Abstract:
A circuit including first and second reference ladders, a selection circuit, first and second analog to digital converters (ADCs), and a summer. The first reference ladder is configured to provide first reference voltages via first taps. The selection circuit is configured to select one of the first reference voltages. The second reference ladder is configured to, based on the selected one of the first reference voltages, provide second reference voltages via second taps. The first ADC is configured to convert the first version of the analog input signal to a first digital signal. The second ADC is configured to, based on the second reference voltages, convert the second version of the analog input signal to a second digital signal. The summer is configured to generate a digital output signal based on the first and second digital signals.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present disclosure is a continuation of U.S. application Ser. No. 13/323,527 (now U.S. Pat. No. 8,742,969), filed Dec. 12, 2011, which is a continuation of U.S. patent application Ser. No. 12/684,735 (now U.S. Pat. No. 8,077,069), filed on Jan. 8, 2010. This application claims the benefit of U.S. Provisional Application No. 61/145,840, filed on Jan. 20, 2009. The entire disclosures of the applications referenced above are incorporated herein by reference. 
         [0002]    The present application is related to co-pending U.S. patent application Ser. No. 12/684,773 entitled “Current Sensing and Background Calibration to Match Two Resistor Ladders”, filed concurrently, and co-pending U.S. patent application Ser. No. 12/684,760 entitled “Reference Pre-Charging for Two-Step Subranging ADC Architecture”, both filed concurrently, the contents of both are incorporated herein in their entirety for all purposes. 
     
    
     BACKGROUND 
       [0003]    Particular embodiments generally relate to analog-to-digital (ADC) architectures and more specifically to two-step subranging ADC architectures. 
         [0004]    A two-step subranging ADC architecture performs an analog-to-digital conversion in two steps.  FIG. 1  depicts a conventional two-step subranging ADC architecture  100 . Architecture  100  includes a coarse ADC  102  and a fine ADC  104 . Coarse ADC  102  includes a coarser or poorer resolution than fine ADC  104  and can quickly determine an approximate subrange that a sample of an input analog signal falls within. This narrows the range of analog voltages in which the sample of the input analog signal may correspond. Fine ADC  104  then further defines the analog voltage from within the subrange selected by coarse ADC  102 . 
         [0005]    The input analog signal is received at a track-and-hold stage (T/H)  106 . Track-and-hold stage  106  tracks the input analog signal and stores an input voltage for the sample of the input analog signal. For example, the input analog signal may be sampled for a half clock cycle and the input voltage from the sample is stored for another half clock cycle. 
         [0006]    Coarse ADC  102  compares the stored voltage to a plurality of coarse references received from a reference ladder  108 . Reference ladder  108  may include a plurality of tap points. Each tap point may be at a different voltage level for each coarse reference. Coarse ADC  102  performs a first comparison of the input voltage to the coarse references to determine a subrange in which the input voltage falls within. 
         [0007]    A result of the first comparison is then used to select finer references or finer subdivisions of the selected subrange for fine ADC  104 . For example, certain switches in a switch matrix  110  are closed to provide a second subrange of fine references to fine ADC  104 . Fine ADC  104  then performs a second comparison of the fine references and the input voltage. 
         [0008]    Encoding and digital correction logic  112  uses the results of the first comparison and the second comparison to determine a first digital code and a second digital code. The first and second digital codes are used to determine a digital output for the sample of the input analog signal. For example, the first and second digital codes may be appropriately weighted, error corrected, and combined to generate the digital output, which may be a digital representation of the sample of the input analog signal. 
         [0009]    The determination of the first digital code and second digital code each needs to be made within a half clock period, T/2, where T is a clock period. When the sampling rate goes up, the time that coarse ADC  102  needs to make a decision becomes a larger part of its half clock period T/2. 
         [0010]    Reference ladder  108  needs time to settle from a voltage level of a previous sample to set up the fine references.  FIG. 2  shows a timing diagram for the conventional two-step subranging architecture  100 . At each clock cycle, the input analog signal is tracked (T) and held (H). During the hold period, coarse ADC  1 - 102  makes its decision within a portion of the T/2 period. Then, in the remaining part of the same T/2 period, coarse output encoding, fine reference selection and subsequent setting of the fine reference takes place. When the sampling rate goes up, coarse output encoding, fine reference selection and subsequent setting of the fine reference combined together take a longer part of the T/2 period, which means less time for reference settling is allotted. 
         [0011]    The reference settling problem is discussed in more detail in  FIG. 3 , which shows a more detailed conventional architecture for a two-step subranging ADC. Reference ladder  1 - 108  includes a plurality of reference segments  202   a - 202   n . Each reference segment includes a coarse tap  204  to coarse ADC  1 - 102 . Coarse taps  204   a - 204   n  provide the coarse references to coarse ADC  1 - 102 . Also, each reference segment  202  includes a plurality of fine taps  206  to provide fine references. When coarse ADC  1 - 102  makes a coarse decision, switch matrix  1 - 110  selects a reference segment  202 . The selected fine references associated with the selected reference segment  202  are then input into fine ADC  1 - 104  through switch matrix  1 - 110 . 
         [0012]    When the two-step subranging ADC architecture  1 - 100  is running at very high sampling rates, the maximum conversion rate is limited by the reference settling speed of reference ladder  1 - 108 . Reference settling becomes a speed bottleneck due to loading from a large number of switches in switch matrix  1 - 110  and comparators in coarse ADC  1 - 102  or fine ADC  1 - 104 . These loadings dynamically disturb the voltage levels of reference ladder  1 - 108  decreasing the bandwidth of reference ladder  1 - 108  and causing a longer settling time. The speed bottleneck becomes a problem as both ADC resolution and conversion rate become higher. To lower the reference settling time, each reference segment  202  may be designed with a very small resistance value. However, designing reference segments with the small resistance value takes up a very large chip area. Also, the resistors of reference segments  202  have their own parasitic capacitance, which limits the return on the lower resistance values. 
       SUMMARY 
       [0013]    In one embodiment, an analog-to-digital converter is provided. A first track and hold stage tracks and stores an input voltage for a sample of an analog input signal. A coarse reference ladder provides a plurality of coarse references. In one embodiment, the coarse reference ladder includes a first coarse reference ladder including a plurality of first taps to provide a plurality of coarse references and a second coarse reference ladder including a plurality of second taps to provide the plurality of coarse references. 
         [0014]    A coarse analog-to-digital converter (ADC) receives the input voltage from the first track and hold stage and the plurality of coarse references. The coarse ADC performs a first comparison of the input voltage and the plurality of coarse references and outputs a coarse output based on the first comparison. A switch matrix includes a plurality of switches and is configured to close a switch corresponding to a coarse reference based on the coarse output. 
         [0015]    A second track and hold stage tracks and stores the input voltage. A fine reference ladder receives the coarse reference from the coarse reference ladder and provides a plurality of fine references. The plurality of fine references are determined based on the coarse reference. A fine ADC receives the input voltage from the second track and hold stage and the plurality of fine references. The fine ADC performs a second comparison of the input voltage and the plurality of fine references and outputs a fine output based on the second comparison. Logic outputs a digital output for the sample of the analog input signal based on the coarse output and the fine output. 
         [0016]    In one embodiment, an apparatus is provided that comprises: a first track and hold stage configured to track and store an input voltage for a sample of an analog input signal; a coarse reference ladder providing a plurality of coarse references; a coarse analog-to-digital converter (ADC) configured to receive the input voltage from the first track and hold stage and the plurality of coarse references, the coarse ADC configured to perform a first comparison of the input voltage and the plurality of coarse references and output a coarse output based on the first comparison; a switch matrix including a plurality of switches, the switch matrix configured to close a switch based on the coarse output, the switch corresponding to a coarse reference; a second track and hold stage configured to track and store the input voltage; a fine reference ladder configured to receive the coarse reference and provide a plurality of fine references determined based on the coarse reference; a fine ADC configured to receive the input voltage from the second track and hold stage and the plurality of fine references, wherein the fine ADC is configured perform a second comparison of the input voltage and the plurality of fine references and output a fine output based on the second comparison; and logic configured to output a digital output for the sample of the analog input signal based on the coarse output and the fine output. 
         [0017]    In another embodiment, an apparatus is provided that comprises: a coarse reference ladder comprising: a first coarse reference ladder including a plurality of first taps to provide a first plurality of coarse references; and a second coarse reference ladder including a plurality of second taps to provide a second plurality of coarse references; a coarse analog-to-digital converter (ADC) configured to receive an input voltage of a sample of an analog input signal and the first plurality of coarse references, the coarse ADC configured to perform a first comparison of the input voltage and the first plurality of coarse references and output a coarse output based on the first comparison; a switch matrix including a plurality of switches, the switch matrix configured to close a switch for a second tap based on the coarse output, the switch corresponding to a second coarse reference; a fine reference ladder configured to receive the coarse reference and output a plurality of fine references from a plurality of third taps of the fine reference ladder, the plurality of fine references determined based on the coarse reference; a fine ADC configured to receive the input voltage and the plurality of fine references from the plurality of third taps, wherein the fine ADC is configured perform a second comparison of the input voltage and the plurality of fine references and output a fine output based on the second comparison; and logic configured to output a digital output for the sample of the analog input sample based on the coarse output and the fine output. 
         [0018]    The following detailed description and accompanying drawings provide a better understanding of the nature and advantages of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  depicts a conventional two-step subranging ADC architecture. 
           [0020]      FIG. 2  shows a timing diagram for the conventional two-step subranging architecture. 
           [0021]      FIG. 3  shows a more detailed conventional architecture for a two-step subranging ADC. 
           [0022]      FIG. 4  depicts an analog-to-digital converter architecture according to one embodiment. 
           [0023]      FIG. 5  shows a conceptual example of the subranges according to one embodiment. 
           [0024]      FIG. 6  depicts another example of subranging ADC reference ladders according to one embodiment. 
           [0025]      FIG. 7  depicts a more detailed example of the ADC architecture according to one embodiment. 
           [0026]      FIG. 8  depicts a timing diagram for the ADC architecture described in  FIG. 7  according to one embodiment. 
           [0027]      FIG. 9  depicts an example of calibration of reference ladders according to one embodiment. 
           [0028]      FIG. 10  shows a more detailed example of the ADC architecture according to one embodiment. 
           [0029]      FIG. 11  depicts a simplified flowchart of a method for calibrating reference ladders according to one embodiment. 
           [0030]      FIG. 12  depicts a waveform showing the pre-charge of an output of a switch matrix according to one embodiment. 
           [0031]      FIG. 13  depicts a simplified flowchart of a method for converting an analog input signal to a digital output signal according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    Described herein are techniques for a two-step subranging analog-to-digital converter (ADC) architecture. In the following description, for purposes of explanation, numerous examples and specific details are set forth in order to provide a thorough understanding of embodiments of the present invention. Particular embodiments as defined by the claims may include some or all of the features in these examples alone or in combination with other features described below, and may further include modifications and equivalents of the features and concepts described herein. 
       Overview of ADC Architecture 
       [0033]      FIG. 4  depicts an ADC architecture  400  according to one embodiment. In one embodiment, architecture  400  is used for ultra high-speed, medium-to-high resolution applications. Although these applications are described, architecture  400  may be used in other applications that require an analog-to-digital conversion. In one embodiment, architecture  400  is a two-step subranging ADC architecture. 
         [0034]    Architecture  400  converts an analog input signal to a digital output signal. The analog input signal is received at a first track-and-hold stage (T/H)  402   a . Track-and-hold stage  402   a  is configured to track the analog input signal for a part of a clock cycle, T, and store an input voltage for another part of the clock cycle. For example, the analog input signal may be tracked for T/2 and the input voltage is stored for another T/2. The stored input voltage is for a sample of the analog input signal. 
         [0035]    A coarse ADC  404  receives the input voltage and performs a comparison of the input voltage to a plurality of coarse references received from a coarse digital-to-analog converter (DAC)  406 . 
         [0036]    In one embodiment, coarse DAC  406  includes a coarse reference ladder  408 , a switch matrix  410 , and a buffer  412 . Coarse reference ladder  408  is separated from a fine reference ladder  414  through buffer  412 . The separation allows for independent optimization of coarse reference ladder  408  and fine reference ladder  414 , which will be described in more detail below. 
         [0037]    Coarse reference ladder  408  may include a plurality of resistors and a plurality of taps. The plurality of taps provide the plurality of coarse references to coarse ADC  404 . The coarse references may be different reference voltage levels. 
         [0038]    Coarse ADC  404  compares the input voltage to the coarse references to determine a coarse decision. The coarse decision may select a coarse reference for a subrange in which the input voltage resides. For example, coarse ADC  404  may choose a midpoint in between a subrange of voltages.  FIG. 5  shows a conceptual example of the subranges according to one embodiment. A plurality of subranges  502   a - 502   d  are shown and a plurality of coarse references are provided. For example, the coarse references may be 1-5V. Coarse ADC  4 - 404  compares the input voltage to coarse references and selects which range of values in which the input voltage resides. For example, the input voltage may reside at a point  506  in subrange  502   c . Coarse ADC  4 - 404  then selects subrange  502   c . The voltage selected may be a midpoint  508  in subrange  502   c . By selecting the midpoint, a slight quantization error, E q  is introduced. As will be explained below, the fine references are used to refine the quantization error using the fine references. 
         [0039]    Referring back to  FIG. 4 , the coarse decision is the result of comparisons between the input voltage and the coarse references. For example, comparators in coarse ADC  404  may compare the input voltage with the different coarse references. Each comparator outputs a logic output based upon the comparison. The value of the logic output is based on whether the coarse reference is higher or lower than the input voltage. For example, a comparator may output a value of 0 if the input voltage has a value that is lower than the coarse reference. Also, a comparator outputs a “1” value if the input voltage has a value higher than the coarse reference. A coarse encoder  414  receives the logic output from the comparators and determines a first digital code. The first digital code is a digital representation of the input voltage. 
         [0040]    A switch in switch matrix  410  is closed such that a coarse reference for subrange  5 - 502  selected by coarse ADC  404  is sent to fine reference ladder  414  through buffer  412 . Buffer  412  separates coarse reference ladder  408  from fine reference ladder  414 . 
         [0041]    The coarse reference is sent to fine reference ladder  414 . Fine reference ladder  414  uses the coarse reference to generate a plurality of fine references for a fine ADC  416 . The plurality of fine references may be within the subrange selected by coarse ADC  404 . For example, referring to  FIG. 5 , a plurality of fine references are provided in between 3V-4V. A fine reference corresponding to the input voltage is then determined. 
         [0042]    Fine ADC  416  receives the plurality of fine references and an input voltage from second track-and-hold stage  402   b . For example, second track-and-hold stage  402   b  tracks the input voltage starting at a T/2 period after the tracking period for first track-and-hold stage  402   a  and stores the input voltage starting at a T/2 period after the storing period for first track-and-hold stage  402   a . By using two track-and-hold stages  402   a  and  402   b , the fine ADC decision may be extended an extra T/2 period. This allows an extended settling time for coarse reference ladder  408  and fine reference ladder  414 . This concept will be described in more detail below. 
         [0043]    Referring back to  FIG. 4 , fine ADC  416  compares the fine references to the input voltage. In one embodiment, comparators of fine ADC  416  output logic outputs of comparisons of the fine references and the input voltage. For example, a 0 or 1 may be output depending on the comparison. A comparator may output a value of 0 if the input voltage has a value that is lower than the reference. Also, a comparator outputs a “1” value if the input voltage has a value higher than the reference. 
         [0044]    A fine encoder  418  receives the logic outputs of the comparison and determines a second digital code. The second digital code is a digital representation of the input voltage. 
         [0045]    Digital error correction logic  420  receives the first digital code from coarse encoder  414  and the second digital code from fine encoder  418 . The first digital code may be received through a flip-flop  422 . Flip-flop  422  may delay the first digital code because of the decision by fine ADC  416  being delayed by a T/2 period. 
         [0046]    Digital error correction logic  420  may include an adder. The adder may add the first digital code and the second digital code to produce a digital output. Additionally, digital error correction logic  420  may weight and error correct the first digital code and the second digital code. In one embodiment, the first digital code may be used to determine the most significant bits (MSB) of the digital output. The second digital code may be used to refine the least significant bits (LSB) of the digital output. The digital output may be a binary code or any other type of code that represents the sample of the analog input in the digital domain. 
         [0047]    Coarse reference ladder  408  and the use of additional track and hold stages  402   a  and  402   b  in architecture  400  will now be described in more detail. The calibration of fine reference ladder  414  and reference precharging will then be described. 
       Coarse Reference Ladder 
       [0048]    Particular embodiments provide two reference ladders for coarse reference ladder  408 . Although two coarse reference ladders are described, any number of coarse reference ladders may be used.  FIG. 6  depicts another example of the subranging ADC reference ladders according to one embodiment. Coarse reference ladder  4 - 408  includes a coarse ADC reference ladder  602  and a coarse DAC ladder  604 . By using two separate ladders, coarse DAC ladder  604  may be free of loading from comparators in coarse ADC  4 - 404 . Additional bandwidth may be gained by coarse DAC ladder  608 . 
         [0049]    In addition to separating coarse reference ladder  4 - 408  into coarse ADC reference ladder  602  and coarse DAC ladder  604 , fine reference ladder  4 - 414  is separated from coarse reference ladder  4 - 408  through buffer  4 - 412 . This allows separate implementation and optimization of coarse ADC reference ladder  602 , coarse DAC ladder  604 , and fine reference ladder  4 - 414 . 
         [0050]    Coarse ADC reference ladder  602  is static. Coarse ADC reference ladder  602  provides a number of reference voltages (e.g., the coarse references) between the voltages V rtop  and V rbot . The reference voltages provided to coarse ADC  4 - 404  do not change making coarse ADC reference ladder  602  static. 
         [0051]    Coarse DAC ladder  604  is dynamic. Each time coarse ADC  4 - 404  selects a different subrange, a different coarse reference is provided to fine ADC  4 - 416 . By using two separate ladders, coarse DAC ladder  604  can settle faster from a previous voltage level to the voltage level selected as the subrange. For example, coarse DAC ladder  604  is free of loading from comparators in coarse ADC  4 - 404 , which allows coarse DAC ladder  604  to settle faster. Additionally, coarse DAC ladder  604  may be implemented with a low impedance, high speed design in contrast to coarse ADC reference ladder  602 , which may be implemented in a high impedance, slow speed design. Coarse ADC reference ladder  602  is static and may not need to be a high speed design. By using a high impedance design, coarse ADC reference ladder  602  consumes less power. However, the high speed design allows coarse DAC ladder  604  to settle faster to set up the fine references based on the subrange selected by coarse ADC  4 - 404 . 
         [0052]    Coarse DAC ladder  604  is also separated from fine reference ladder  4 - 414  by buffer  4 - 412 . The use of buffer  4 - 412  instead of coarse DAC ladder  604  to drive fine reference ladder  4 - 414  prevents a large loading from fine ADC  4 - 416  on coarse DAC ladder  604 . For example, loading from the comparators found in fine ADC  4 - 416  is prevented. This improves settling speed and slew rate of coarse DAC ladder  604 . 
         [0053]    Fine reference ladder  4 - 414  is dynamic because different fine references are being selected based on the subrange selected by coarse ADC  4 - 404 . When different subranges are selected, the fine references are at different voltage levels and this causes shifts in voltage at fine reference ladder  4 - 414 . However, because fine reference ladder  4 - 414  is separated from coarse DAC ladder  604  by buffer  4 - 412 , coarse DAC ladder  604  is not disturbed by the change in voltage levels at fine reference ladder  4 - 414 . 
         [0054]    Fine reference ladder  4 - 414  may also be floating in that there may not need to be a fixed resistance ratio between fine reference segments and coarse reference segments. A reference segment may be a unit resistor between taps of coarse reference ladder  4 - 408  or fine reference ladder  4 - 414 . Coarse reference ladder  4 - 408  or fine reference ladder  4 - 414  may each include multiple unit resistors that divide the ladder into the different voltage subranges. The unit resistors of floating fine reference segments may be implemented in different orientations and sizes from coarse reference ladder  4 - 408 . Calibration is used to match unit resistors of fine reference ladder  4 - 414  to coarse reference ladder  4 - 408 , which will be described below. Conventionally, a fixed resistance ratio between coarse reference ladder  4 - 408  and fine reference ladder  4 - 414  lead to ultra low resistance segments in a high-speed design if coarse reference ladder  4 - 408  uses low resistance segments. The very low resistance values may lead to parasitic effects. Also, physical implementation of low resistance segments may require large areas and have other process parasitics (e.g., interface and contacts resistance). Using floating fine references avoids these problems as low impedance resistors may be used but very small resistor segments can be avoided. 
         [0055]    Fine reference ladder  4 - 408  may be floating, but the voltage of fine reference ladder  4 - 414  is a fixed ratio of the voltage for coarse reference ladder  4 - 408 . A calibration is used to ensure that the voltage ratio is fixed between fine reference ladder  4 - 414  and coarse reference ladder  4 - 414 . Accordingly, very small resistor segments that are used in coarse DAC ladder  604  do not need to be used in fine reference ladder  4 - 414 . More details of the calibration of fine reference ladder  4 - 414  will be described below. 
       Example Implementation of ADC Architecture Using Multiple Track and Hold Stages 
       [0056]      FIG. 7  depicts a more detailed example of ADC architecture  4 - 400  according to one embodiment. A first track-and-hold stage  4 - 402   a  includes an amplifier  702   a , a switch  704   a , and a capacitor  706   a . Although this implementation of track-and-hold stage  4 - 402   a  is described, other implementations may be appreciated. Capacitor  706   a  is used to store the input voltage. Switch  704   a  is toggled between the track stage and the hold stage. The switch may be closed to charge capacitor  706   a  and then opened when the voltage is stored. 
         [0057]    A second track-and-hold stage includes an amplifier  702   b , switch  704   b , and capacitor  706   b . Amplifier  702   b  is gain matched with amplifier  702   c . The matching ensures that the input voltage that is being input into coarse ADC  4 - 404  is matched with the voltage being tracked and stored by track-and-hold stage  4 - 402   b.    
         [0058]    Coarse DAC  4 - 406  includes coarse ADC reference ladder  6 - 602  and coarse DAC ladder  6 - 604 . Coarse DAC  4 - 406  and coarse DAC ladder  6 - 604  each include a plurality of unit resistors.  31  coarse taps of coarse ADC reference ladder  6 - 602  in between the unit resistors are provided to coarse ADC  4 - 404 . However, any number of coarse taps may be used. In this case, architecture  400  may be a 9-bit resolution ADC. 
         [0059]    Coarse ADC (CADC)  4 - 404  receives a clocking signal, strobec. At each clock cycle, coarse ADC  4 - 404  makes a coarse decision. For example, when a coarse reference is selected by coarse ADC  4 - 404 , coarse encoder (CENC)  4 - 426  provides a control signal to switches  708  included in switch matrix  4 - 410  to close one of the switches corresponding to the coarse reference selected. In one embodiment, a 32-bit signal is sent to open or close switches  708 . 
         [0060]    The selected coarse reference is sent through a buffer  702   d  from coarse DAC ladder  6 - 604 . A buffer  702   e  is gain matched with buffer  702   d . This ensures that the input voltage into fine ADC  4 - 416  from buffer  702   e  is gain matched with the reference selected by coarse ADC  4 - 404 . 
         [0061]    A plurality of fine taps and a plurality of fine switches  712  are included in fine reference ladder  4 - 414 . In one example, based on the signal received, different switches in fine reference ladder  4 - 414  are closed to send 31 fine references to fine ADC  4 - 416 . Fine ADC  4 - 416  may also receive the input voltage from buffer  702   e.    
         [0062]    Fine ADC (FADC)  4 - 416  makes a fine decision at each clock cycle of a clocking signal, strobef. For example, fine ADC  4 - 416  outputs logic outputs from comparisons of the input voltage and the fine references. Fine encoder  4 - 418  uses the logic outputs to determine a second digital code. Digital correction logic  4 - 420  receives the second digital code and the first digital code through a flip-flop  4 - 422 . The first digital code may be used to determine the 5 most significant bits for the digital output and the second digital code may be used to refine the 5 least significant bits of the first digital code. For example, digital correction logic  420  combines and error corrects the first digital code and second digital code into a 9-bit digital output. 
         [0063]      FIG. 8  depicts a timing diagram for architecture  4 - 400  described in  FIG. 7  according to one embodiment. Because two track-and-hold stages  4 - 404   a  and  4 - 404   b  are used, additional time for reference settling is provided. Conventionally, as described in  FIG. 3 , the coarse decision, fine reference bit encoding, and fine reference settling are all are done in half of a clock cycle (T/2). However, in particular embodiments, this total time is extended by another half clock period  ˜ T/2 to be approximately a full clock period before fine ADC  4 - 416  needs to start making a comparison. 
         [0064]    At  802  and  804 , the signals for first track-and-hold stage  4 - 404   a  and second track-and-hold stage  4 - 402   b  are shown. First track-and-hold stage  4 - 404   a  tracks and holds a sample for a clock period, T, and then second track-and-hold stage  4 - 402   b  tracks and holds the same sample for another clock period, T. For example, first track-and-hold stage  4 - 402   a  tracks and holds a new sample S1 and then second track-and-hold stage  4 - 402   b  tracks and holds the new sample. While first track and hold stage  4 - 402   a  is tracking the new sample S1, second track and hold stage  4 - 402   b  is holding a current sample S0. The delay in tracking and holding between first track-and-hold stage  4 - 402   a  and second track-and-hold stage  4 - 402   b  is approximately T/2. 
         [0065]    At  806 ,  808 , and  810 , the signals for coarse ADC  4 - 404 , coarse DAC  4 - 406 , and fine ADC  4 - 416  are shown, respectively. Coarse ADC  4 - 404  makes a coarse decision at  812  for the sample S1. The fine references need to be set up after the coarse decision is made. That is, coarse DAC reference ladder  6 - 604  settles. Additionally, a precharge of the output of coarse DAC ladder  6 - 604  is performed at  814 . A time period shown at  816  shows the time taken to make the coarse decision. 
         [0066]    Fine ADC  4 - 416  then makes a fine decision for the sample S1 at  818 . Thus, instead of determining the first digital code and the second digital code, respectively, within consecutive T/2 periods, the fine decision time is extended to another T/2 period. That is, the coarse decision determination starts in a first T/2 period, a second T/2 period passes, and the fine decision determination is started after the second T/2 period. As shown at  820 , fine reference ladder  4 - 414  settles and makes the fine decision in a second time period. Fine ADC  4 - 416  has approximately a full clock period before fine ADC  4 - 416  has to start a comparison to determine the fine decision after the coarse decision determination starts. This allows the ADC conversion rate to be higher. 
       Calibration of Fine Reference Ladder 
       [0067]      FIG. 9  depicts an example of calibration of reference ladders according to one embodiment. The calibration is described with respect to coarse reference ladder  4 - 408  and fine reference ladder  4 - 414 ; however, it will be understood that the calibration described herein may be used with respect to other designs. For example, other designs that require multiple reference ladders may use the calibration described. Also, although reference ladders that provide references are discussed, the calibration may be used on any resistor ladders. 
         [0068]    In one embodiment, a master reference ladder  900  is part of coarse reference ladder  4 - 408 . For example, master reference ladder may be a reference segment (unit resistor) that is selected as the subrange by coarse ADC  4 - 404 . Master reference ladder  900  is a precise ladder. For example, master reference ladder  900  is built using larger valued unit resistors, R A , where a voltage, V A , is stable across the unit resistor R A . 
         [0069]    Fine reference ladder  4 - 414  includes a first fine reference ladder  902  and a second fine reference ladder  904 . Second fine reference ladder  904  may be a separate part of or included in fine reference ladder  4 - 414 . Second fine reference ladder  904  includes a unit resistor, R C  and first fine reference ladder  902  includes a unit resistor, R B . Unit resistor R B  includes one or more unit resistors R C . 
         [0070]    Second fine reference ladder  904  is separated from first fine reference ladder  902  using a buffer component  906 . For example, buffer component  906  may be one or more cascode devices. Buffer component  906  attenuates noise from a signal path that is from coarse reference ladder  4 - 408  to first fine reference ladder  902 . Because first fine reference ladder  902  is in the signal path, it may produce noise. Buffer component  906  provides a high-impedance shielding from the signal path that may filter or attenuate the noise from first fine reference ladder  902 . 
         [0071]    Dynamic events occur at first fine reference ladder  902  that may cause the noise. For example, the voltage V B  may be dynamically changing. In one example, as different subranges are selected in fine reference ladder  4 - 414 , different voltage levels are across first fine reference ladder  902 . In contrast, the voltage V C  is not changing and second reference ladder  904  is quiet compared to first reference ladder  902 . Because of the shielding from buffer  906 , the current through second fine reference ladder  904  is also almost quiescent and is isolated from dynamic events at first fine reference ladder  902  because any noise from the events is absorbed by buffer  906 . 
         [0072]    A calibrator  908  performs a calibration of voltages across second fine reference ladder  904  and master reference ladder  900 . The quiet voltage across second fine reference ladder  904  can also be used to calibrate the voltage across first fine reference ladder  902 . Using a quiet voltage instead of a noisy voltage provides a more accurate calibration. In one embodiment, calibrator  908  uses a low-offset, low speed calibration loop in the background to perform the calibration. 
         [0073]    Calibrator  908  senses the voltage V C  from second fine reference ladder  904  at a sense port  910 . Also, calibrator  908  senses the voltage across the whole or a segment of first reference ladder  4 - 408  at a reference port  912 . The voltage V A  is a multiple k 1  of V C , where k 1  is a constant. Calibrator  908  adjusts the current to adjust V C  to be multiple k 1  of V A . For example, the current may be adjusted using a current source  910 . 
         [0074]    First fine reference ladder  902  and second fine reference ladder  904  are matched together using a fixed ratio. For example, first reference ladder  902  is built using units of second fine reference ladder  904 , or vice versa. If a unit resistor, R C , is used in second fine reference ladder  904 , first fine reference ladder  902  is built using multiple unit resistors of R C . 
         [0075]    By using multiple units of R C , the voltage V B  may be a fixed ratio of V A . For example, the voltage V C  is: 
         [0000]    
       
      
       V 
       C 
       =k 
       1 
       V 
       A  
      
     
         [0000]    If first fine reference ladder  902  and second fine reference ladder  904  having good matching, then: 
         [0000]        V   B   =k   2   V   C   =k   2   k   1   V   A   =k   3   V   A . 
         [0000]    Thus, V B  is a fixed ratio of V A , where k 1 , k 2 , and k 3  are constants. 
         [0076]    Accordingly, first fine reference ladder  902  may be matched to coarse reference ladder  4 - 408  through the calibration. First fine reference ladder  902  and second fine reference ladder  904  may be implemented using different orientation and size resistors from coarse reference ladder  4 - 408 . Also, matching is kept over all corners and long term drifts using background calibration without disturbing the signal path or having calibration affected by the signal path. 
         [0077]      FIG. 10  shows a more detailed example of architecture  4 - 400  according to one embodiment. As shown, coarse DAC ladder  6 - 604  includes a plurality of unit resistors R A . Second fine reference ladder  9 - 904  includes a unit resistor R C  and first fine reference ladder  9 - 902  includes a plurality of unit resistors R C . As discussed above, coarse ADC  4 - 404  receives an input voltage and selects a coarse reference. A subrange  5 - 502  in coarse DAC ladder  6 - 604  is selected to send the coarse reference to fine reference ladder  4 - 414  through buffer  7 - 702   d . Buffer component  7 - 906  is coupled to a circuit such that it attenuates noise from first fine reference ladder  9 - 902 . A current based on a voltage level of the coarse reference is sent to buffer  7 - 702 . The voltage V A  is a voltage drop across a unit resistor of coarse DAC ladder  6 - 604 . The voltage V B  is matched to a fixed ratio of the selected voltage V A  using the calibration. 
         [0078]    The voltage V A  is sent to calibrator  9 - 908 . Calibrator  9 - 908  also senses the voltage V C  across second fine reference ladder  9 - 904 . Calibrator  9 - 908  calibrates the current across fine reference ladder  4 - 414  using current source  9 - 910 . As discussed above, the voltage V B  is calibrated to a multiple V A . 
         [0079]      FIG. 11  depicts a simplified flowchart  1100  of a method for calibrating reference ladders according to one embodiment. At  1102 , calibrator  9 - 908  receives a first voltage across master reference ladder  9 - 900 . At  1104 , calibrator  9 - 908  receives a second voltage across second fine reference ladder  9 - 904 . The second voltage being received is buffered from disturbances in a third reference ladder. 
         [0080]    At  1106 , calibrator  9 - 908  adjusts a current through second fine reference ladder  9 - 904  to adjust the second voltage to be a first voltage ratio with the first voltage. The adjustment of the current adjusts a third voltage across first fine reference ladder  9 - 902  to be a second voltage ratio of the first voltage to the third voltage. 
         [0081]    The use of the calibration is described in co-pending U.S. patent application Ser. No. 12/684,773 entitled “Current Sensing and Background Calibration to Match Two Resistor Ladders”, filed concurrently, the contents of which is incorporated herein in its entirety for all purposes. 
       Reference Precharge 
       [0082]    Referring back to  FIG. 4 , in one embodiment, a pre-charge of the voltage level at the output of switch matrix  410  is provided. The voltage level is pre-charged to a level of the input voltage. This allows the movement of the voltage at the output of switch matrix  410  to be performed more quickly. For example, the previous voltage level at the output of switch matrix  410  may be the voltage of the last analog input sample. The voltage level needs to be moved from the previous voltage level to the coarse reference selected by coarse ADC  404 . For example, the coarse reference selected by coarse ADC  404  is the input voltage plus a quantization error E q . The quantization error E q  is the error from the closest digital code that approximates the input voltage. 
         [0083]    A time period is taken where coarse ADC  404  is making the coarse decision. During this time period, the voltage level at the output of switch matrix  410  may be pre-charged to the input voltage (V in ). When the coarse decision is made, the voltage only needs to be changed to V in +E q . For example, the selected coarse reference is a voltage that is for a subrange that includes input voltage. Thus, if it is known the output of switch matrix  410  will be around V in +E q , the output of switch matrix  410  may be pre-charged to the input voltage V in . The adjusting of the input voltage V in  may be performed faster because adjusting an E q  amount is a much smaller adjustment than from the previous sample&#39;s voltage level. 
         [0084]    As shown in  FIG. 4 , a switch  424  is provided to allow the precharge of the output of switch matrix  4 - 410 . Switch  424  may be closed to precharge the output of switch matrix  410  while coarse ADC  404  is making the coarse decision. When the coarse decision is made, switch  424  is opened to allow the output of switch matrix  410  to settle to V in +E q . In this case, a switch in switch matrix  410  is closed and the coarse reference is sent to buffer  4 - 412 . 
         [0085]      FIG. 7  also shows the precharge according to one embodiment. As shown, switch  4 - 424  is used to precharge output lines of coarse DAC  6 - 604 . When coarse ADC is making the coarse decision, switch  4 - 424  may be closed and switches  708  may be open. This allows input lines  710  to be precharged to the input voltage V in . In one embodiment, all input lines  710  are precharged. Thus, when the coarse reference is selected, the selected input line  710  is precharged. When the coarse decision is made, a switch  708  is closed to send the coarse reference to fine reference ladder  4 - 414 . Also, switch  4 - 424  is opened to allow the selected input line  710  to settle to V in +E q . 
         [0086]      FIG. 12  depicts a waveform showing the pre-charge of an output of switch matrix  4 - 410  according to one embodiment. At  1202 , the voltage at the output of switch matrix  4 - 410  is V 1 . This is the voltage of the previous input voltage sample. At  1204 , coarse ADC  4 - 404  is strobed. At this point, coarse ADC  4 - 404  may start to make a coarse decision. For example, at  1206 , coarse ADC  4 - 404  performs a comparison of the input voltage and the plurality of coarse references. At  1208 , the first digital code is determined based on the comparison. The first digital code is used to select a switch in switch matrix  4 - 410 . 
         [0087]    The output of switch matrix  4 - 410  is pre-charged during a period at  1206 . When a switch is selected, instead of the voltage at the output of switch matrix  4 - 410  being at V 1 , the voltage is substantially around V. The voltage then needs to settle at the coarse reference of the input voltage V in  plus the coarse quantization error E q . 
         [0088]    The use of the pre-charge is described in more detail in co-pending U.S. patent application Ser. No. 12/684,760 entitled “Reference Pre-Charging for Two-Step Subranging ADC Architecture”, filed concurrently, the contents of which is incorporated herein in its entirety for all purposes. 
       Method Using Particular Embodiments 
       [0089]      FIG. 13  depicts a simplified flowchart  1300  of a method for converting an analog input signal to a digital output signal according to one embodiment. At  1302 , first track and hold stage  4 - 402   a  tracks and stores an input voltage for a sample of an analog input signal. At  1304 , coarse reference ladder  4 - 408  provides a plurality of coarse references. In one embodiment, coarse reference ladder  4 - 408  includes first coarse ADC reference ladder  6 - 602  and second coarse reference ladder  6 - 604 . 
         [0090]    At  1306 , coarse ADC  4 - 404  receives the input voltage from first track and hold stage  4 - 402   a  and the plurality of coarse references. At  1308 , coarse ADC  4 - 404  performs a first comparison of the input voltage and the plurality of coarse references and outputs a coarse output based on the first comparison. At  1310 , switch matrix  4 - 410  closes a switch corresponding to a coarse reference based on the coarse output. An input line has been precharged to the input voltage. 
         [0091]    At  1312 , second track and hold stage  402   b  tracks and stores the input voltage. At  1314 , fine reference ladder  4 - 414  receives the coarse reference from the coarse reference ladder and provides a plurality of fine references. The plurality of fine references are determined based on the coarse reference. At  1316 , fine ADC  4 - 416  receives the input voltage from second track and hold stage  4 - 402   b  and the plurality of fine references. At  1318 , fine ADC  4 - 416  performs a second comparison of the input voltage and the plurality of fine references. At  1320 , fine ADC  4 - 416  outputs a fine output based on the second comparison. At  1322 , a digital output is output for the sample of the analog input signal based on the coarse output and the fine output. 
         [0092]    As used in the description herein and throughout the claims that follow, “a”, “an”, and “the” includes plural references unless the context clearly dictates otherwise. Also, as used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise. 
         [0093]    The above description illustrates various embodiments of the present invention along with examples of how aspects of the present invention may be implemented. The above examples and embodiments should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the present invention as defined by the following claims. Based on the above disclosure and the following claims, other arrangements, embodiments, implementations and equivalents may be employed without departing from the scope of the invention as defined by the claims.