Abstract:
The present invention is related to a pipelined analog-to-digital converter (ADC) utilizing a power distribution scheme selectively delivering both constant and variable reference currents in selected proportions to a plurality of stages and operational transconductive amplifiers (OTAs). This permits the ADC to maintain an optimized speed over power consumption ratio over a wide data rate range. Since the invention is capable of supporting a large operating range while maintaining very low power consumption relative to the data rate, the pipelined ADC in accordance with the present invention is particularly adaptable to a large number of applications.

Description:
[0001]    This application is a Continuation of U.S. application Ser. No. 11/538,229, filed on Oct. 3, 2006, which is incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention is related to analog-to-digital converters (ADCs). More particularly, the present invention is directed to an ADC having a pipelined architecture with an improved power distribution scheme for data rate programmability. 
       BACKGROUND 
       [0003]    An ADC is an electronic circuit that converts a continuous analog signal, such as a voltage signal, to a discrete digital number. Due to the popularity of consumer electronic devices, there are a large number of applications that employ ADCs with medium resolution, such as 10 bits, and medium to high data rates ranging from a few MHz to several tens of MHz. Among these applications are wireless communication systems, mobile phones, video components, imaging components, local area network transceivers, and the like. 
         [0004]    Pipelined ADCs have multiple stages which successively process an analog input signal. The principal in pipelined ADCs is to find a set of reference voltages whose sum equals the signal sample being converted. This is realized by subtracting different reference voltages from the input sample until the residue value becomes zero, indicating that the sum of the subtracted references equals the original signal sample. In the pipeline, the analog residue or remainder value may be amplified by an amplifier between the subtraction steps in order to increase accuracy prior to being fed to the next stage in the pipeline. 
         [0005]      FIG. 1  is a high-level block diagram of a conventional 10-bit pipelined ADC. There are two basic types of components within the ADC: 1) pure analog components which include a reference voltage generator, a bias current generator, a track and hold (T&amp;H) stage, front end pipeline stages ( 1  through  4 ) and back end pipeline stages ( 5  through  8 ); and 2) digital and mixed analog/digital components including a phases generator, a 2 bit ADC flash unit, delay lines, and a Redundant Signed Digital (RSD) error correction unit. 
         [0006]    In the pipelined architecture shown in  FIG. 1 , the ADC has nine stages which process the analog input signal. However, the number of stages in a pipelined architecture can be any number depending on the desired resolution of the ADC. The greater the number of stages in an ADC the higher the resolution. From the left side of  FIG. 1 , the analog input signal first enters the (T&amp;H) stage. Subsequent pipeline stages  1  through  8  process the T&amp;H output and drive a 2 bit ADC on the right. In each of the nine stages, an operational transconductive amplifier (OTA) is the active consuming analog cell. As an example, the ADC shown in  FIG. 1  uses an external s-bits bus (speed&lt;s−1:0&gt;), where s may be any number, in order to program the ADC with a specific amount of active current that is proportional to the targeted data rate of the ADC. 
         [0007]    Referring to  FIG. 2 , a block diagram of a single pipeline stage is shown. The single pipeline stage includes a sample and hold (S/H) unit for providing a constant analog signal, an ADC, a digital-to-analog converter (DAC), a summer and an OTA. The input V j  is the sample number  j  coming out from the previous pipeline stage as a new input to the present stage. It is sampled and held, and also quantized with a low resolution 2 bit ADC. The resulting digital word D j  &lt;1:0&gt; is converted back to analog using a 2 bit DAC and subtracted from the original held value to create a residue. The residue is then amplified by G j  to generate an output voltage V j+1  to the next pipeline stage given by Equation (1) as follows: 
         [0000]        V   j+1   =Gj ·( V   j   −Vj   DA ( D   j )).  Equation (1) 
         [0008]    When considering the switched capacitor implementation, the S/H function as well as the DAC, the voltage subtraction and the residue amplification may be performed by a single operation known by those skilled in the art as a multiplying DAC or MDAC. The MDAC performs the operation in a period that is set by half a clock period, due to the switched capacitor circuit implementation. During a first half clock period the sample value is stored in a first capacitor. During a second half clock period the sample value is amplified by an OTA and multiplied by a gain value. 
         [0009]    Since the 2 bit ADC-DAC implementation has a very low resolution, it is designed to provide a very high speed response. A limiting element of the pipeline stage, in terms of speed, is the OTA, which performs and provides the V j+1  output. Setting the gain G j  with a high enough resolution not to degrade the final ADC converter resolution, results in high power consumption. Additionally, the overall speed of the OTA is directly dependent upon the power consumption. The more power that is provided to the OTA, the faster the speed of the OTA and the conversion data rate of the ADC device. However, the increased speed of the OTA results in higher power consumption of the ADC. This is a significant drawback, particularly for wireless devices and other consumer electronic devices which rely upon a battery as a power source. 
         [0010]    Accordingly, what is needed is a versatile pipelined ADC which can operate at a desired resolution over a wide operating range without the drawback of high power consumption of current pipelined ADCs. Moreover, an ADC that is independent of circuit process variations for providing a reliable resolution is desirable. 
       SUMMARY 
       [0011]    The present invention is related to a pipelined ADC utilizing a power distribution scheme selectively delivering both constant and variable reference currents in selected proportions to a plurality of stages and OTAs. This permits the ADC to maintain an optimized speed over power consumption ratio over a wide data rate range. Since the invention is capable of supporting a large operating range while maintaining very low power consumption relative to the data rate, the pipelined ADC in accordance with the present invention is particularly adaptable to a large number of applications. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    A more detailed understanding of the invention may be had from the following description, given by way of example and to be understood in conjunction with the accompanying drawings wherein: 
           [0013]      FIG. 1  is a block diagram of a conventional pipelined ADC; 
           [0014]      FIG. 2  is a block diagram of a conventional single pipeline stage; 
           [0015]      FIG. 3  is a block diagram of a pipelined ADC in accordance with the present invention; 
           [0016]      FIG. 4  is a block diagram of the current programming scheme per pipeline stage in accordance with the present invention; 
           [0017]      FIG. 5  is a schematic diagram of a fully differential folded cascode OTA in accordance with the present invention; and 
           [0018]      FIG. 6  is a process for providing analog to digital conversion having a selectable data rate in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    The present invention would be described with reference to the drawings wherein like numerals represent similar elements throughout. For purposes of describing the present invention, the phrase low, medium, or high voltage levels may be used. It will be appreciated that the words “low”, “medium”, and “high” are relative terms and not necessarily a fixed voltage. Accordingly, the phrase low, medium, or high voltage level may be any voltage and may vary, for example, based on the processing technology and/or the material in which an electronic device is implemented. The word “level” may represent a fixed voltage or a voltage range, as desired. Moreover, predetermined voltage levels in the description forthcoming can be any voltage level and may be dependent on the design, structure, and materials used to configure a circuit element. A node, a voltage at a node, or a current at a node may be used interchangeably and a load capacitance may be a parasitic capacitance in the description forthcoming. It should be understood by those of skill in the art that the equations and variables set forth below are exemplary and should not be understood to limit the invention. 
         [0020]    A block diagram of a pipelined ADC in accordance with the present invention is shown in  FIG. 3 . The ADC  200  includes a bias current unit  201 , a track and hold (T&amp;H) unit  204 , a plurality of pipeline stages including pipeline stages  1  through  4   206   0−3 , pipeline stages  5  through  8   206   4−7 , and a flash ADC unit  216 . Although stages  1  through  4  and  5  through  8  are shown as separated into two distinct groups this is only for simplicity of explanation. Each stage may be provided with an independent reference current level by bias current unit  201 , as will be explained in detail hereinafter, and an independent operating point. The ADC circuit  200  may also have any number of pipeline stages depending upon the desired resolution of the ADC circuit  200 . It should be understood by those of skill in the art that pipelined ADCs may comprise many other components to support the various resolutions of the ADC circuit  200 . However, for simplicity, only those portions of the pipelined ADC in accordance with the present invention are described in detail hereinafter. 
         [0021]    The bias current unit  201  includes a reference current generator  202  which may be a proportional to the absolute temperature (PTAT) reference current generator, a constant mode current generator  205  and an active speed current generator  210 . The reference current generator  202  generates and provides reference current, which is proportional to temperature in the case of a PTAT reference current, to the constant mode current generator  205  and the active speed current generator  210 . A PTAT reference current generator  202  may be desirable in order to make the present invention insensitive to temperature effects. Considering a programmed ADC speed, the use of a PTAT reference current generator would maintain the desired OTA bandwidth constant over any temperature variations providing compliance for any desired operational temperature range for the present invention. 
         [0022]    The constant mode current generator  205  generates constant mode bias current ic to the individual pipeline stages  206   0−7 . The active speed current generator  210  selectively generates variable or programmable bias current ia to the individual pipeline stages  206   0−7 . In general, the bias current unit  201  generates and distributes current to the T&amp;H unit  204  and the individual pipeline stages  206   0−7  via 2 separate channels; a first channel  212  for conducting constant mode current ic and a second channel  214  for conducting variable current ia. 
         [0023]    The first channel  212  comprises a bus of nine lines; one line for each active component; (i.e. the T&amp;H unit  204  and the pipeline stages  206   0−7 ). Accordingly, constant mode current ic is distributed via one of the lines to the T&amp;H unit  204  and the pipeline stages  206   0−7 . The constant mode current ic, generated by the constant mode current generator  205 , is a fixed current which depends on the mode of operation of the ADC. 
         [0024]    The second channel  214  comprises a bus of nine lines; one line for each active component. Accordingly, active speed current ia is distributed via one of the lines to the T&amp;H unit  204  and the pipeline stages  206   0−7 . The active speed current ia may be used to provide increased flexibility and greater bandwidth to the T&amp;H unit  204 , as desired. The mode&lt;1:0&gt; input to bias current unit  201  may be used to selectively control the operation of the currents ic and ia. 
         [0025]    The active speed current ia is proportional to the desired speed &lt;s−1:0&gt;, where s may be any number, which may offer 2 S  codes of programmability. Although up to two bits of programmability is shown in  FIG. 3 , s may be any value used to provide higher desired levels of analog to digital resolution, as desired. 
         [0026]    Still referring to  FIG. 3 , the T&amp;H unit  204  samples an analog input signal, maintains that signal level for a predetermined duration, and passes the signal to the pipelined stages  206   0−7  for sequential processing of the signal. The final pipeline stage transfers the last analog signal to the flash ADC unit  216  for quantization of the pipeline remaining residue. In accordance with the present invention, the bias current unit  201  selectively delivers constant mode and active speed current ic, ia, respectively, to each of the active components  204  and  206   0−7 . Since each stage may be selectively provided with a constant mode and an adaptive active speed currents ic, ia respectively, the pipelined ADC  200  can be programmed to provide any desired data rate while minimizing power requirements. Power is conserved since the use of an unnecessarily high active current may be prevented by independently adjusting the current level of each pipelined stage. 
         [0027]    Referring to  FIG. 4 , a schematic diagram of the bias current unit  201  which generates and selectively outputs current for each pipelined stage  206   0−7  in accordance with the present invention is provided, The bias current unit  201  includes the reference current generator  202 , which may be a PTAT reference current to compensate for any temperature effects, and a plurality of bias stages  250   0−8 ; one for each pipelined stage  206   0−7  and the T&amp;H unit  204 . 
         [0028]    The constant mode current generators  205   0−8  and the active speed current generators  210   0−8  may comprise separate and distinct components as shown in  FIG. 4 . In  FIG. 4 , each current generator may comprise a portion of a bias stage  250 . The reference current generator  202  distributes a reference current ib i  [i=0 . . . 8] to each of the plurality bias stages  250   0−8  that provide current to each pipelined stage  206   0−7  and the T&amp;H unit  204 . Although nine bias stages are shown in  FIG. 4 , any number of bias stages may be used depending upon the desired number of pipelined stages. Each bias stage  250   0−8  receives ib i , which is used as a current reference iref as shown by  203   0−8 , and transmits both constant mode current ic i  and active speed current ia i  to its corresponding pipeline stage  206   0−7  and T&amp;H unit  204 . 
         [0029]    The transfer function of the local bias stage number i is given as follows: 
         [0000]        ic   i   =c·iref;   Equation (2) 
         [0000]        ia   i =( a   i +1)· iref   Equation (3) 
         [0000]    where iref is the reference current provided by unit  202 , c is a constant integer dependent upon the desired OTA operating point, and a i  is a programmable or variable number, such as an integer, dependent upon the desired variable program speed &lt;s−1:0&gt;. Since a i  is a variable, the ADC can be used to provide an active current that is proportional to any desired data or conversion rate. 
         [0030]    Table 1 provides purely as an example a case where s=2. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Programmed Speed 
                   
                   
                 Current ia 1   
               
             
          
           
               
                   
                 speed&lt;1&gt; 
                 speed&lt;0&gt; 
                 a 1   
                 (unit in iref) 
               
               
                   
                   
               
               
                   
                 0 
                 0 
                 0 
                 1 
               
               
                   
                 0 
                 1 
                 1 
                 2 
               
               
                   
                 1 
                 0 
                 2 
                 3 
               
               
                   
                 1 
                 1 
                 3 
                 4 
               
               
                   
                   
               
             
          
         
       
     
         [0031]    Since a i  is a variable, each OTA in the pipeline ADC  200  may be programmed using any amount of current ia i  as given in Equation (3) and shown in Table 1. Since the OTA in accordance with the present invention may be programmable, the OTA can provide a bandwidth large enough to be compatible with any desired ADC data rate. 
         [0032]    Each pipelined stage  206   0−7  in accordance with the present invention may be configured similar to the general configuration shown in  FIG. 2 . However, the present invention includes an OTA  300  which is adapted to take advantage of the two-part current distribution scheme shown in  FIGS. 3 and 4  for adjusting the power consumption to any desired data rate. With the power consumption adapted to the desired ADC data rate, this data rate is considered as the maximum data rate (MDR) of the ADC. Referring to  FIG. 5 , a transistor level schematic of an OTA  300  in accordance with the present invention is shown. Although, there are a plurality of bias stages  250   0−8  in a pipelined ADC having a corresponding plurality of OTAs  300 , only one OTA  300  is shown for simplicity. 
         [0033]    The OTA  300  is a fully differential folded cascode topology using an n-type metal-oxide semiconductor (NMOS) transistor input pair. As those that are skilled in the art would realize, the p-type metal-oxide semiconductor (PMOS) type transistor input pair version could also be considered. The output of the OTA  300  may be loaded by single-ended load capacitors C 1 . 
         [0034]    The Common Mode FeedBack box (CMFB) provides regulation of the common mode output voltage ensuring that either vop=von=vcm when vip=vin at the input of the OTA  300  or (vop+von)/2=vcm for all other cases. The two separate input currents, the active speed current ia at node  302  and the constant mode current ic at node  304  are received from a respective bias stage  250   0−8 . The constant mode current ic is derived from Equation (2) and is used to set the internal cascode voltage nodes vcas  304 ,  306 , and  308  in order to maximize the dynamic output range of the OTA  300 . The active speed current ia is derived from Equation (3) and is used to provide a desirable unity gain bandwidth for OTA  300 . 
         [0035]    A metric used to measure the performance of circuits  200  and  300  is the ratio of the maximum data rate (MDR) to the power dissipation, sometimes referred to as the figure of merit (FOM) of an ADC device, given by Equation (4) as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     F 
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                     O 
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                         D 
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                         R 
                       
                       POWER 
                     
                     . 
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     4 
                     ) 
                   
                 
               
             
           
         
       
     
         [0036]    The MDR of ADC  200  is given by Equation (5) as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     M 
                      
                     
                         
                     
                      
                     D 
                      
                     
                         
                     
                      
                     R 
                   
                   ≈ 
                   
                     A 
                     · 
                     
                       
                         
                           k 
                           * 
                           
                             
                               ( 
                               
                                 
                                   a 
                                   i 
                                 
                                 + 
                                 1 
                               
                               ) 
                             
                             · 
                             iref 
                           
                         
                         Cl 
                       
                       . 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     5 
                     ) 
                   
                 
               
             
           
         
       
     
         [0037]    In Equation (5), A is a fixed parameter dependent upon a desired processing technology and the desired resolution (i.e. number of bits) of an ADC, and k is the current amplification factor of OTA  300 . Therefore, MDR is directly proportional to the current amplification factor k, the reference current iref, and the local active current variable a i  for each stage as set forth in Equation (3) and shown in  FIG. 4 . MDR is inversely proportional to the equivalent capacitance, including any parasitic capacitances, Cl seen at the output of circuit  300 . 
         [0038]    In Equation (4), the dissipated POWER metric is calculated by summing the current consumed by each pipeline stage  206   0−7  and T&amp;H unit  204 . Since each pipeline stage  206   0−7  and T&amp;H unit  204  may include an OTA similar to  300 , the total dissipated POWER is given by Equation (6) as follows: 
         [0000]      POWER=1000 *P*idd*V   DD  (in mW).  Equation (6) 
         [0039]    In Equation (6), P is the total number of OTA circuits  300  used in ADC  200 , V DD  is the supply voltage level, and current idd is the supply current level at node  310  for each stage given by Equation (7) as follows: 
         [0000]        idd=ic   i +(4 k+ 2)* ia   i .  Equation (7) 
         [0040]    In Equation (7), k is the current amplification factor of OTA  300 , ic i  is the constant mode current at node  304 , and ia i  is the active speed current at node  302 . 
         [0041]    Substituting Equations (2) and (3) into Equation (7) gives Equation (8) as follows: 
         [0000]        idd =( c+ 2(2 k+l )( a   i   +l ))*iref.  Equation (8) 
         [0042]    In Equation (8), the variables c, k, a i , and iref are the same as those provided in Equations (2), (3) and (7) above. Since the supply current idd of each OTA stage  300  is dependent upon the programmable variable a i , both the OTA unity gain bandwidth and power consumption may be optimized for OTA  300  and ADC  200 , respectively. 
         [0043]    Substituting Equations (5), (6), and (8) into Equation (4) and assuming 2(2k+1)&gt;&gt;c, the FOM may be given by Equation (9) as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     F 
                      
                     
                         
                     
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                     O 
                      
                     
                         
                     
                      
                     M 
                   
                   ≅ 
                   
                     
                       A 
                       · 
                       k 
                     
                     
                       
                         2000 
                         · 
                         P 
                         · 
                         Vdd 
                         · 
                         
                           ( 
                           
                             
                               2 
                                
                               k 
                             
                             + 
                             1 
                           
                           ) 
                         
                         · 
                         C 
                       
                        
                       
                           
                       
                        
                       1 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     9 
                     ) 
                   
                 
               
             
           
         
       
     
         [0044]    In Equation (9), c, a i  and iref have been removed from the expression for the FOM. The ADC  200  has been made independent of the programmed data rate and corresponding current consumption which results in ADC  200  performing at a desirably optimized constant speed over power ratio. 
         [0045]      FIG. 6  is a process  400  for providing steps for analog to digital conversion having a selectable data rate and optimized power dissipation in accordance with the present invention. An ADC is RESET and the MODE and SPEED are preset to a default value providing a default bias current setting which includes a set of constant bias current and a set of (preset) variable or active bias current (step  402 ). A variable analog signal level is sampled (step  404 ) to provide a constant analog signal. The constant analog signal level is provided to an analog to digital converter for producing a first digital output number which is then converted back to a converted analog signal via a digital to analog converter (step  406 ). A first analog remainder is provided by subtracting the second analog signal from the sampled analog signal level (step  408 ). The first analog remainder signal is used to produce a second digital output number and a second analog remainder (step  410 ). The first and second digital output numbers are summed to produce a digital output signal (step  412 ). Changing the SPEED programming to generate a new set of variable bias current, for producing the digital output signal to the desired data rate while dissipating a minimum amount of power (step  414 ) may then be performed. 
         [0046]    Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention. The present invention may be implemented in a computer program or firmware tangibly embodied in a computer-readable storage medium having machine readable instructions for execution by a machine, a processor, and/or any general purpose computer for use with or by any non-volatile memory device. Suitable processors include, by way of example, both general and special purpose processors. 
         [0047]    Typically, a processor will receive instructions and data from a read only memory (ROM), a RAM, and/or a storage device having stored software or firmware. Storage devices suitable for embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, read only memories (ROMs), magnetic media such as internal hard disks and removable disks, magneto-optical media, and optical media such as CD-ROM disks and digital versatile disks (DVDs). Types of hardware components, processors, or machines which may be used by or in conjunction with the present invention include Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs), microprocessors, or any integrated circuit.