency noise is reduced. The auto-zero phase may be repeated to provide dynamic offset cancellation because the outputs are re-sampled and the offset similarly canceled as noise or other conditions change over time. 
     When the operational amplifier  600  is used in an image sensor with an electronic rolling shutter, the auto-zero period can overlap or partially overlap with a row sampling time. This gives a long enough period of time for the auto-zero phase. The auto-zero procedure repeats in every row operation before pixel amplification starts. After the auto-zero procedure is done, the operational amplifier  600  can be used as an almost offset-free operational amplifier to read out the signals stored in columns of the whole row. Meanwhile, row-wise noise can also be reduced due to the reduction of noise from the operational amplifier  600 . 
     Alternative offset cancellation schemes are shown in  FIGS. 8-10 .  FIG. 8  is a schematic diagram of a circuit  800  for offset cancellation constructed in accordance with an embodiment described herein. The circuit  800  of  FIG. 8  is similar to the circuit  700  of  FIG. 7 , except that two pre-charged capacitors  810 ,  820  with respective switches  830 ,  840  (controlled by the auto-zero signal AZ) are inserted in the offset voltage feedback loops  805 ,  806  to set the output common mode voltage OUTP, OUTN to, or close to, a predetermined value. The switches  830 ,  840  can be made small to prevent charge leakage of the offset capacitors Cofp, Cofn since the auto-zero phase may be much longer than a clock period. The pre-charged capacitors  810 ,  820  are charged by applying a voltage across their respective terminals. 
       FIG. 9  is a schematic diagram of a circuit  900  for offset cancellation constructed in accordance with another embodiment described herein. The circuit  900  of  FIG. 9  is similar to the circuit  700  of  FIG. 7 , except that two buffers  910 ,  920  are inserted in the offset voltage feedback loops  905 ,  906  and the offset voltage sampling capacitors Cofsp, Cofsn are placed at the input of the buffers  910 ,  920 . The buffers  910 ,  920  help prevent the operational amplifier output signals OUTP, OUTN from disturbing the biasing voltages sampled and held on the offset voltage sampling capacitors Cofsp, Cofsn for offset cancellation, especially in the case that multiple amplifying phases follow one single auto-zero phase (as when circuit  800  is used in a CMOS imaging sensor). 
       FIG. 10  is a schematic diagram of a circuit  1000  for offset cancellation constructed in accordance with another embodiment described herein. The circuit  1000  of  FIG. 10  is similar to the circuit  700  of  FIG. 7 , except that two assistant operational amplifiers  1010 ,  1020  each having differential inputs are inserted in the offset voltage feedback loops  1005 ,  1006  so that an output common mode voltage can be set to a predetermined value. The assistant operational amplifiers  1010 ,  1020  have respective switches  1030 ,  1040  (controlled by the auto-zero signal AZ) connected to the non-inverting inputs, and each receive the output common mode voltage at an inverting input. 
       FIG. 11  is a graph  1100  of two charts  1110 ,  1120  of two simulation test results. Both charts  1110 ,  1120  simulate a 0 V and a 10 mV operational amplifier offset voltage, showing voltage with respect to time. Chart  1110  shows a result of a simulation using the operational amplifier  600  described in  FIG. 6  and the circuit  700  described in  FIG. 7 . Chart  1120  shows a result of a simulation using the conventional operational amplifier  500  described in  FIG. 5 . For the charge amplifier using the operational amplifier  600 , the simulations show almost no difference (&lt;1 uV difference) between the two situations (i.e., with a 0 V or a 10 mV operational amplifier offset voltage), such that only one line  1111  is visible, representing two overlapping lines. 
     However, for the charge amplifier using the conventional telescopic operational amplifier  500 , a 10 mV operational amplifier offset voltage causes about 49.8 mV difference between the two situations (i.e., with a 0 V or a 10 mV operational amplifier offset voltage), where the 0 V offset result is shown the solid line  1121 , and the 10 V offset result is shown the dashed line  1122 . The difference of 49.8 mV, which approximately equals the operational amplifier offset voltage divided by a feedback factor, is the expected result for the conventional amplifier  500 . The simulation results verify the effectiveness of offset voltage cancellation for the amplifier  500 . 
       FIG. 12  is an embodiment of a camera system  1200 , which can use an imaging device constructed in accordance with an embodiment described herein, such as imaging device  100  ( FIG. 1 ) using the differential telescopic operational amplifier  600  in place of the differential amplifier  162 . The amplifier  600  may be used in camera system  1200  with any of the circuits  700 ,  800 ,  900 ,  1000  to provide offset cancellation for the amplifier  600 . Camera system  1200 , for example, is a still or video camera system, which generally comprises a lens  1230  for focusing an image on the pixel array  140  ( FIG. 1 ) when shutter release button  1235  is depressed, a central processing unit (CPU)  1205 , such as a microprocessor for controlling camera operations, that communicates with one or more input/output (I/O) devices  1210  over a bus  1215 . Imaging device  100  also communicates with the CPU  1205  over bus  1215 . The system  1200  also includes random access memory (RAM)  1220 , and can include removable memory  1225 , such as flash memory, which also communicate with CPU  1205  over the bus  1215 . Imaging device  100  may be combined with a processor, such as a CPU, digital signal processor, or microprocessor, with or without memory storage on a single integrated circuit or on a different chip than the processor. 
     The camera system  1200  is one example of a system having digital circuits that could include image sensor devices. Without being limiting, such a system could instead include a computer system, scanner, machine vision, vehicle navigation, video phone, surveillance system, auto focus system, star tracker system, motion detection system, image stabilization system, and other image acquisition and processing system. 
     The processes and devices in the above description and drawings illustrate examples of methods and devices of many that could be used and produced to achieve the features, and advantages of embodiments described herein. For example, embodiments include combining amplifier  500  with any of circuits  700 ,  800 ,  900 ,  1000 . In addition, the amplifier  600  is not limited to the specific p-type and n-type transistors shown in  FIG. 6 . In addition, embodiments are not limited to using a CMOS imaging device. Thus, the embodiments are not to be seen as limited by the foregoing description of the embodiments, but only limited by the appended claims.