Abstract:
A digital-to-analog converter having a compact “M-2M” binary-weighted ladder structure, where “M” represents an effective resistance inversely proportional to the W/L line ratio of an n-type or p-type metal oxide semiconductor (MOS) device. A reduction in the number of ladder components is accomplished by utilizing the MOS device&#39;s inherent switching function in combination with the device&#39;s resistive behavior. The ladder is comprised of a plurality of “2M” rungs, one rung for each binary bit, and each rung is comprised of a complementary pair of upper and lower MOS devices series-connected at a common node. Each device in the pair has an effective resistance of 2M ohms and only one is enabled at any given time depending upon the value of the associated binary bit. Permanently enabled MOS devices having an effective resistance of M ohms and interconnecting the common nodes of adjacent binary bit device pairs make up an “M” runner. The rungs are connected in parallel across upper-level and lower-level voltage references and an analog voltage corresponding to a digital word is generated at the common node of the most significant bit device pair.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application Ser. No. 60/132,856, filed May 6, 1999; the disclosure of which is hereby incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     N/A 
     FIELD OF THE INVENTION 
     This invention relates generally to digital-to-analog converters and, more particularly, to digital-to-analog converters incorporating binary-weighted resistor arrays or ladders. 
     BACKGROUND OF THE INVENTION 
     A variety of circuits for the conversion of a digital word into an analog voltage are known. A digital-to-analog converter (DAC) typically employs a constant impedance resistor string or a binary-weighted resistor array to accomplish the conversion. A constant impedance resistor string is comprised of 2 N  series-connected resistors, where N equals the number of bit of the digital word to be converted. A voltage reference is placed across the string to thereby generate a series of monotonically increasing voltages. The value of the digital word determines which one of these voltages is selected as the analog output. 
     Binary-weighted resistor arrays require fewer resistors for a specified number of bits (2N versus 2 N ) and one common array is the “R-2R” ladder  10  illustrated in FIG.  1 . The R-2R ladder is comprised of a “runner” of series-connected resistors  2 , each having a resistance of R ohms, and a, plurality of “rungs,” one for each binary bit of the digital word. A rung includes a resistor  4  of 2R ohms and a switch  6  that is controlled by its associated binary bit signal. A voltage reference, V ref , is placed across the R-2R ladder producing binary-weighted currents that are typically summed and converted into an output voltage by an operational amplifier  8 . 
     Unlike the constant impedance resistor string, a binary-weighted resistor array is not an inherently monotonic structure wherein the analog output voltage is guaranteed to monotonically increase as the value of the digital word increases. To avoid the output noise caused by non-monotonic performance, the array resistors must be tightly matched. For example, the resistor matching for the i th  bit of an n-bit converter should be within R/(2 n−i ) ohms. 
     Both the constant impedance resistor string, due to the large number of resistors required, and the binary-weighted array, due to the difficulty in matching the array resistors, are effectively limited in the number of bits that they can convert. To convert digital words having a large number of binary bits, some digital-to-analog converters divide the digital word into a most significant bit (MSB) segment and a least significant bit (LSB) segment and process each segment separately. A segmented DAC structure, described in U.S. Pat. No. 5,648,780, is illustrated in FIG. 2. A first divider stage  20  is responsive to the MSB segment and employs a constant impedance resistor string  22  to generate a series of reference voltages. A decoder  26  generates a plurality of switch control signals  27  to control the switch pairs of a switch string  24  and thereby select, based on the binary value of the MSB segment, an upper-level voltage reference and lower-level voltage reference appearing at a pair of output nodes  28 ,  29 . 
     A second, last divider stage  30  employs a binary-weighted resistor ladder  32  to produce a voltage at an output node  38 . The output voltage depends upon both the established voltage reference range placed across the ladder  32  and the binary value of the LSB segment which controls a plurality of switches  34 . If the digital word to be converted is very large, one or more additional divider stages (not shown) may be placed between the first and last divider stages for the bits of the intermediate segments of the digital word. 
     To achieve the smallest possible package size, a DAC is commonly implemented as a monolithic integrated circuit. Typically, metal oxide semiconductor field effect transistor (MOSFET) devices are used for switches and diffused or implanted structures are used for the resistors. Unfortunately, a monolithic DAC still tends to be relatively large because the switch and resistive devices consume a significant amount of die space. The continued miniaturization of electronic equipment has resulted in a need for an even more compact monolithic DAC. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the invention, a monolithic digital-to-analog converter (DAC) is comprised of a binary-weighted array that uses a single n-type or p-type metal oxide semiconductor (MOS) device for each of the resistor/switch combinations that typically make up such arrays. Because a MOS device has a relatively high equivalent resistance per unit area and, furthermore, because the switching function is inherent in a MOS device, a much more compact layout can be achieved by using a single MOS device for each resistor/switch combination. 
     More particularly, the binary-weighted array has an “M-2M” ladder structure, where “M” refers to the effective resistance of a MOS device when enabled. The M-2M ladder includes one “2M” ladder “rung” for each binary bit, and each rung includes a complementary pair of upper and lower MOS devices series-connected at a common node. Each complementary MOS device has an effective resistance of 2M ohms when enabled and only one device in the pair is enabled at any given time depending upon the value of the associated binary bit. The rungs are connected in parallel and a voltage reference is placed across the pairs. 
     Permanently enabled MOS devices serve as resistors that interconnect the common nodes of adjacent LSB binary bit device pairs. These interconnecting MOS devices make up the “M” ladder “runner” and each has an effective resistance of M ohms. The voltage generated at the common node of the most significant bit is the analog output, which is isolated from the DAC output terminal by a unity gain buffer amplifier. 
     To handle digital words having a large number of bits, the DAC may segment the word into a MSB segment and LSB segment and also include an inherently monotonic resistor string. A series of incremental voltages are developed across the string and the binary bits of the MSB segment are used to select an upper-limit voltage reference and lower-limit voltage reference for the “M-2M” array. Because the upper and lower voltage references are limited to a relatively narrow range, the non-linear effects of a MOS device are largely avoided in the “M-2M” array. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be more fully understood by reference to the following Detailed Description of the Invention in conjunction with the drawings, of which: 
     FIG. 1, discussed above, is an illustration of an “R-2R” resistor ladder used in a conventional DAC; 
     FIG. 2, discussed above, is an illustration of a prior art segmented DAC; 
     FIG. 3 is an illustration of a segmented DAC incorporating an “N-2N” NMOS device array in accordance with the invention; and 
     FIG. 4 is an illustration of the operation of the segmented DAC of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A digital-to-analog converter (DAC) in accordance with the invention is illustrated in FIG.  3 . The DAC includes a first divider stage  20  for conversion of the MSB segment of a digital word and a second, last divider stage  40  for conversion of the LSB segment. 
     The first divider stage  20  includes a resistor string  22  made up of a plurality of series-connected resistors coupled across a reference voltage, V ref , and ground. 
     Preferably, the ladder resistors  22   a - 22   h  are comprised of polycrystalline silicon and should have a resistance of approximately one-tenth the below-discussed load impedance of the second divider stage  40 . The first divider stage  20  also includes a plurality of upper/lower switch pairs  24   a - 24   h  arranged to connect the ladder resistors to a pair of output nodes  28 ,  29 . 
     A first stage decoder  26  is responsive to the binary bits of the MSB segment and generates a plurality of switch control signals  27  for controlling the upper/lower switch pairs  24   a - 24   h  which interconnect the resistors of the first stage resistor ladder  22  to the output nodes  28 ,  29 . One method for implementing such a decoder is described in the above-referenced U.S. Pat. No. 5,648,780. The first divider stage  20  generates an upper-limit voltage reference, V refu , and a lower-limit voltage reference, V refl , at the two output nodes  28 ,  29 , respectively, for use by the last divider stage  40 . 
     The last divider stage  40  includes a plurality of complementary n-type MOS (NMOS) device pairs  42   a ,  42   b ,  42   c , one for each binary bit of the LSB segment. Each pair  42   a - 42   c  is comprised of an upper NMOS device  50   a ,  50   b ,  50   c  and a lower NMOS device  60   a ,  60   b ,  60   c  series-connected at a common node  43   a ,  43   b ,  43   c  via the source pin  56   a ,  56   b ,  56   c  of the upper NMOS device  50   a - 50   c  and the drain pin  62   a ,  62   b ,  62   c  of the lower NMOS device  60   a - 60   c . These pairs  42   a - 42   c  are connected in parallel with the drain pin  52   a ,  52   b ,  52   c  of each upper NMOS device  50   a - 50   c  connected to the upper-level voltage reference, V refu , and the source pin  66   a ,  66   b ,  66   c  of each lower NMOS device  60   a - 60   c  connected to the lower-level voltage reference, V refl . The gate pin  54   a ,  54   b ,  54   c  of each upper NMOS device  50   a - 50   c  is connected to its corresponding binary bit signal B 1 , B 2 , B 3 . The gate pin  64   a ,  64   b ,  64   c  of each lower NMOS device  60   a - 60   c  is connected to the inverse of its corresponding binary bit signal B 1 /, B 2 /, B 3 /. The substrate base pin of each device  58   a - 58   c ,  68   a - 68   c  is connected to its corresponding source pin  56   a - 56   c ,  66   a - 66   c.    
     The NMOS device pairs  42  of adjacent binary bits are interconnected at their common nodes by interconnecting NMOS devices  44   ab ,  44   bc . As described below, all interconnecting NMOS devices  44  are permanently enabled (i.e., switched “on”) with an effective resistance of N ohms. The last divider stage  40  also includes an additional permanently enabled NMOS device  46  having an effective resistance of 2N ohms and connected between the common node  43   a  and the lower-level voltage reference, V refl , to implement a ½ LSB shift. The common node  43   c  of the highest bit, B 3 , of the LSB segment serves as the output node  38  for the second divider stage  40 . A unity-gain buffer amplifier  36  isolates the output node  38  from the DAC output terminal  39 . 
     When a bit signal B 1 , B 2 , B 3  is false, the upper NMOS device  50   a - 50   c  is switched “off” and an essentially open circuit exists between the drain  52   a - 52   c  and the common node  43   a - 43   c . At the same time, the lower NMOS device  60   a - 60   c  is switched “on” and its effective resistance determines the amount of current flowing through the device. The characteristics of an NMOS device, including its effective resistance, are described below. Alternatively, when a bit signal B 1 -B 3  is true, the lower NMOS device  60   a - 60   c  is switched “off” to become an open circuit, and the upper NMOS device  50   a - 50   c  is switched “on” to establish a current flow through the device as determined by its effective resistance. 
     The operation of the DAC is illustrated in FIG. 4, where the digital word to be converted is 100010 (MSB . . . LSB) and the reference voltage, V ref , is 2.50 volts. The upper/lower switch pair  24   e  of the first stage  20  that is associated with the MSB segment value “ 100 ” is active and the upper-level and lower-level voltage references produced at the two output nodes  28 ,  29  are 1.56 volts and 1.25 volts, respectively. 
     In the second divider stage  40 , an LSB segment value of “ 010 ” enables the lower NMOS device  60   a  of the B 1  pair, the upper NMOS device  50   b  of the B 2  pair, and the lower NMOS device  60   c  of the B 3  pair. Therefore, these devices are replaced in the circuit diagram with their effective resistances, 2N ohms. The B 1  upper NMOS device  50   a , the B 2  lower NMOS device  60   b , and the B 3  upper NMOS device  50   c  are all disabled, so these devices are shown as open circuits. The interconnecting devices are the permanently enabled NMOS devices  44  represented by their effective resistances, N ohms. The resulting binary-weighted currents produce a voltage of 1.33 volts at the output node  43   c  that corresponds to the digital word 100010 for a voltage reference, V ref , of 2.50 volts. 
     The present invention takes advantage of the resistive behavior of the NMOS device which presents a resistance N ohms, that is inversely proportional to its width-to-length (W/L) ratio when enabled. The sheet resistance of an NMOS device is typically 4K ohms per square (L/W=1 square unit), and very long (high L) devices of narrow width (low W) provide an accurate, high-value resistance. However, very narrow devices should be avoided because small random photolithographic errors make device matching very difficult. Because the N-2N structure is not inherently monotonic, the devices must be sized appropriately to achieve the device matching necessary to ensure monotonic performance. Larger area NMOS devices are also preferred because they allow for easier device matching and lower flicker noise. Furthermore, the input impedance of the array should be approximately ten times the output impedance of the first divider stage to prevent input impedance variations from causing non-monotonicity. For example, a device may have a 5/80 width-to-length ratio and an equivalent resistance of approximately 65K ohms. 
     To operate an NMOS device in a resistive mode, the drain voltage, V D , must be kept at a level much lower than the gate voltage, V G , that is, V G &gt;&gt;V D . Preferably, V G  is set at approximately 7.0 volts, while V D  is kept between 0.0 and 1.0 volts. (Note that when the opposite occurs, i.e. V D &gt;&gt;V G , the drain current becomes constant and the output impedance increases as the device becomes non-linear.) Therefore, the voltage across the N-2N array (i.e., V refu −V refl ) should be kept small, preferably less than 1.5 volts, to minimize the drain-to-source voltage (V DS ) and bulk-to-source voltage (V BS ) and thereby avoid non-linear effects. In addition, the switching, or threshold, voltage, V t , which is typically 0.7 volts, can vary from device to device and this mismatch may be caused by, inter alia, a variation in oxide thickness, oxide charge, photolithographic effects, and the stress profile of the silicon. A “one-sigma” statistical mismatch of 2 mV is typically for NMOS devices. Ultimately, the mismatch negatively impacts the other performance parameters of the DAC. To minimize the effects of the mismatch, the “on” voltage level of the switching signals, B 1 , B 1 /, B 2 , B 2 /, B 3 , B 3 /, should be much higher than the drain voltage, V D . Furthermore, to ensure a uniform impedance per unit area when the NMOS devices are enabled, the gates of the devices should be switched between a fixed common voltage and ground. Those NMOS devices that are not switched should have their gates permanently tied to the same fixed common voltage. 
     Because an NMOS device offers a relatively high equivalent resistance per unit area, while also providing the switching function inherent in such devices, a compact layout less than one-half the size of conventional R-2R arrays may be achieved. The preferred approach is to use two parallel NMOS devices having a total effective resistance proportional to L/2W ohms for the “N” element and a single NMOS device having an effective resistance proportional to L/W ohms for the “2N” element. This provides for better device matching. However, the alternative approach may be used if a higher input impedance is desired wherein a single NMOS device with an effective resistance proportional to L/W ohms for the “N” element and two series NMOS devices having a total effective resistance proportional to 2L/W ohms for the “2N” element. 
     The DAC may be further enhanced by the addition of one or more NMOS devices of higher resistance across each of the NMOS devices of the complementary upper/lower device pairs  42 . These devices are enabled or disabled to precisely adjust, or “trim,” the device resistance to match the specified binary bit weight. This trimming may be hard-wired or set on a power-up calibration. A trimmed DAC is monotonic out to several more bits, while the added trimmer devices do not contribute significantly to the consumption of valuable die space due to their small resistance values. 
     Those skilled in the art will recognize that the “N-2N” array of FIG. 3 has a balanced, or “differential”, structure in that the array is connected across the upper and lower voltage references and is not referenced to ground. However, alternative balanced or unbalanced array structures may be employed that achieve some or all of the advantages of the present invention. 
     Those skilled in the art will also recognize that where the digital word to be converted contains more or less than six bits, either, or both, the first and last divider stages may be adapted to handle more or less than three bits. When the above-discussed limitations prevent an increase in the size of the divider stages sufficient to accommodate the desired number of bits, the DAC may be readily adapted to have more than the illustrated two stages by adding one or more intermediate stages between the first stage  20  and the second stage  40 . The added intermediate stages may be a constant impedance resister string having resistors of matched resistance values or, alternatively, may be a binary-weighted array. Preferably, any such intermediate stage is a constant impedance resister string, since both an upper-level voltage reference output and a lower-level voltage reference output must be generated by each intermediate stage for the following stage and this requirement may add complexity to the above-described “2N-N” array which is configured to generate only a single voltage output. 
     The disclosed digital-to-analog converter may be used in a wide variety of applications. Furthermore, it should be appreciated by those skilled in the art that the digital-to-analog converter may alternatively employ p-type MOS (PMOS) devices to compose a “P-2P” array. In a converter employing PMOS devices, the voltage polarities are generally the reverse of those shown. For example, whereas the gate voltage for a logic “one” level is positive with respect to the source for an NMOS device, the gate voltage for a logic “one” level is negative with respect to the source for a PMOS device. The resistance of a PMOS device is typically twice the resistance of an NMOS device having the same area. This may make the use of a PMOS device preferable in circuits where minimal loading of the prior stage is desired. 
     Having described a preferred embodiment of the invention, it will be apparent to one of skill in the art that other embodiments incorporating its concepts may be used. Accordingly, the invention should be limited only by the spirit and scope of the appended claims.