Abstract:
Because of variations in open loop gain and bandwidth in successive approximate register (SAR) analog-to-digital converters (ADCs), designing amplifiers with the desired characteristics is difficult. Here, a multipath amplifier is provided that accounts for the variations in open loop gain and bandwidth. Preferably, a number of cascaded amplifiers are provided that can auto-zero to account for offset voltages so as to allow the multipath amplifier to be stable over the desired open loop gains and bandwidths.

Description:
CROSS-RELATED APPLICATIONS 
       [0001]    This application claims priority from Indian Patent Application No. 2979/CHE/2009, filed Dec. 3, 2009, which is hereby incorporated by reference for all purposes. 
       TECHNICAL FIELD 
       [0002]    The invention relates generally to an amplifier and, more particularly, to a multipath amplifier that is preferably used with a recyclic successive approximation register (SAR) analog-to-digital converter (ADC). 
       BACKGROUND 
       [0003]    In a recyclic SAR ADC, the open loop gain can change from 2 4  to 2 12 , and bandwidth requirements can also change from cycle to cycle. Thus, it is difficult to provide an amplifier that is stable with such variations in both gain and bandwidth. Some examples of conventional circuit are: U.S. Pat. No. 6,275,104; U.S. Pat. No. 6,486,820; U.S. Pat. No. 6,577,185; U.S. Pat. No. 7,193,545; U.S. Pat. No. 7,227,481; U.S. Pat. No. 7,342,449. 
       SUMMARY 
       [0004]    A preferred embodiment of the present invention, accordingly, provides an apparatus is provided. The apparatus comprises a first amplifier stage including: a first preamplifier having a plurality of input terminals and an output terminal, wherein at least one of the input terminals of the first preamplifier receives an input signal for the first amplifier stage; a first output amplifier having an input terminal and an output terminal, wherein the input terminal of the first output amplifier is coupled to the output terminal of the first preamplifier; a first switch that is coupled between the output terminal of the first output amplifier and at least one of the input terminals of the first preamplifier; and a first capacitor that is coupled to the first switch; and a second amplifier stage including: a second preamplifier having a plurality of input terminals and an output terminal, wherein at least one of the input terminals is coupled the output terminal of the first preamplifier stage; a second output amplifier having an input terminal and an output terminal, wherein the input terminal of the second output amplifier is coupled to the output terminal of the second preamplifier; a second switch that is coupled between the output terminal of the second output amplifier and at least one of the input terminals of the second preamplifier; and a second capacitor that is coupled to the second switch. 
         [0005]    In accordance with a preferred embodiment of the present invention, the apparatus further comprises a third stage including: a third preamplifier having a plurality of input terminals and an output terminal, wherein at least one of the input terminals is coupled the output terminal of the second preamplifier stage; a third output amplifier having an input terminal and an output terminal, wherein the input terminal of the third output amplifier is coupled to the output terminal of the third preamplifier; a third switch that is coupled between the output terminal of the third output amplifier and at least one of the input terminals of the third preamplifier; and a third capacitor that is coupled to the third switch. 
         [0006]    In accordance with a preferred embodiment of the present invention, the first preamplifier further comprises a pair of amplifiers, wherein each amplifier from the pair includes a pair of differential input terminals and a pair of differential output terminals. 
         [0007]    In accordance with a preferred embodiment of the present invention, the second preamplifier further comprises a pair of amplifiers, wherein each amplifier from the pair includes a pair of differential input terminals and a pair of differential output terminals. 
         [0008]    In accordance with a preferred embodiment of the present invention, each of the first and second output terminals includes a pair of differential input terminals and a pair of differential output terminals. 
         [0009]    In accordance with a preferred embodiment of the present invention, an apparatus is provided. The apparatus comprises a first amplifier having a first input terminal, a second input terminal, a first output terminal, and a second output terminal, wherein the first and second input terminals of the first amplifier receive an input signal; a second amplifier having a first input terminal, a second input terminal, a first output terminal, and a second output terminal, wherein the first output terminal of the second amplifier is coupled to the first output terminal of the first amplifier, and wherein the second output terminal of the second amplifier is coupled to the second output terminal of the first amplifier; a third amplifier having a first input terminal, a second input terminal, a first output terminal, and a second output terminal, wherein the first input terminal of the third amplifier is coupled to the first output terminal of the first amplifier, and wherein the second input terminal of the third amplifier is coupled to the second output terminal of the first amplifier; a first switch that is coupled between the first output terminal of the third amplifier and the first input terminal of the second amplifier; a second switch that is coupled between the second output terminal of the third amplifier and the second input terminal of the second amplifier; a fourth amplifier having a first input terminal, a second input terminal, a first output terminal, and a second output terminal, wherein the first input terminal of the fourth amplifier is coupled to the first output terminal of the first amplifier, and the second input terminal of the fourth amplifier is coupled to the second output terminal of the first amplifier; a fifth amplifier having a first input terminal, a second input terminal, a first output terminal, and a second output terminal, wherein first output terminal of the fifth amplifier is coupled to the first output terminal of the fourth amplifier, and wherein the second output terminal of the fifth amplifier is coupled to the second output terminal of fourth amplifier; a sixth amplifier having a first input terminal, a second input terminal, a first output terminal, and a second output terminal, wherein the first input terminal of the sixth amplifier is coupled to the first output terminal of the fourth amplifier, and wherein the second input terminal of the sixth amplifier is coupled to the second output terminal of the fourth amplifier; a third switch that is coupled between the first output terminal of the sixth amplifier and the first input terminal of the fifth amplifier; and a fourth switch that is coupled between the second output terminal of the and the second input terminal of the fifth amplifier. 
         [0010]    In accordance with a preferred embodiment of the present invention, the apparatus further comprises: a seventh amplifier having a first input terminal, a second input terminal, a first output terminal, and a second output terminal, wherein the first input terminal of the seventh amplifier is coupled to the first output terminal of the first amplifier, and the second input terminal of the seventh amplifier is coupled to the second output terminal of the first amplifier; a eighth amplifier having a first input terminal, a second input terminal, a first output terminal, and a second output terminal, wherein first output terminal of the eighth amplifier is coupled to the first output terminal of the seventh amplifier, and wherein the second output terminal of the eighth amplifier is coupled to the second output terminal of seventh amplifier; a ninth amplifier having a first input terminal, a second input terminal, a first output terminal, and a second output terminal, wherein the first input terminal of the ninth amplifier is coupled to the first output terminal of the seventh amplifier, and wherein the second input terminal of the ninth amplifier is coupled to the second output terminal of the seventh amplifier; a fifth switch that is coupled between the first output terminal of the ninth amplifier and the first input terminal of the eighth amplifier; and a sixth switch that is coupled between the second output terminal of the and the second input terminal of the eighth amplifier. 
         [0011]    In accordance with a preferred embodiment of the present invention, the apparatus further comprises a fifth switch that is coupled between the first and second input terminals of the first amplifier. 
         [0012]    In accordance with a preferred embodiment of the present invention, the apparatus further comprises: a first capacitor that is coupled between the first input terminal of the second amplifier and ground; a second capacitor that is coupled between the second input terminal of the second amplifier and ground; a third capacitor that is coupled between the first input terminal of the fifth amplifier and ground; and a fourth capacitor that is coupled between the second input terminal of the fifth amplifier and ground. 
         [0013]    In accordance with a preferred embodiment of the present invention, an successive approximate register (SAR) analog-to-digital converter (ADC) is provided. The SAR ADC comprises an ADC; a DAC that is coupled to the ADC; a residue amplifier including: a first preamplifier having a plurality of input terminals and an output terminal, wherein at least one of the input terminals of the first preamplifier is coupled to the DAC; a first output amplifier having an input terminal and an output terminal, wherein the input terminal of the first output amplifier is coupled to the output terminal of the first preamplifier; a first switch that is coupled between the output terminal of the first output amplifier and at least one of the input terminals of the first preamplifier; and a first capacitor that is coupled to the first switch; and a second preamplifier having a plurality of input terminals and an output terminal, wherein at least one of the input terminals is coupled the output terminal of the first preamplifier stage; a second output amplifier having an input terminal and an output terminal, wherein the input terminal of the second output amplifier is coupled to the output terminal of the second preamplifier; a second switch that is coupled between the output terminal of the second output amplifier and at least one of the input terminals of the second preamplifier; and a second capacitor that is coupled to the second switch; and SAR logic that is coupled to the residue amplifier and to the DAC. 
         [0014]    In accordance with a preferred embodiment of the present invention, the SAR ADC further comprises a conversion flash circuit coupled between the residue amplifier and the SAR logic. 
         [0015]    In accordance with a preferred embodiment of the present invention, the residue amplifier further comprises: a third preamplifier having a plurality of input terminals and an output terminal, wherein at least one of the input terminals is coupled the output terminal of the second preamplifier stage; a third output amplifier having an input terminal and an output terminal, wherein the input terminal of the third output amplifier is coupled to the output terminal of the third preamplifier; a third switch that is coupled between the output terminal of the third output amplifier and at least one of the input terminals of the third preamplifier; and a third capacitor that is coupled to the third switch. 
         [0016]    In accordance with a preferred embodiment of the present invention, each of the first and second output amplifier further comprises a first input terminal, a second input terminal, a first output terminal, and a second output terminal, and wherein the first switch further comprises a pair of first switches, and wherein the second switch further comprises a pair of second switches, and wherein the first capacitor further comprises a pair of first capacitors, and wherein the second capacitor further comprises a pair of second capacitors. 
         [0017]    In accordance with a preferred embodiment of the present invention, the first preamplifier further comprises: a first amplifier having a first input terminal, a second input terminal, a first output terminal, and a second output terminal, wherein the first and second input terminals of the first amplifier receive an input signal; and a second amplifier having a first input terminal, a second input terminal, a first output terminal, and a second output terminal, wherein the first output terminal of the second amplifier is coupled to the first output terminal of the first amplifier, and wherein the second output terminal of the second amplifier is coupled to the second output terminal of the first amplifier, and wherein each of the first and second input terminals of the second amplifier is coupled to at least one of the pair of first switches. 
         [0018]    In accordance with a preferred embodiment of the present invention, the second preamplifier further comprises: a third amplifier having a first input terminal, a second input terminal, a first output terminal, and a second output terminal, wherein the first input terminal of the third amplifier is coupled to the first output terminal of the first amplifier, and the second input terminal of the third amplifier is coupled to the second output terminal of the first amplifier; and a fourth amplifier having a first input terminal, a second input terminal, a first output terminal, and a second output terminal, wherein first output terminal of the fourth amplifier is coupled to the first output terminal of the third amplifier, and wherein the second output terminal of the fourth amplifier is coupled to the second output terminal of third amplifier, and wherein each of the first and second input terminals of the fourth amplifier is coupled to at least one of the pair of second switches. 
         [0019]    In accordance with a preferred embodiment of the present invention, the DAC further comprises: a set of resolution capacitors, wherein each resolution capacitor from the first set of resolution capacitors is coupled to first preamplifier; a set of resolution switches, wherein each resolution switch is coupled to the ADC, receives a reference voltage, and is coupled to at least one of the resolution capacitors; a set of feedback capacitors, wherein each feedback capacitor is coupled to the first preamplifier; a set of feedback switches, wherein each feedback switch is coupled to the residue amplifier, receives a common mode voltage, and is coupled to at least one of the feedback capacitors. 
         [0020]    In accordance with a preferred embodiment of the present invention, the SAR ADC further comprises a sample flash circuit that is coupled between the ADC and SAR logic. 
         [0021]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0023]      FIG. 1  is a block diagram of an example of ADC in accordance with a preferred embodiment of the present invention; 
           [0024]      FIG. 2  is a circuit diagram of an example of the reside amplifier of  FIG. 1 ; and 
           [0025]      FIG. 3  is an example of a timing diagram for the circuit of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Refer now to the drawings wherein depicted elements are, for the sake of clarity, not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
         [0027]    Referring to  FIG. 1  of the drawings, the reference numeral  100  generally designates an ADC in accordance with a preferred embodiment of the present invention. The ADC  100  generally comprises ADC  102 , digital-to-analog converter (DAC)  104 , residue amplifier  106 , conversion flash  108 , SAR oscillator  110 , SAR logic  112 , sampling flash  114 , and reference system  116 . DAC  104  generally comprises capacitor arrays  120 - 1 ,  120 - 2 ,  120 - 3 ,  122 - 1 ,  122 - 2 , and  122 - 3  (which each include several resolution capacitors), switch arrays  118 - 1 ,  118 - 2 ,  118 - 3 ,  124 - 1 ,  124 - 2 , and  124 - 3  (which each include several resolution switches), feedback capacitors CFB 1  through CFB 6 , and feedback switches SFB 1  through SFB 6 . 
         [0028]    In operation (which can be seen in  FIG. 3 ), the ADC  100  is generally able to perform multi-stage resolution using a recyclic or recycling architecture. ADC  102  receives and analog input signal AIN and generates differential signals INP and INM. During a sampling phase, the signals INP and INM are sampled into their respective capacitor arrays  120 - 1 ,  120 - 2 ,  120 - 3 ,  122 - 1 ,  122 - 2 , and  122 - 3  through their respective switch arrays  118 - 1 ,  118 - 2 ,  118 - 3 ,  124 - 1 ,  124 - 2 , and  124 - 3  and into sampling flash  114 . For example, this sampling phase can be, for example, from about 50 ns to about 100 ns long. 
         [0029]    Once complete the sampling phase is complete, the ADC enters the conversion phase. Following the sampling phase, the first 4+1 (for example) bits are resolved by the sampling flash  114 , and the remainder of the conversion phase generally occurs in three segments. During the first segment (which can be, for example, from about 20 ns to about 40 ns in length), the residue amplifier  106  applies feedback to the feedback capacitors CFB 1  and CFB 4  through feedback switches FSB 1  and SFB 4 , and the SAR logic  112  applies the appropriate control signals to switch arrays  120 - 1  and  124 - 1  (to apply the reference voltages REFP and REFM) and feedback switches FSB 1  and SFB 4  (to apply the common mode voltage VCM) in accordance with a conventional SAR algorithm. This first segment resolves the next 4+1 (for example) bits. During the second segment (which can be, for example, from about 20 ns to about 40 ns in length), the residue amplifier  106  applies feedback to the feedback capacitors CFB 2  and CFB 5  through feedback switches FSB 2  and SFB 5 , and the SAR logic  112  applies the appropriate control signals to switch arrays  120 - 2  and  124 - 2  (to apply the reference voltages REFP and REFM) and feedback switches FSB 2  and SFB 5  (to apply the common mode voltage VCM) in accordance with a conventional SAR algorithm. This second segment resolves the next 4+1 (for example) bits. During the third segment (which can be, for example, from about 30 ns to about 60 ns in length), the residue amplifier  106  applies feedback to the feedback capacitors CFB 3  and CFB 6  through feedback switches FSB 3  and SFB 3 , and the SAR logic  112  applies the appropriate control signals to switch arrays  120 - 3  and  124 - 3  (to apply the reference voltages REFP and REFM) and feedback switches FSB 3  and SFB 6  (to apply the common mode voltage VCM) in accordance with a conventional SAR algorithm. This third segment resolves the next 4+1 (for example) bits. For each of the segments, too, the last bit resolved for the previous segment is retaken; thus, a total of 16 bits (for example) can be resolved. Additionally, ADC  100  can be arranged to have more or less bits resolved for each segment, and the total number of stages can be increased or decreased. 
         [0030]    In a typical SAR ADC, a single bit is resolved through a comparator for every conversion clock cycle, but in ADC  100 , the residue amplifier  106  resolved multiple bits for each conversion clock cycle. Designing the residue amplifier  106  to perform this function, though, includes taking in to consideration the closed loop gain changes, tolerable gain error, settling requirements, changing capacitive loads, and so forth. 
         [0031]    Considering that it is very difficult to design an amplifier that performs well with very different requirements, residue amplifier  106 , which is shown in more detail in  FIG. 2 , employs cascaded amplifier to perform as desired. Residue amplifier  106  generally comprises preamplifiers  202 ,  208 , and  214 , output amplifiers  220 ,  222 , and  224 , capacitors C 1  through C 6 , and switches S 1  through S 7 . These components operate to provide three feedback paths (for each segment). 
         [0032]    Initially, however, the residue amplifier  106  can be auto-zeroed to generally eliminate offset components by adjusting the voltages stored at capacitors C 1  through C 6 . During auto-zeroing, switches S 1  through S 7  are closed. Under these conditions, the voltages stored across capacitors C 1  and C 2  (VST 1 ), capacitors C 3  and C 4  (VST 2 ), and capacitors C 4  and C 5  (VST 3 ) would be as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0000]    where A 1  through A 9  are the gains of amplifiers  204 ,  206 ,  220 ,  212 ,  210 ,  222 ,  218 ,  216 , and  224  (respectively),  204 ,  206 ,  220 ,  212 ,  210 ,  222 ,  218 ,  216 , and  224  are the offset voltages of amplifiers  204 ,  206 ,  220 ,  212 ,  210 ,  222 ,  218 ,  216 , and  224  (respectively), V 1  is the output voltage of amplifier  304 , and V 2  is the output voltage of amplifier  212 . Thus, when switches S 1  through S 7  are open and an input voltage VIN is applied to amplifier  204 , the output voltages VOUT 1 , VOUT 2 , and VOUT 3  will be as follows: 
         [0000]        VOUT 1 =VINA 1 A 3 +VST 1 A 2 A 3 +VOS 1 A 1 A 3 +VOS 2 A 2 A 3 +VOS 3 A 3;  (4)
 
         [0000]        VOUT 2 =VINA 1 A 4 A 6 +VOS 1 A 1 A 4 A 6 +VOS 2 A 2 A 4 A 6 +VST 1 A 2 A 4 A 6 +V 1 A 4 A 6 +Vos 4 A 4 A 6 +VOS 5 A 5 A 6 +VST 2 A 5 A 6 +VOS 6 A 6;  (5)
 
         [0000]      and 
         [0000]        VOUT 3 =VINA 1 A 4 A 7 A 9 +VOS 1 A 1 A 4 A 7 A 9 +VOS 2 A 2 A 4 A 7 A 9 +VST 1 A 2 A 4 A 7 A 9 +VOS 4 A 4 A 7 A 9 +VOS 5 A 5 A 7 A 9 +VST 2 A 4 A 7 A 9 +VOS 7 A 7 A 9 +VOS 8 A 8 A 9 VST 3 A 8 A 9 +VOS 9 A 9.  (6)
 
         [0000]    Each of equations (4), (5), and (6) can then be reduced as follows because the offset voltages VOS 1  through VOS 9  for amplifiers  204 ,  206 ,  220 ,  212 ,  210 ,  222 ,  218 ,  216 , and  224  are compensated for by the auto-zeroing: 
         [0000]      VOUT1=VINA1A3;  (4)
 
         [0000]      VOUT2=VINA1A4A6;  (5)
 
         [0000]      and 
         [0000]      VOUT3=VINA1A4A7A9.  (6)
 
         [0033]    Thus, these output voltages VOUT 1 , VOUT 2 , and VOUT 3  operate as feedback voltage for the DAC  104 . Preamplifier  204 , capacitors C 1  and C 2 , and amplifier  220  preferably operate the feedback path for the first segment. Preamplifiers  204  and  208 , capacitors C 1  through C 4 , and amplifier  222  preferably operate as the feedback path for the second segment, and preamplifiers,  202 ,  208 , and  214 , capacitors C 1  through C 6 , and amplifier  224  preferably operate as the feedback path for the third segment. 
         [0034]    Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.