Abstract:
Multi-step sub-ranging analog-to-digital converters (ADCs) utilize a plurality of converter modules to generate sub-ranges of bits for a digital output signal during a multi-phase conversion cycle. Each subsequent converter utilizes reference voltage levels corresponding to conversions performed by prior converters during the timing phases of the conversion cycle. Settling time for these reference voltages, which limits the speed and/or accuracy of each conversion, may be reduced by pre-charging the reference input nodes of the subsequent converters using the analog input signal during a timing phase in which a prior converter is converting the analog input signal to generate one of the sub-ranges for the digital output signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
   This application claims the benefit of the filing date of U.S. provisional application No. 60/703,989, filed on Jul. 29, 2005, the teachings of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to electronics, and, in particular, to multi-step sub-ranging analog-to-digital converters. 
   2. Description of Related Art 
     FIG. 1  illustrates a block diagram of a conventional two-step sub-ranging analog-to-digital converter (ADC)  100 . ADC  100  comprises a front-end Sample-and-Hold (S/H) module  101 , a coarse converter  103 , a fine converter  104 , a reference ladder  105 , a reference switch network  102 , and an encoder  106 . An analog input signal  111  is received by S/H module  101 , which provides a stable input signal  112  for application to coarse converter module  103  and fine converter module  104 . 
   Input signal  112  is converted by coarse converter  103  during a first phase of a conversion cycle based on a subset of reference voltages  113  provided by reference ladder  105 . Coarse converter  103  generates one or more of the most significant bit (MSB) values ( 115 ) for the digital representation  117  of analog input signal  111 . During a second phase of the conversion cycle, fine converter  104  converts input signal  112  to generate one or more of the least significant bit (LSB) values ( 116 ) for the digital representation of analog input signal  111 . During this second phase, reference switch network  102  provides a different subset of reference voltages  114  to fine converter  104 , where reference voltages  114  are selected based upon a control signal  120  generated by coarse converter  103  corresponding to the MSB values generated during the first phase of the conversion cycle. Encoder  117  combines the MSB values generated by coarse converter  103  and the LSB values generated by fine converter  104  to generate digital output signal  117 . 
   Coarse converter  103  and fine converter  104  may be constructed using any suitable ADC circuits that provide the desired resolutions and accuracies. For example, in one possible embodiment, each converter is constructed using a set of analog comparators. Each of these comparators compares input signal  112  with a different reference voltage. Assume, for example, that ADC  100  generates an 8-bit digital output signal  117 , where coarse converter  103  generates the four MSBs and fine converter  104  generates the four LSBs of output signal  117 . In that case, reference ladder  105  generates 2 8 −1 or 255 different (e.g., equally spaced) reference voltages that span the dynamic range of ADC  100 . Assume, for ease of explanation, that the dynamic range of ADC  100  is from 0V to 256 mV, and that reference ladder  105  generates 255 reference voltages from 1 mV to 255 mV at 1-mV increments. 
   Continuing with this 8-bit ADC example, 4-bit coarse converter  103  and 4-bit fine converter  104  are both implemented with 15 comparators. During the first phase of the conversion cycle, coarse converter  103  receives 15 “coarse” reference voltages (e.g., corresponding to 16 mV, 32 mV, 48 mV, . . . , 240 mV), where each of the 15 comparators in coarse converter  103  compares input signal  112  to a different one of these 15 coarse reference voltages. The largest of these 15 coarse reference voltages that is smaller than input signal  112  (as determined by the comparator outputs) identifies the 4 MSBs of digital output  117 . Assume, for example, that this “largest smaller” coarse reference voltage is 144 mV. 
   Coarse converter  103  generates control signal  120  based on this largest coarse reference voltage. Based on control signal  120 , reference switch network  102  selects  15  “fine” reference voltages from the  255  different reference voltages  113  generated by reference ladder  105  to provide to fine converter  104 . Continuing with the example in which the “largest smaller” coarse reference voltage is 144 mV, reference switch network  102  would select the following 15 fine reference voltages for use by fine converter  104 : 145 mV, 146 mV, 147 mv, . . . , 159 mV. During the second phase of the conversion cycle, fine converter  104  receives the 15 selected fine reference voltages, where each of the 15 fine-converter comparators compares input signal  112  to a different one of these 15 fine reference voltages. The largest of these 15 fine reference voltages that is smaller than input signal  112  identifies the 4 LSBs of digital output  117 . 
     FIG. 2  illustrates a signal flow diagram for one of the fine reference voltages selected by reference switch network  102  of  FIG. 1  for use by fine converter  104  during the second phase of the conversion cycle. One of the design challenges of two-step sub-ranging ADC  100  of  FIG. 1  is the settling accuracy of reference switch network  102  when changing fine reference voltages  114  for fine converter  104  for different input signals. This settling error has to be lower than a certain level for a specific resolution requirement of ADC  100 . One major contributor to the settling error is a “memory effect” caused by electric charge stored at each reference input node  203  of fine converter  104 . The memory effect occurs because of parasitic capacitance  205  between each output node of reference switch network  102  and the corresponding reference input node to fine converter  104 . The reference settling process, and therefore the final settling accuracy, depends on the fine reference voltage levels of the previous conversion cycle stored on parasitic capacitances ( 205 ) at the interface between reference switch network  102  and the reference input nodes of fine converter  104 . 
   For example, the settling error would be higher if the differences between the current fine reference voltages and the previous fine reference voltages become larger. In particular, because of the memory effect from parasitic capacitance  205 , more time would be needed for the voltages at the reference input nodes of fine converter  104  to settle to the current fine reference voltages from the previous reference voltages. This memory effect causes an input-dependent settling error that lowers the observed Signal-to-Noise Ratio (SNR), and therefore the Effective Number of Bits (ENOB), of ADC  100 . This problem becomes more severe in high-speed applications, where the input slew rate for analog input signal  111  of  FIG. 1 , and therefore the slew rate of reference switching network  102 , is high and the time for reference switching is short. Other contributors to the reference settling error may include signal-dependent non-linearity and charge injection of the switch devices used in reference switch network  102 . 
   Attempts to reduce these errors caused by the memory effect of the previous fine reference voltages stored at the reference input nodes of fine converter  104  include increasing driving current within reference ladder  105 . This increase in the drive current attempts to lower the RC time constant of reference ladder  105 , which drives reference switch network  102 , and by lowering the resistance of switches within reference switch network  102 . This approach increases the power consumption of ADC  100  and requires larger switch devices within reference switch network  102 , which results in more charge injection. 
   Another previous attempt to reduce the reference settling error was to use two interleaved fine converters  104 , each working at half the conversion rate of ADC  100 . Although this architecture relaxes the time for the fine reference voltages to settle for each fine converter and therefore lowers the settling error, the addition of the second interleaved fine converter significantly increases the complexity of ADC  100 , demands more die area for the entire circuit, and introduces errors caused by the interleaving operations, e.g., the “ping-pong” noise between the two fine converters. 
   SUMMARY OF THE INVENTION 
   Problems in the prior art are addressed in accordance with the principles of the present invention by utilizing a reference voltage pre-charge process using the addition of a switched signal path from an output of a sample-and-hold module to a switched reference voltage used by a fine converter module. 
   In one embodiment, the present invention is an analog-to-digital converter (ADC) for converting an analog input signal into a digital output signal. The ADC comprises a coarse converter adapted to convert the analog input signal into one or more coarse bits for the digital output signal during an initial phase of a conversion cycle for the ADC, at least one fine converter adapted to convert the analog input signal into one or more fine bits for the digital output signal during a subsequent phase of the conversion cycle, a reference-voltage supply adapted to generate and apply one or more fine reference voltages to one or more reference input nodes of the fine converter for use by the fine converter during the subsequent phase of the conversion cycle, and an encoder module adapted to combine the coarse and fine bits to generate the digital output signal. During the initial phase of the conversion cycle, the ADC is adapted to apply a pre-charge signal based on the analog input signal to at least one reference input node of the fine converter to pre-charge the at least one reference input node. 
   In another embodiment, the present invention is a method for converting an analog input signal into a digital output signal. The method converts, by a coarse converter, the analog input signal into one or more coarse bits for the digital output signal during an initial phase of an ADC conversion cycle, generates and applies one or more fine reference voltages to one or more reference input nodes of at least one fine converter for use by the fine converter during a subsequent phase of the conversion cycle, converts, by the fine converter, the analog input signal into one or more fine bits for the digital output signal during the subsequent phase of the conversion cycle, and combines the coarse and fine bits to generate the digital output signal. During the initial phase of the conversion cycle, a pre-charge signal based on the analog input signal is applied to at least one reference input node of the fine converter to pre-charge the at least one reference input node. 
   In yet another embodiment, the present invention is an integrated circuit comprising an ADC for converting an analog input signal into a digital output signal. The ADC comprises a coarse converter adapted to convert the analog input signal into one or more coarse bits for the digital output signal during an initial phase of a conversion cycle for the ADC, at least one fine converter adapted to convert the analog input signal into one or more fine bits for the digital output signal during a subsequent phase of the conversion cycle, a reference-voltage supply adapted to generate and apply one or more fine reference voltages to one or more reference input nodes of the fine converter for use by the fine converter during the subsequent phase of the conversion cycle, and an encoder module adapted to combine the coarse and fine bits to generate the digital output signal. During the initial phase of the conversion cycle, the ADC is adapted to apply a pre-charge signal based on the analog input signal to at least one reference input node of the fine converter to pre-charge the at least one reference input node. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which like reference numerals identify similar or identical elements. 
       FIG. 1  illustrates a block diagram of a conventional two-step sub-ranging analog-to-digital converter (ADC). 
       FIG. 2  illustrates a signal flow diagram for one of the fine reference voltages in the ADC of  FIG. 1 . 
       FIG. 3  illustrates a block diagram of a two-step sub-ranging ADC according to an embodiment of the present invention. 
       FIG. 4  illustrates a signal flow diagram for one of the fine reference voltages in the ADC of  FIG. 3 . 
       FIG. 5  illustrates a timing diagram for the ADC of  FIG. 3 . 
   

   DETAILED DESCRIPTION 
     FIG. 3  illustrates a block diagram of a two-step sub-ranging analog-to-digital converter (ADC)  300  according to an embodiment of the present invention. ADC  300  operates in the manner described in reference to ADC  100  of  FIG. 1  with the addition of pre-charge switch  301  and pre-charge signal  320 , which are used to “pre-charge” each reference input node of fine converter  104 . In possible implementations of ADC  300 , pre-charge signal  320  may be either a buffered or an un-buffered signal when driving the reference input node of fine converter  104 . 
   In particular, while coarse converter  103  is converting input signal  112 , pre-charge switch  301  provides input signal  112  as pre-charge signal  320  to pre-charge the fine reference input nodes. Once coarse converter  103  has completed its operation, pre-charge switch  301  switches off pre-charge signal  320 , and reference switch network  102  uses control signal  120  to select and apply the set of fine reference voltages used by fine converter module  104 . Continuing with the previously described 8-bit ADC example, where the coarse reference voltages applied to coarse converter  103  are separated by 16-mV increments, input signal  112  will be within 16 mV of each fine reference voltage provided by reference switch network  102  to fine converter  104  during the second phase of the conversion cycle. By applying input signal  112 , during the first phase, to each reference input node of fine converter  104 , the process of settling the input nodes from the previous fine reference voltage levels to the fine reference voltage levels for the current conversion cycle will begin during the first phase, thereby reducing the settling time of the second phase for most situations. Note that, if the MSB values generated by coarse converter  103  do not change from the previous conversion cycle, then the settling time might increase for some or even all of the reference input nodes. However, in this case, the difference between the previous and current reference voltage levels will still be on the order of the resolution of the coarse converter. 
   In another possible implementation of ADC  300 , reference ladder  105  generates only a subset of the full range of reference voltages (such as only the coarse reference voltages provided to coarse converter  103 ). In that case, reference switch network  102  selects one or more of the reference voltages from reference ladder  105  based on control signal  120 , and fine converter  104  uses those one or more selected reference voltages to generate (e.g., by interpolating between comparators) a set of fine reference voltages for converting stable input signal  112 . 
   In this embodiment, a two-stage ADC circuit is utilized. One skilled in the art will recognize that a multi-stage converter that uses any number of converter modules (i.e., a coarse converter and one or more progressively finer converters) to generate different sets of bit values for the converted input signal may be employed without deviating from the spirit and scope of the present invention as recited in the attached claims. Moreover, the division of digital output signal  117  into multiple sets of bit values does not require the different sets to have the same number of bits. 
     FIG. 4  illustrates a signal flow diagram for one of the fine reference voltages in ADC  300  of  FIG. 3 , and  FIG. 5  illustrates a timing diagram for ADC  300 .  FIG. 4  shows the new signal path from S/H module  101  through pre-charge switch  301  to reference input node  403  of fine converter  104 . Although not shown in  FIG. 4 , input signal  112  is also applied (in a non-switched manner) to the signal input node of fine converter  104 , as illustrated in  FIG. 3 . 
   As represented in  FIG. 5 , during the first phase ( 511 ) of the conversion cycle for an initial input signal (i.e., Input 1), while coarse converter  103  converts input signal  112  into the corresponding MSB values ( 421 ), switch S 1  of  FIG. 4  is closed to apply pre-charge signal  320  to pre-charge parasitic capacitance  405  at reference input node  403  of fine converter  104 , while switch S 2  in reference switch network  102  is open. This reference voltage pre-charge process pulls the voltage of reference input node  403  of fine converter  104  to the level of input signal  112  and therefore to a level in the neighborhood of the fine reference voltage for the upcoming second phase. 
   At the beginning of the second phase ( 512 ) of the conversion cycle for Input 1, switch S 1  turns off and switch S 2  turns on ( 422 ) to apply the appropriate fine reference voltages to enable fine converter  104  to convert input signal  112  into the corresponding LSB values ( 431 ). At the end of the second phase, the MSB and LSB values are combined together and latched out as final conversion results  117  of the ADC for Input 1. 
   The above-described timing sequence is repeated to generate a digital conversion output value  117  for each successive input signal value  111 . 
   Pre-charge of reference input node  403  of fine converter  104  reduces the settling error during the reference switching process in that it eliminates the input-dependent charge stored on input node  403  from the previous conversion cycle and replaces it with a voltage tracking the current input level that corresponds to a voltage close to the current fine reference level. Now the reference voltage settling process is more accurate and uniform since the difference between the initial voltage level, which is pre-charged to the current input level, and the current fine reference voltage level, is kept to within a known maximum value. This reduction in the memory effect is significant in high-speed applications where the slew rate of analog input signal  111 , and therefore the slew rate of the reference switching voltage, is high and correspondingly, the time allowed for the reference switching is short. 
   Unlike the previous attempts mentioned above, this invention does not noticeably increase the power consumption and/or the die area of the ADC, since the settling accuracy of the pre-charge is simply not critical and the size of the switch devices for pre-charge could be made minimum. By utilizing a reference voltage pre-charge process, the disclosed embodiments for ADC circuits significantly reduce the reference settling error due to the memory effect with almost no area and power consumption penalty. 
   As noted above, a multi-stage ADC may be constructed using more than two converter modules. For example, an ADC could have a coarse converter and first and second fine converters, with the second fine converter being finer than the first, where the three converters generate three different sets of bit values for the digital output. In that case, depending on the particular implementation, while the coarse converter generates the first set of bit values (i.e., the MSB values), the input signal could be applied to pre-charge the reference input nodes at either or both of the two fine converters. Then, while the first fine converter generates the second set of bits (i.e., a set of intermediate bit values), the input signal could continue to pre-charge the reference input nodes at the second fine converter, which would next generate the LSB values. 
   Although the present invention has been described in the context of ADCs having coarse and fine converters implemented using comparator-based converters, the present invention can be implemented using any suitable, and possibly different, types of converter modules for the coarse and fine converters. 
   Although the present invention has been described in the context of ADCs having a reference-voltage supply consisting of a reference ladder and a reference switch network, the present invention can be implemented in the context of ADCs having other configurations of reference-voltage supplies designed to generate the reference voltages for the coarse and fine converters. 
   Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.” 
   The present invention may be implemented as circuit-based processes, including possible implementation as a single integrated circuit (such as an ASIC or an FPGA), a multi-chip module, a single card, or a multi-card circuit pack. As would be apparent to one skilled in the art, various functions of circuit elements may also be implemented as processing steps in a software program. Such software may be employed in, for example, a digital signal processor, micro-controller, or general-purpose computer. 
   It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the scope of the invention as expressed in the following claims. 
   Although the steps in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those steps, those steps are not necessarily intended to be limited to being implemented in that particular sequence.