fig3 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 fig1 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 . fig4 illustrates a signal flow diagram for one of the fine reference voltages in adc 300 of fig3 , and fig5 illustrates a timing diagram for adc 300 . fig4 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 fig4 , input signal 112 is also applied ( in a non - switched manner ) to the signal input node of fine converter 104 , as illustrated in fig3 . as represented in fig5 , 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 fig4 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 .