Abstract:
A data converter ( 20 ) comprising an input (I 0 ′-I 3 ′) for receiving a digital word and an output (V OUT2 ) for providing an analog voltage level in response to the digital word. The data converter further comprises a plurality of bit lines (BL 0 ′-BL 3 ′) formed with an alignment in a first dimension and a plurality of word lines (WL 0 ′-WL 4 ′) formed with an alignment in a second dimension different than the first dimension. Further, the data converter comprises a string ( 12 ′) comprising a plurality of series connected resistive elements (R 0 ′-R 14 ′). The string comprises a plurality of voltage taps (T 0 ′-T 15 ′), and at least a majority of the plurality of series connected resistive elements are formed with an alignment in the first dimension. The data converter also comprises a plurality of switching transistors (ST 0 ′-ST 15 ′) coupled between the plurality of voltage taps and the output. Lastly, for each of a majority of the plurality of word lines the word line is coupled to at least one gate of a switching transistor located on a first side of the word line and to at least one gate of a switching transistor located on a second side of the word line, wherein the second side is opposite the first side.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
         [0001]    Not Applicable.  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not Applicable.  
         BACKGROUND OF THE INVENTION  
         [0003]    The present embodiments relate to data converters, and are more particularly directed to converters using resistor strings.  
           [0004]    Data converters may be used in various types of electronic circuits, or may be formed as a single integrated circuit device. Such converters typically take one of two forms, either as a digital-to-analog converter (“DAC”) or an analog-to-digital converter (“ADC”). For the DAC, its operation converts an input digital signal to an output analog signal, typically where the amplitude of the output analog signal corresponds directly to the magnitude of the input digital signal. Conversely, the ADC converts an input analog signal to an output digital signal, typically where the value of the output digital signal corresponds directly to the amplitude of the input analog signal. In many configurations, both DACs and ADCs implement a resistor string that includes a number of series-connected resistors, where each resistor provides a voltage tap at each of its ends. Typically, the overall string is biased at opposing ends by two different reference voltages, where for example one such voltage is a positive voltage and the other is ground. Also in this regard, in an effort to maintain the linearity between the digital input and the analog output, a common concern in the art is to endeavor to ensure that each resistor in the string has as close to the same resistance value as all other resistors in the string. Accordingly, the resistor string forms a series voltage dividing network and each of the voltage taps is accessible as part of the operation for the data conversion (i.e., either from digital to analog, or analog to digital).  
           [0005]    For further background to converters and by way of example, FIG. 1 illustrates a typical configuration of a prior art DAC  10 , and is detailed briefly below. In addition, since the primary focus of the preferred embodiments described later is directed to resistor strings as used in either a DAC or an ADC, the following discussion provides one example of such a string as used in a DAC, but is not unduly lengthened by also providing a detailed analysis of an ADC. Instead, such an understanding is left to one skilled in the art.  
           [0006]    [0006]FIG. 1 illustrates a typical configuration of a prior art DAC  10 , and is detailed briefly here with additional detail ascertainable by one skilled in the art. By way of example and as appreciated later, DAC  10  is a 4-input 16-output DAC, while numerous other dimensions may exist for different DAC configurations. In general and as detailed below, DAC  10  is operable to receive a 4-bit input word, designated from least significant bit to most significant bit as I 0 -I 3 , and in response to the magnitude of that input to output a corresponding analog voltage. Before detailing this operation, it is first instructive to examine the devices and connections of DAC  10 . In this regard, DAC  10  includes a series-connected resistor string designated generally at  12 , and which forms a meander in that it serpentines back and forth. Additionally, DAC  10  is generally an array in nature, having a number of bit lines in the vertical dimension and a number of word lines in the horizontal dimension, and with repeated device patterns thereby forming cells in the array. Since the example of DAC  10  presents a 4-input 16-output DAC, the array of DAC  10  includes four bit lines BL 0  through BL 3 , and four word lines WL 0  through WL 3 . Also for the current example of a 4-to-16 DAC, resistor string  12  includes fifteen resistive elements R 0  through R 14  formed in the horizontal dimension. Resistive elements R 0  through R 14  may be formed using various techniques as discussed in greater detail later, where regardless of the technique used to form the resistive elements ideally each resistive element has as close to the same resistance value as all other resistors in the string. Moreover, a voltage source V REF1  is applied across resistor string  12 , and may be of any suitable biasing voltage, which for current applications is typically on the order of 2.0 volts. For DAC  10 , string 12 is biased between V REF1  and ground, but it should be understood that in other configurations two different non-ground potentials may be connected at opposing ends of string  12 . When ground is connected to one end of the string, it is easily appreciated that this difference of the potentials at the ends of the string equals V REF1 . In any event, given the equal resistance of each element in the string, V REF1  is uniformly divided across the resistive elements of string  12 .  
           [0007]    Looking to the detailed connections with respect to resistive elements R 0  through R 14 , each resistive element provides two taps from which a voltage may be measured as detailed below. For example, looking to resistive element R 0 , it provides a tap T 0  and a tap T 1 , while resistive element R 1  shares the same tap T 1  and provides another tap T 2 , and so forth. Each tap has a switching device connected between it and a corresponding output bit line. In the current example, each of these switching devices is an n-channel field effect transistor, and is labeled for convenience by combining the abbreviation ST (i.e., switching transistor) with the same numeric identifier corresponding to the tap to which a source/drain of the transistor is connected. For example, a source/drain of transistor ST 0  is connected to tap T 0 , a source/drain of transistor ST 1  is connected to tap T 1 , and so forth. Further, the switching transistors are arranged so that a like number of taps are coupled via corresponding switching transistors to a corresponding one of the bit lines. In the current example of DAC  10 , four taps are coupled in this manner to a corresponding bit line. For example, taps T 0  through T 3  are coupled, via corresponding switching transistors ST 0  through ST 3 , to bit line BL 0 . As another example, taps T 4  through T 7  are coupled, via corresponding switching transistors ST 4  through ST 7 , to bit line BL 1 . Each bit line BL 0  through BL 3  is coupled via a respective column access transistor, CAT 0  through CAT 3 , to a column decoder  14 . More particularly and for reasons evident below, column decoder  14  is coupled to receive the two most significant bits (MSBs) of the 4-bit word input to DAC  10 , and in response column decoder  14  controls the gates of column access transistors CAT 0  through CAT 3 . Lastly, it should be understood that column decoder  14  operates in response to an overall system supply voltage V S1  which in the prior art is typically on the order of 5.0 volts.  
           [0008]    Returning now to switching transistors ST 0  through ST 15 , and given the array nature of DAC  10 , it is further appreciated that the switching transistors are arranged so that a like number of switching transistors are controlled, via connection to their gates, by a corresponding word line which is further connected to row decoder  16 . Like column decoder  14 , row decoder  16  is also responsive to the system supply voltage V S1 . Returning to the connectivity between row decoder  16  and the switching transistors, and given the current example of DAC  10 , the gates of four switching transistors are coupled to each corresponding word line. For example, the gates of switching transistors ST 0 , ST 7 , ST 8 , and ST 15  are coupled to word line WL 0 . As another example, the gates of switching transistors ST 1 , ST 6 , ST 9 , and ST 14  are coupled to word line WL 1 . Lastly in this regard, and for reasons evident below, row decoder  16  is coupled to receive the two least significant bits (LSBs) of the 4-bit word input to DAC  10  (i.e., bits I 1  and I 0 ), and is also controlled in response to the least significant bit (“lsb”), I 2 , of the two MSBs input to column decoder  14 . More particularly, each least significant bit I 0  and I 1  is coupled as an input to a corresponding exclusive OR gate EOG 0  and EOG 1  as a first input, while the second input of exclusive OR gates EOG 0  and EOG 1  is connected to receive  12  (i.e., the least significant bit of the two MSBs input to column decoder  14 ). In response to these bits, row decoder  16  controls the gates of switching transistors ST 0  through ST 15  as detailed below.  
           [0009]    The operation of DAC  10  is now described, first in general and then more specifically through the use of a few examples. A 4-bit digital word is connected to inputs I 0  through I 3  and ultimately causes signals to pass to column decoder  14  and row decoder  16 . Generally, row decoder  16  includes sufficient logic circuitry or the like to respond by asserting one of word lines WL 0  through WL 3 , thereby providing an enabling voltage to the gates of the four switching transistors coupled to the asserted word line. Similarly, column decoder  14  includes sufficient logic circuitry or the like to respond by enabling one of column access transistors CAT 0  through CAT 3 , thereby causing the enabled transistor to pass the voltage from the corresponding one of bit lines BL 0  through BL 3  to output V OUT1 . In a simple case, the result of the above operations may be viewed by correlating the value of the 4-bit input to one of the sixteen decimal tap numbers. For example, if the 4-bit digital word equals 0001 (i.e., decimal value one), then DAC  10  enables a switching transistor and a column access transistor to couple the voltage at tap T 1  to V OUT1 .  
           [0010]    By way of detailed illustration of the operation of DAC  10 , the example of an input equal to 0001 is now traced through DAC  10  in greater detail. From the input of 0001, its two MSBs are coupled to column decoder  14  and, thus, the value of 00 is received by column decoder  14 . In response, column decoder  14  enables the gate of the column access transistor having a numeric identifier equal to the value of the MSBs. Here, the MSBs of 00 equal a decimal value of zero and, thus, column decoder  14  couples a voltage of V S1  to the gate of column access transistor CAT 0 , thereby enabling that transistor. Turning now to row decoder  16 , it responds to the value of the two LSBs of the 4-bit input. However, note that these two LSBs pass through exclusive OR gates and, therefore, their values are unchanged when passed to row decoder  16  if the lsb equals 0, or their complements are passed to row decoder  16  if the lsb equals 1. Returning then to the example of a 4-bit input equal to 0001, the two LSBs equal 01 and the lsb of the two MSBs equals 0. Thus, the unchanged LSBs equal to 01 reach row decoder  16 , and row decoder  16  in response asserts the word line having a decimal numeric identifier equal to the value of the two LSBs as received from gates EOG 0  and EOG 1 . In the present example, therefore, row decoder  16  asserts word line WL 1  high to a value of V S1  which, therefore, enables each of switching transistors ST 1 , ST 6 , ST 9 , and ST 14 . Recall also that column decoder  14  in this example enables column access transistor CAT 0 . As a result, the voltage from tap T 1  passes via switching transistor ST 1  to bit line BL 0 , and then passes via column access transistor CAT 0  to V OUT1 . Lastly, it is noted that the voltage at tap T 1  is divided across one resistive element (i.e., R 0 ) and, thus, for an input equal to 0001, the analog output voltage using voltage division is 1/15*V REF1 .  
           [0011]    To further illustrate in detail the operation of DAC  10 , consider now the example of an input equal to 0111 as traced through DAC  10 . At the outset, from the general operation described above, one skilled in the art will expect that since the decimal value of 0111 equals seven, then the tap selected by DAC  10  for output is tap T 7 . This expectation is now confirmed through a detailed examination of this example. From the input of 0111, its two MSBs of 01 are coupled to column decoder  14 . In response, column decoder  14  enables the gate of the column access transistor having a decimal numeric identifier equal to the two MSB values of 01 and, hence, the gate of column access transistor CAT 1  is enabled. Turning now to row decoder  16 , note first that the lsb of the two MSBs in this example equals one; consequently, gates EOG 0  and EOG 1  cause the complements of the two LSBs to reach row decoder  16 . Thus, the complements of the 11 LSBs are 00 and, therefore, the value of 00 reaches row decoder  16 . In response, row decoder  16  asserts word line WL 0  high since that word line has a numeric identifier equal to the value of the two complemented LSBs. When word line WL 0  is asserted, it enables each of switching transistors ST 0 , ST 7 , ST 8 , and ST 15 . Recall also that column decoder  14  in this example enables column access transistor CAT 1 . As a result, the voltage from tap T 7  passes via switching transistor ST 7  to bit line BL 1 , and then passes via column access transistor CAT 1  to output V OUT1 . Lastly, it is noted that the voltage at tap T 7  is divided across seven of the fifteen resistive elements (i.e., R 0  through R 6 ) and, thus, for an input equal to 0111, the analog voltage output using voltage division is equal to 7/15*V REF1 . Accordingly, the digital input of 0111 has been converted to an analog voltage which equals this divided voltage. Given this as well as the preceding example, one skilled in the art will further appreciate that with different digital inputs, any of the switching transistors of DAC  10  may be enabled along with enabling one of the column access transistors, and for each such combination of transistors there is a corresponding output which represents a divided voltage between zero volts or any value incrementing up from zero volts by 1/15V REF1  (assuming equal resistance among the resistive elements), and up to an output equal to V REF1 .  
           [0012]    Having detailed DAC  10 , attention is now focused in greater detail to resistive elements R 0  through R 14 . Specifically, in the prior art such elements are often formed as metal devices in an effort to achieve considerably low resistance for reasons such as achieving various device speeds. For example, a single metal resistive element may have a resistance on the order of sixty milliohms per square and may support device speeds on the order of hundreds of megahertz. However, it has been observed in connection with the present inventive embodiments that various configurations do not require such small resistance values, where such configurations may well arise in devices operating at lower frequencies. In addition, it is often the goal of an integrated circuit to be made smaller, and this goal may well apply to a DAC, either alone or in combination with other circuitry on the same single integrated circuit However, the present inventors recognize that the relatively larger size of metal resistive elements may conflict with the goal of reducing DAC size, or at least may complicate the efforts of obtaining such a goal without an inventive layout to accommodate those resistors. Accordingly, the present inventors recognize as developed below in the preferred embodiments that by either avoiding metal resistive elements or by providing an alternative cell layout some of the area and complexities that accompany such devices also may be reduced or substantially eliminated. In view of these drawbacks and goals, there arises a need to provide an improved converter configuration, as is achieved by the preferred embodiments discussed below.  
         BRIEF SUMMARY OF THE INVENTION  
         [0013]    In the preferred embodiment, there is a data converter comprising an input for receiving a digital word and an output for providing an analog voltage level in response to the digital word. The data converter further comprises a plurality of bit lines formed with an alignment in a first dimension and a plurality of word lines formed with an alignment in a second dimension different than the first dimension. Further, the data converter comprises a string comprising a plurality of series connected resistive elements. The string comprises a plurality of voltage taps, and at least a majority of the plurality of series connected resistive elements are formed with an alignment in the first dimension. The data converter also comprises a plurality of switching transistors coupled between the plurality of voltage taps and the output. Lastly, for each of a majority of the plurality of word lines the word line is coupled to at least one gate of a switching transistor located on a first side of the word line and to at least one gate of a switching transistor located on a second side of the word line, wherein the second side is opposite the first side. Other circuits, systems, and methods are also disclosed and claimed.  
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0014]    [0014]FIG. 1 illustrates a schematic of a prior art digital-to-analog converter (“DAC”).  
         [0015]    [0015]FIG. 2 illustrates a schematic of an inventive DAC including a resistor string, where the majority of the cells within the DAC include four access transistors, and where two of those transistors are controlled by a common word line.  
         [0016]    [0016]FIG. 3 a  illustrates a plan view of a cell from the DAC of FIG. 2 according to a first embodiment.  
         [0017]    [0017]FIG. 3 b  illustrates a plan view of the cell from the DAC of FIG. 2, but at an earlier step in the manufacturing process than in FIG. 3 a.    
         [0018]    [0018]FIG. 3 c  illustrates a cross sectional view of a resistive element from FIG. 3 a.    
         [0019]    [0019]FIG. 3 d  illustrates the plan view of the cell in FIG. 3 a  with additional components to demonstrate additional connections.  
         [0020]    [0020]FIG. 4 illustrates a plan view of a partially fabricated cell from the DAC of FIG. 2 according to a second embodiment.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    [0021]FIG. 1 was described in the preceding Background Of The Invention section of this document and in connection with the prior art.  
         [0022]    [0022]FIG. 2 illustrates a schematic of a DAC  20  according to a preferred embodiment. DAC  20  includes some components that are schematically comparable in some respects to components in DAC  10  of FIG. 1, but as detailed below there are also various differences in various of such components in the preferred embodiments. In any event, to illustrate and introduce these components, they are shown with the same reference identifiers as in FIG. 1, with the exception that an apostrophe is added to the identifier in FIG. 2. Briefly reviewing these elements with respect to DAC  20 , and assuming the reader is familiar with DAC  10 , DAC  20  is a 4-to-16 DAC that receives a 4-bit input word I 0 ′-I 3 ′. DAC  20  includes a series-connected resistor string  12 ′ which includes resistive elements R 0 ′ through R 14 ′ formed according to techniques described later, while it is noted here that each element preferably has a like resistance. The ends of resistive elements R 0 ′ through R 14 ′ give rise to an array of corresponding taps T 0 ′ through T 15 ′, and as detailed later these taps in one preferred embodiment are formed at the same time as resistive elements R 0 ′ through R 14 ′ and no separate physical contact is required between a tap and its corresponding resistive element. String  12 ′ is biased in total by a reference voltage V REF2 , which may be any suitable voltage (e.g., 2.0 volts). DAC  20  further includes a number of bit lines BL 0 ′ through BL 3 ′. Each bit line is connected to a corresponding one of column access transistors CAT 0 ′ through CAT 3 ′, where each such column access transistor has a first source/drain connected to one corresponding bit line and a second source/drain connected to an output V OUT2 . DAC  20  also includes a number of word lines WL 0 ′ through WL 3 ; however, DAC  20  includes an additional word line which, for sake of consistency in identifiers, is referred to as word line WL 4 .′ Word lines WL 0 ′ through WL 4 ′ are connected to sets of switching transistors, as further described below.  
         [0023]    Turning now to a first item of contrast between DAC  20  and DAC  10  of the prior art, note that the schematic illustrations of FIGS. 1 and 2 are also intended to illustrate the array layouts as pertaining to switching transistors. Further in this regard, DAC  20  includes various switching transistors, which are preferably n-channel field effect transistors. In the preferred embodiment, and in contrast to the prior art, the switching transistors of DAC  20  are connected in pairs between corresponding pairs of taps. For example, such a switching transistor pair is shown relative to taps T 0 ′ and T 7 ′; more particularly, switching transistor ST 0 ′ has a first source/drain connected to tap T 0 ′, switching transistor ST 7 ′ has a first source/drain connected to tap T 7 ′, and a second source/drain of switching transistor ST 0 ′ is connected to a second source/drain of switching transistor ST 7 ′. As demonstrated below, in one preferred embodiment each of these electrical connections (i.e., between two switching transistors and by each transistor to a corresponding tap) is achieved by forming a mutual device region and does not require a separate physical connection. Having demonstrated these electrical connections by way of example, Table 1 below illustrates all pairs of switching transistors and the corresponding taps to which each source/drain of each transistor in the pair is connected.  
                               TABLE 1                                   Switching transistor pair   Tap connection   Tap connection                           ST0′, ST7′   ST0′ to T0′   ST7′ to T7′           ST1′, ST6′   ST1′ to T1′   ST6′ to T6′           ST2′, ST5′   ST2′ to T2′   ST5′ to T5′           ST3′, ST4′   ST3′ to T3′   ST4′ to T4′           ST11′, ST12′   ST11′ to T11′   ST12′ to T12′           ST13′, ST10′   ST13′ to T13′   ST10′ to T10′           ST14′, ST9′   ST14′ to T14′   ST9′ to T9′           ST15′, ST8′   ST15′ to T15′   ST8′ to T8′                      
 
         [0024]    As a second item of contrast between DAC  20  and DAC  10  of the prior art, each switching transistor in a pair, in addition to being electrically connected to one another, is both physically and electrically connected to one of bit lines BL 0 ′ through BL 3 ′ via a corresponding contact. For example, for the switching transistor pair between taps T 0 ′ and T 7 ′ (i.e., switching transistors ST 0 ′ and ST 7 ′), the second source/drain of each is connected via a contact C 0,7  to bit line BL 1 ′. Note for sake of reference that the subscripts used for this contact, as well as the other comparable contacts discussed in this document, identify the switching transistor(s) to which the contact is connected (i.e., to the gate of the transistor). Having demonstrated this connection by way of example, Table 2 below illustrates all pairs of switching transistors and the corresponding bit lines to which the second source/drain of each transistor in the pair is connected via a contact.  
                           TABLE 2                                   Switching transistor pair   Contact connection                           ST0′, ST7′   C 0,7  to bit line BI1′           ST1′, ST6′   C 1,6  to bit line BL0′           ST2′, ST5′   C 2,5  to bit line BL1′           ST3′, ST4′   C 3,4  to bit line BL0′           ST11′, ST12′   C 11,12  to bit line BL2′           ST13′, ST10′   C 13,10  to bit line BL3′           ST14′, ST9′   C 9,14  to bit line BL2′           ST15′, ST8′   C 8,15  to bit line BL3′                      
 
         [0025]    As a third item of contrast between DAC  20  and DAC  10  of the prior art, the majority of word lines connect to the gates of two switching transistors per cell, where a first of the switching transistors is on one side of the word line while a second of the switching transistors is on an opposite side of the word line. This aspect is further appreciated by now detailing the specific connections of the word lines, and is appreciated in yet additional detail later with respect to subsequent Figures illustrating the layout of a single cell. For each of word lines WL 1 ′, WL 2 ′, and WL 3 ′, that is, for those word lines that are not on the upper or lower perimeter of the DAC array, they are shown to have the connections described immediately above. For example, in one cell word line WL 1 ′ is connected to the gate of switching transistor ST 8 ′ above the word line and to switching transistor ST 9 ′ below the word line; to further illustrate this connection and to demonstrate a layout aspect shown later, this connection is via a contact C 8,9 , where again subscripts are used in the reference to identify the switching transistors to which the contact is connected (i.e., to the gates of those transistors). Similarly, in another cell word line WL 1 ′ has a contact C 0,1  connected to the gate of switching transistor ST 0 ′ above the word line and to switching transistor ST 1 ′ below the word line. Looking to word line WL 2 ′, it has a contact C 13,14  connected to the gates of switching transistors ST 13 ′ and ST 14 ′ located on opposite sides of that word line, and it also has a contact C 5,6  connected to the gates of switching transistors ST 5 ′ and ST 6 ′ where those transistors again are on opposite sides of the word line. Word line WL 3 ′ is shown to have comparable connections given its contact C 10,11  connected to the gates of switching transistors ST 10 ′ and ST 11 ′, and its contact C 2,3  connected to the gates of switching transistors ST 2 ′ and ST 3 ′. Lastly, it is noted that the illustration only includes a total of five word lines; however, for a larger DAC which includes a greater number of word lines, then the preferred implementation as applied to such a DAC will result in more than three word lines being connected in the fashion just-described with respect to word lines WL 1 ′, WL 2 ′, and WL 3 ′.  
         [0026]    As yet another item of contrast between DAC  20  and DAC  10  of the prior art and as introduced in part in the preceding discussion, the word lines in DAC  20  that are on the upper and lower perimeter of the DAC array connect to the gate of only a single switching transistor per cell. For example, at the top of the array of DAC  20  is word line WL 0 ′, and it connects via a contact C 15  to the gate of switching transistor ST 15 ′ in one array cell and via a contact C 7  to the gate of switching transistor ST 7  in another array cell. Similarly, at the bottom of the array of DAC  20  is word line WL 4 ′, and it connects via a contact C 12  to the gate of switching transistor ST 12 ′ in one array cell and via a contact C 4  to the gate of switching transistor ST 4 ′ in another array cell.  
         [0027]    Another item of contrast between DAC  20  and DAC  10  of the prior art is detailed later relating to the preferred embodiment for forming resistive elements R 0 ′ through R 14 ′, but is introduced now given the general layout of those elements. More particularly, in DAC  20  string  12 ′ serpentines back and forth to form a meander having a number of columns that are primarily in the vertical dimension, with horizontal portions referred to as “turns” connecting each column of resistive elements. Further, although the majority of resistive elements are shown schematically in a vertical sense in FIG. 2, note that due to the formation of those elements as detailed below that some resistive elements will be formed at least in part in a turn that extends in the horizontal dimension. Also due to the layout having a majority of elements in the vertical dimension, the voltage swing sampled to a bit line from any set of resistive elements aligned in the vertical dimension is only a fraction of the total bias voltage (e.g., of V REF2 ), whereas if the resistive elements were aligned horizontally then the voltage swing would be much greater, thereby increasing device loading.  
         [0028]    As a final item of contrast between DAC  20  and DAC  10  of the prior art, the location of the paired switching transistors among the string meander lends itself to a more compact design as detailed later, thereby reducing the overall complexity and size of DAC  20  and gaining the benefits as known in the art which accompany the reduction in device size. In addition and as detailed later, in one preferred embodiment resistive elements R 0 ′ through R 14 ′ are formed as diffusion devices, thereby reducing overall power consumption. Still further, this choice also furthers a preferred layout because the resistive elements may then be formed in the same plane as the switching transistors. Lastly, note that the symmetry of DAC  20  is such that once a cell geometry is defined, it may be repeated for virtually all sets of taps and corresponding switching transistors in DAC  20 , although some cells may require additional considerations by one skilled in the art such as those relating to a turn in meander  12 ′.  
         [0029]    The operation of DAC  20  is comparable in various respects to that of DAC  10  described earlier and, thus, is reviewed here with a lesser level of detail given the previous explanation of FIG. 1. Thus, decoders  14 ′ and  16 ′ respond to bits I 0 ′-I 3 ′ of the digital input word; in this regard, however, note that the different layout of DAC  20  may cause one skilled in the art to create different types of decoding methodologies for column decoder  14 ′ and row decoder  16 ′ and, indeed, for this reason, the additional logic gates shown in FIG. 1 are not shown in FIG. 2. In any event, in response to a digital input word, one word line is asserted thereby enabling the switching transistors having their gate connected to that word line. Consequently, each enabled switching transistor provides a conductive path between the tap connected to the first source/drain of the switching transistor and the contact connected to the second source/drain of the switching transistor. The contact connection thereby connects the tap voltage to one of the bit lines BL 0 ′ through BL 3 ′. Thus, during an overlapping time period, column decoder  14 ′ enables one of column access transistors CAT 0 ′ through CAT 3 ′ and the enabled one of those transistors passes the voltage along its corresponding bit line to V OUT2 . To accomplish these operations, one skilled in the art may select various alternative circuit configurations for both column decoder  14 ′ and row decoder  16 ′.  
         [0030]    The operation of DAC  20  described above is now explored by way of an example, followed by an additional Table that demonstrates the operation for accessing any of voltage taps T 0 ′ through T 15 ′. By way of example, assume that a 4-bit input word I 0 ′-I 3 ′ is presented to access tap T 7 ′. As a result, row decoder  16 ′ decodes bits I 0 ′ and I 1 ′ so that word line WL 0 ′ is asserted. In response, switching transistor ST 7 ′ is enabled (as is switching transistor ST 15 ′). Consequently, switching transistor ST 7  provides a conductive path from tap T 7 ′ to contact C 0,7 , and further therefore to bit line BL 1 ′. Next, column decoder  14 ′ decodes bits  12 ′ and I 3 ′ so that column access transistor CAT 1 ′ is enabled. In response, therefore, a conductive path is provided from bit line BL 1 ′ to V OUT2 , thereby passing the voltage from tap T 7 ′ to V OUT2 . Thus, this example should further explain the operation of DAC  20 , which is further demonstrated in the following Table 3 which depicts for each voltage tap the combination of asserted word line and selected bit line to pass the voltage on a given tap to V OUT2 .  
                               TABLE 3                                       Asserted word               Voltage tap   line   Selected bit line                           T0′   WL1′   BL1′           T1′   WL1′   BL0′           T2′   WL3′   BL1′           T3′   WL3′   BL0′           T4′   WL4′   BL0′           T5′   WL2′   BL1′           T6′   WL2′   BL0′           T7′   WL0′   BL1′           T8′   WL1′   BL3′           T9′   WL1′   BL2′           T10′   WL3′   BL3′           T11′   WL3′   BL2′           T12′   SL4′   BL2′           T13′   WL2′   BL3′           T14′   WL2′   BL2′           T15′   WL0′   BL3′                      
 
         [0031]    [0031]FIG. 3 a  illustrates a plan view of a cell C 1  to be repeated such that it forms one of numerous cells in DAC  20  and to further demonstrate the preferred and compact design according to the preferred embodiment; further, FIGS. 3 b  through  3   d  illustrate related views to further depict this design. Turning first then to FIG. 3 a,  its illustration includes the DAC cell that includes resistive elements R 0 ′ and R 6 ′; thus, FIG. 3 a  further illustrates taps T 0 ′ and T 7 ′ at the top of these resistive elements and taps T 1 ′ and T 6 ′ at the bottom of these resistive elements. Additional aspects of FIG. 3 a  such as the switching transistors and word lines are discussed later, but first attention is directed to the formation of the resistive elements, as is further appreciated by examining FIG. 3 b.    
         [0032]    [0032]FIG. 3 b  illustrates the same plan perspective as FIG. 3 a,  but before various manufacturing steps are complete in order to focus on a lower plane of the device. More particularly, FIG. 3 b  illustrates a plan view of a diffusion region designated generally as DR 1 , where in the preferred embodiment region DR 1  is formed by diffusing N+ dopants into a semiconductor device such as a semiconductor substrate. In an alternative embodiment, however, region DR 1  could be formed as a p-type diffusion region (in an appropriate well or semiconductor substrate). In any event, by forming region DR 1 , it forms the eventual regions for various different devices or device components, including resistive elements R 0 ′ and R 6 ′ Moreover, FIG. 3 c  illustrates a cross-sectional view of either resistive element R 0 ′ or R 6 ′. From the FIG. 3 c  perspective, region DR 1  forms the width of a resistive element, and in the preferred embodiment is isolated on both sides by forming oxide regions OX in trenches adjacent each side of the resistive element, where such a formation is achieved using shallow trench technology. The shallow trench technology is preferred because it assists in maintaining a uniform width for each resistive element and, in doing so, thereby increases the chance of maintaining the same resistance value for each of those elements. This may be in contrast to other isolating technologies, such as LOCOS, where lateral encroachment leads to reduced diffusion surface width and may make resistor matching more difficult. Lastly with respect to FIG. 3 c,  note that a silicide region SR is shown over diffusion region DR 1 . In the preferred embodiment, such a region may be formed to further reduce the resistance of the resistive element (as well as the others) in which case the majority of the current passing through the device will pass through region SR.  
         [0033]    Looking to the remainder of FIG. 3 b,  region DR 1  also forms other device components. More particularly, in the horizontal dimension region DR 1  defines the source/drain regions for the switching transistors formed relative to resistive elements R 0 ′ and R 6 ′ Specifically, recall it was noted earlier that in the preferred embodiment the resistive elements are formed in the same plane as the switching transistors. This same plane is defined by diffusion region DR 1 . Similarly, the area at the top of resistive element R 0 ′, which may be electrically referred to as tap T 0 ′, also provides a first source/drain S/D 1  for switching transistor ST 0 ′. Similarly, the area at the bottom of resistive element R 0 ′, which may be electrically referred to as tap T 1 ′, also provides a first source/drain S/D 1  for switching transistor ST 1 ′. Similar observations may be made with respect to resistive element R 6 ′. Thus, the area at the top of resistive element R 6 ′ is electrically tap T 7 ′ and provides a first source/drain S/D 1  for switching transistor ST 7 ′, while the area at the bottom of resistive element R 6 ′ (i.e., electrically tap T 6 ′) provides a first source/drain S/D 1  for switching transistor ST 6 ′.  
         [0034]    Continuing with FIG. 3 b,  diffusion region DR 1  also forms the second source/drain regions for the switching transistors described in the preceding paragraph. Thus, between taps T 0 ′ and T 7 ′ there is a mutual area of diffusion which forms the second source/drain S/D 2  for switching transistor ST 7 ′ and which also forms the second source/drain S/D 2  for switching transistor ST 0 ′. Indeed, at this point attention is returned to FIG. 3 a  and it may be seen that a gate conductor GC(ST 7 ′) is formed vertically between source/drain regions S/D 1 (ST 7 ′) and S/D 2 (ST 7 ′). Thus, this combination of gate conductor and source/drain regions (as well as an insulator below the gate, but not seen from the figure&#39;s perspective) form switching transistor ST 7 ′ shown schematically in FIG. 2. Indeed, a similar layout is shown for the other three switching transistors formed by diffusion region DR 1  in FIG. 3 b,  and given the added gate conductors in FIG. 3 a.  Thus, gate conductor GC(ST 0 ′) combines with source/drain regions S/D 2 (ST 0 ′) and S/D 1 (ST 0 ′) to form switching transistor ST 0 ′, gate conductor GC(ST 6 ′) combines with source/drain regions S/D 2 (ST 6 ′) and S/D 1 (ST 6 ′) to form switching transistor ST 6 ′, and gate conductor GC(ST 1 ′) combines with source/drain regions S/D 2 (ST 1 ′) and S/D 1 (ST 1 ′) to form switching transistor ST 1 ′.  
         [0035]    Having demonstrated the location of the resistive elements, taps, and switching transistors in FIGS. 3 a  and  3   b,  the remaining illustrations in FIG. 3 a  are now explored. First, note that the above-described gate conductors are preferably formed in a polysilicon layer overlying diffusion region DR 1 , with an oxide (not shown) layer formed between the two. Next, an additional insulator layer (not shown) is formed over the polysilicon layer, and then a first metal layer is patterned, where typically in the art such a layer is referred to as metal-1. In the preferred embodiment the metal-1 layer is formed to create the word lines for DAC  20 , and in FIG. 3 a  word lines WL 1 ′ and WL 2 ′ are shown from this layer. Next, metal contacts are formed for DAC  20  and, more particularly in FIG. 3 a,  two such contacts C 0,1  and C 5,6  are shown. Recall from FIG. 2 that these contacts connect the gates of switching transistors to the appropriate word line. For example, contact C 0,1  in FIG. 3 a  connects word line WL 1 ′ to the gate conductors beneath it, namely, to gate conductor GC(ST 0 ′) and gate conductor GC(ST 1 ′). As the other example illustrated in FIG. 3 a,  contact C 5,6  in FIG. 3 a  connects word line WL 2 ′ to the gate conductor beneath it, namely, to gate conductor GC(ST 6 ′). Finally in this regard, note that due to the cell boundaries as illustrated in FIG. 3 a,  certain other like connections are not shown. For example, gate conductor GC(ST 7 ′) of FIG. 3 a  is connected to word line WL 0 ′ as shown in FIG. 2, but this connection is not shown in FIG. 3 a  although it is achieved in a manner comparable to the contacts shown in FIG. 3 a.  As another example, while contact C 5,6  in FIG. 3 a  is shown to connect to gate conductor GC(ST 6 ′) for switching transistor ST 6 ′, it also connects to a gate conductor for switching transistor ST 5 ′, but again that connection is not shown in FIG. 3 a  given the boundary of cell C 1 .  
         [0036]    Given the layout illustrations and discussion thus far, note two additional observations with respect to the switching transistors in the preferred embodiment. As a first observation, note that a portion of a diffused area in diffusion region DR 1  which forms an end of a resistive element is effectively a shared diffusion region in that it also provides a source/drain for a switching transistor (e.g., T 0 ′ and S/D 1 (ST 0 ′)). Accordingly, this shared region further reduces the size of each cell and, hence, the size of the DAC. As a second observation, note that a mutual diffusion area forms a source/drain for two different switching transistors (e.g., S/D 2 (ST 7 ′) and S/D 2 (ST 0 ′)). As a result of this latter aspect, there are various benefits. For example, no additional electrical contact is required between these two source/drains to achieve the electrical communication required by the schematic of FIG. 2. In addition, capacitance is reduced. Still further, the cell size is reduced as compared to a configuration where each switching transistor requires an independent area to form its source/drain. As a final observation, since diffusion region DR 1  forms both the resistive elements and the potential conductive paths for the switching transistors, each end of a resistive element, that is, each DAC tap, is also connected in this manner to a source/drain of its corresponding switching transistor without requiring an additional component to make this connection. Once more, therefore, such an approach reduces device complexity as well as device size.  
         [0037]    [0037]FIG. 3 d  illustrates the same plan perspective as FIG. 3 a,  but after additional manufacturing steps are complete in order to illustrate the complete device cell C 1 . More particularly, FIG. 3 d  illustrates the preferred layout to complete the remaining electrical connections shown for the cells in FIG. 2 and not discussed above in FIGS. 3 a  and  3   b.  First, note that FIG. 3 d  shows the addition of bit lines BL 0 ′ and BL 1 ′. In the preferred embodiment, an additional insulator layer (not shown) is formed over the metal-1 layer, and then a second metal layer (i.e., metal-2) is patterned to form bit lines BL 0 ′ and Bl 1 ′. Generally, bit lines BL 0 ′ and Bl 1 ′ are vertically disposed above diffusion region DR 1 . However, as detailed below, the bit lines also include short horizontal portions to facilitate the remaining required electrical connections discussed earlier with respect to FIG. 2.  
         [0038]    To discuss the remaining aspects of FIG. 3 d,  attention is directed by way of example to switching transistors ST 0 ′ and ST 7 ′ as shown in FIGS. 3 a,    3   b,  and  3   d.  Specifically, at the same time the metal-1 layer is used to form word lines WL 1 ′ and WL 2 ′, it is also patterned to form a pad P 0,7 . Two aspects are now noted with respect to the location of pad P 0,7 . First, pad P 0,7  overlies the mutual source drain regions of switching transistors ST 0 ′ and ST 7 ′ (i.e., S/D 2 (ST 0 ′) and S/D 2 (ST 7 ′)). This location permits a contact C 0,7  to be formed to electrically connect pad P 0,7  to those mutual source/drain regions. Contact C 0,7 , as shown in FIG. 2, permits the connection, therefore, of these mutual source/drain regions to bit line BL 1 ′. Completing this connection leads to a second aspect of pad P 0,7 , namely, note that it extends in the direction of bit line BL 1 ′. In addition, bit line BL 1 ′ includes a relatively short horizontal portion HP 0,7  that extends in the direction of pad P 0,7  and, indeed, horizontal portion HP 0,7  in the metal-2 layer overlies pad P 0,7  in the metal-1 layer. Finally, note that an electrical via contact V 0,7  is shown in horizontal portion HP 0,7 , thereby electrically connecting it to the underlying pad P 0,7  in the metal-1 layer. From these connections, therefore, note that pad P 0,7  connects the mutual source/drain regions of switching transistors ST 0 ′ and ST 7 ′ to pad P 0,7 , and pad P 0,7  is connected by via contact V 0,7  to bit line BL 1 ′. Thus, as shown in Table 3, above, either tap T 0 ′ or T 7 ′ may be electrically accessed by bit line BL 1 ′ by enabling either switching transistor ST 0 ′ or ST 7 ′, respectively. Finally, having demonstrated these various aspects with respect to switching transistor ST 0 ′ and ST 7 ′, similar observations may be made with respect to switching transistor ST 1 ′ and ST 6 ′. Particularly, a metal-1 layer pad P 1,6  is formed over the mutual source/drain regions of switching transistor ST 1 ′ and ST 6 ′ and is electrically connected to those regions by a contact C 1,6 , and a metal-2 horizontal portion HP 1,6  of bit line BL 0 ′ overlies pad P 1,6  and is electrically connected thereto by an electrical via contact V 1,6 . As a result and as also shown in Table 3, above, tap T 1 ′ or T 6 ′ may be electrically accessed by bit line BL 0 ′ by enabling either switching transistor ST 1 ′ or ST 6 ′, respectively.  
         [0039]    [0039]FIG. 4 illustrates a plan view of an alternative embodiment for a cell C 2  to be used in of DAC  20  and which permits the use of a different material for the resistive elements. Specifically, recall that FIGS. 3 a  through  3   d  illustrated a preferred embodiment which implements resistive elements formed in a diffusion region DR 1 . The alternative embodiment of FIG. 4 shares various aspects with that of FIGS. 3 a  through  3   d,  but permits connection to a resistive element of a different type. Looking now to the common aspects, the cell of FIG. 4 includes a diffusion region DR 2  to form switching transistors that are oriented in the same manner with respect to each of the gate conductors, word lines, bit lines with horizontal portions, contacts to switching transistor source/drain regions, and via contacts shown in FIG. 3 d.  However, in this alternative embodiment, note that diffusion region DR 2  does not include resistive elements. Instead, these elements may be formed in a different manner, as further discussed below.  
         [0040]    Examining the resistive elements and connections to those elements in FIG. 4, note that diffusion region DR 2  includes horizontal portions extending generally outward from the cell in a manner to connect to corresponding resistive elements. For example, along the left of FIG. 4, diffusion region DR 2  includes a horizontal portion HP T7′  and a horizontal portion HP T6′ . Given the reach of these portions, the cell further includes a resistive element R 6 ′, but in this embodiment resistive element R 6 ′ is formed in the polysilicon layer at the same time as the gate conductors. Moreover, because resistive element R 6 ′ is not formed in the same layer as the switching transistors, then an additional contact is required to connect each tap to a source/drain of a switching transistor; as examples, a contact C T7′  connects one end of resistive element R 6 ′ to a source/drain region of switching transistor ST 7 ′, and a contact C T6′  connects an opposite end of resistive element R 6 ′ to a source/drain region of switching transistor ST 6 ′. Finally with respect to FIG. 4, note that the use of horizontal portions of diffusion region DR 2  could permit connection to yet other types of resistive elements. In all such cases, however, note that while there is increased flexibility in choice of resistive material, there is a tradeoff in that device size is increased and additional contacts are required.  
         [0041]    From the above, it may be appreciated that the present embodiments provide numerous advantages over the prior art, many of which have been set forth above and additional ones of which will be ascertained by one skilled in the art. For example, the preferred configurations may apply either to DACs or ADCs. In addition, while the present embodiments have been described in detail, various substitutions, modifications or alterations could be made to the descriptions set forth above without departing from the inventive scope. Indeed, some of these type of variations have been set forth above, and still others may be ascertained. For example, while resistor strings with relatively small numbers of resistors have been illustrated, the present teachings apply equally if not more beneficially to strings with larger numbers of resistors. Thus, the preceding as well as other ascertainable examples should further illustrate the inventive scope, where that scope is defined by the following claims.