Patent Publication Number: US-3877021-A

Title: Digital-to-analog converter

Description:
5 I Unlted States Patent 1 1 1111 3,877,021  
 Raamot Apr. 8, 1975 i [54] DlGlTAL-TO-ANALOG CONVERTER 3.581.303 5/l97l Kelly 340/347 DA I 3.602.799 8/1971 Guillen 323/4 [75] Invent Raamoi Prmcetoni Ni 3.010.953 10/1971 Gordon et 111.... 340/347 DA 73 Assignec; western Ehctric Company, New 3.685.045 8/1972 Pastoriza 340/347 DA York, N.Y. OTHER PUBLICATIONS Filed? 1973 Breedlove, Ladder Decoder,&#34; IBM Technical Disclo- [211 APPL 337,347 sure Bulletln, Vol. 3, No. 7, 12/1960, p. 17.  
  Related Application D Primary E.\&#39;aminerTh0mas J. Sloyan I Continuation of 5 p i 1971. Attorney, Agent, or Firm-B. W. Sheffield; 1. L.  
 abandoned, which is a continuation-in-part of Ser. Stave&#34; No. 805,543, March 10. 1969, abandoned.  
 152 vs. 01. 340/347 DA; 307/00; 323/22 T [571 ABSTRACT [51] Int. Cl. H03k 13/04 The invention comprises a digital-to-analog converter [58] Field of Search 340/347 DA; 307/33, 60, having a high degree of resolution. An unregulated /297; 32l/I7. I8; 2 T, I voltage source supplies current to a network of binaryweighted resistors through a corresponding plurality of I References Cited selectively actuated switching circuits. A novel paral- UNITED STATES PATENTS lel-current voltage regulating circuit is connected to a 3.247397 4/1966 Kopek etal 340 347 DA highly accufme and regPlates&#39; 3.343.073 9/1967 Mesenhimcr 323/15 Voltage PP I0 1n ry-we1g hted res1stor after 3.375.435 3/1968 Baugher 333 22 T X the operation of the assoclated swltchmg clrcult. An 3,396,380 8/1968 Ohashi 340/347 DA 18-bit converter having an accuracy greater than 4 3.403393 9/1968 Skrenes 340/347 DA parts per million has been realized. 3,475.74) 10/1969 Plice 340/347 DA g 1 3.544.094 12/1970 Hanson et aI. 340/347 DA 5 ClaIms, 10 Drawing Figures CIRCUIT REGULATION CIRCUIT P.:&#34;* .TEHTEUAPR W5 A 2.877, 021 A FIG. lb PR/oR ART DIGITAL INPUT 8 S S E 000 OPEN OPEN OPEN O=.GROUND OOI OPEN OPEN CLOSED 2/22 E 0 0 OPEN CLOSED OPEN 4/22 E Ol I OPEN CLOSED CLOSED 6/22 E,  
 IOO CLOSED OPEN OPEN 8/22 E, lOl CLOSED OPEN CLOSED IO/22 E, ||o CLOSED CLOSED OPEN l2/22 E Ill CLOSED CLOSED CLOSED l4/22 E PR/0R ART DIGITAL l INPUT AMI/M06 OUTPUT I R E refi 2 D F/GZ la 5 4R V E, INVENTOR ART i a J. RAAMOT ATTORNEY L. REFERENCE VOLTAGE 0 Ev R Q L y l UNREGULATED VOLTAGE R SHEET 3 (IF 4 FIG. 6  
 PARALLEL CURRENT REGULATION CIRCUIT PARALLEL CURRENT REGULATION CIRCUIT PARALLEL CURRENT REGULATION CIRCUIT PARALLEL CURRENT REGULATION CIRCUIT PARALLEL CURRENT REGULATION CIRCUIT SWITCHING CIRCUIT (30 /5 SWITCHING J CIRCUIT 3 SWITCHING CIRCUIT (30 1 SWITCHING J CIRCUIT J30 SWITCHING CIRCUIT SWITCHING CIRCUIT PARALLE L CURRENT REGULATION CIRCUIT INPUT ,+E ref R I02 W s yofl ||9 L&#39; ref D IOI gme  #0 v /(4O SWITCHING PARALLEL CURRENT CIRCUIT REGULATION CIRCUIT 2R i I SWITCHING PARALLEL CURRENT C CIRCUIT REGULATION CIRCUIT 2 (3O M40 SWITCHING PARALLEL CURRENT CIRCUIT REGULATION cIRcUIT 32R 1 SWITCHING PARALLEL CURRENT ll9 CIRCUIT REGULATION CIRCUIT ISO I (40 BAckoRouNDoF rHEiNvENTloN 1. Field of the Invention i 1 This invention relates to a digital-to-analog converter and more particularly to a digital-&#34;to-analog converter having a high degree of resolution.  
  2. Description of the Prior Art A digital-to-arialog converter is a device for converting electrical signals in digital form into their analog equivalent. Such converters are widely used as an interface between a digital computer and a utilization device which cannot be directly operated by digital signals. For example, a digital-to-analog converter is used to convert digital signals generated by a computer into analog signals to control a machine tool in a computercontrolled machine-tool operation.  
  Digital&#39;comp&#39;uters are, of course, inherently accurate machines, their accuracy being limited solely by the size of the digital words that can be accommodated in the registers and accumulators of the computer. Double and triple length words may be used for extended precision, in some instances. The commercial digitalto-analogconverters which are currently available, however, are not capable of converting digital input signals into their analog equivalent with the same degree of resolution inherently possessed by the digital input signals. Prior art digital-to-analog converters typi- Cally comprise a network of binary-weighted resistors connected to a common summing resistor. Aseries of switches associated with the binary-weightedresistors selectively connect a regulated voltage to each resistor in the network, in accordance with the digits of the binary word to be converted. The correspondingly weighted currents which&#39;flow in the binary-weighted resistors are summed in&#39;a summing resistor or in&#39;a load to produce&#39;thedesired analog signal.  
  Digital-to-analog converters of this type suffer from several disadvantages; First, the energizing voltage for the network of binary-weighted resistors must be highly regulated. This is difficult to do because the voltage source must supply a considerable&#39;amount&#39;of current andmust have a relatively high potential, if-any reasonable degree of resolutionis required. While voltage sources having a high degree of regulation accuracy are commercially available, these sources are accurate only when supplying a constant or slowly varying load. When connected to a rapidly and constantly varying load, as is found for example in a digital-to-analog converter, the regulating accuracy drops to a much lower figure, typically 0.01%. This drop in accuracy is caused tively expensive and the above-mentioned limitation i the degree of voltage regulation which can be obtained of necessity limits the r e soluti on o f-priorart digital-to analog converter to approximately. 13 bits. Additionally, in the prior art,. theregulation of the energizing voltage source alwaysoccursbefdre the-source ,is  
 switched to energize the &#34;binary-weightedresistors. Thus, if electromechanical switching devices are used, contact resistance also becomes limiting. On the other 2 hand, if-semiconductor switching is used, the&#39; wide manufacturing, variations between semiconductors of the same type andthe consequent unpredictable voltage drops through the transistors when conducting, are also limiting. The useof field-effect transistors promises to improve this situation slightly, but by not more than onebit, at the very best. Thus, practically speaking. prior&#39;art digital-to-analog converters are inherently limited tea resolution of less than 14 bits.  
  The novel circuitry of the digital-to-analog&#39;converter disclosed herein, however, can attain an accuracy and degree of resolution which are both at least one order of magnitude better than that obtained in the prior art. An experimental 18-bit digital-to-analog converter has been constructed and operated. The resolution of this experimental converter is limited only by the state of the art, which at the present time can achieve resistor values and reference voltage sources accurate to one part per million. As the art progresses it will be possible to increase the resolution of di&#39;gital-tofanalog converters constructed according to the p rinciplesof this invention as well as those constructed according to the teachings of the prior art, but the novel circuitry disclosed herein is suchth&#39;at theinstant digital-to-ahalog converter will always have an accuracy at least one order of magnitude greater than the prior -,art. A preferred embodiment pf the.inventionlcomprises a network of binary-weighted resistors connected to a summing load. An unregulated voltage source is connected through a correspondingseries of switching cir cuits to each resistor in thenetwork. These switching circuits are selectively energized, in accordance with the digits of the binary numbentobeconverted. and connect the unregulated voltage source to each binaryweighted resistor in the network to thereby generate the analog signal. 7:  
  An important feature ofrt he invention is the highly accurateregulation of the unregulated energizing fvoltage after this&#39;unregulatedvoltage has been switched&#39;to energize each binary-weighted resistor. This regulation is accomplished by a-novelparallel-current voltage regulating circuit comprising a highly accurate source of reference potential, and anoperational amplifier having a&#39;feedback loop connected to.v supply a compensatory regulating current to the. resistor, inparallel with the current supplied by the unregulated source, thereby maintaining the potential developed across the binaryweighted resistor equal to the potential of the reference voltage. M  
  An alternate embodiment of the invention uses a difference amplifier instead of an operational amplifier in the parallel-current voltage regulation circuit. This substitutionsimplifies the circuitry to such an extent that the components for a low precision, high speed digitalto-analog converter having-,for example, a six-bit digital input, may be; incorporated on a single monolithic circuit chip; Six-bit converters implemented on single circuit chips exist in the prior art, however, these converters do not use parallel-current voltage regulation after switching. Typical priorart circuits require an operational amplifier in the ou tpu t circuit to provide a reasonable output voltagelswing, but this limits the settlingtime of the circuit. Such an operational amplifier is riotn&#39;eeded in the output circuit of a digital-to-analog converter which, according to the present invention, uses parallel-current voltage regulation; and which,  
 chit according toFlG. 3a which therefore, can be built to exhibit a much faster settling time.  
  BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1a is a schematic diagram of atypical priorart digital-to-analog converter; v  
  FIG. lb is achartshowing&#39;the operation of the con; verter shown in FIG. la and is helpful in, understanding the operation of both the prior art converter shown in FIG. la and the instant invention;  
  FIG. is a schematic drawing ofa portion of another prior art digital-to-analog converter which illustrates the use of tran sistorized switching circuitry; H FIG. 3a is a schematic drawing of a parallel-current voltage regulating circuit suitable for use with the present invention; v I g FIGI3bis a schematic drawing illustrating an alternativear&#39;rangement for the regulating circuit shown in FIG. 30? 1 v FIG. 4ais a simplified schematic drawing of the regulating circuit&#39;ish oiwn&#39; in FIG. 3a which is useful in the I analysis thereof;  
  another Lsimplified schematic drawing of circuit shown in FIG. 3a which is also useful in tl ie mathematical analysis thereof;  
  FIG/&#39;5 is a s chematic drawing of an alternative embodiment of th&#39;e&#39; parallel-current voltage regulating cirpro&#39;vidsan even greater&#39;dgi&#39;ee of&#39; regulation;  
  FIG. 6f &#39;i&#39;s&#34;a schematic drawing of an illustrative embod iinent of &#39;a digi&#39;tal-to-analog converter according to the present&#39;inv&#39;ention; and I FIG. 7 is a sche&#39;maticdrawing of a low precision, high-speed, 6-bit digital-to-analog converter, using difference amplifiers for regulation.  
 DESCRIPTION OF THE PREFERRED I EMBODIMENT FIG; lti&#39;is a -schematic drawing of a 3-bit digital-toana&#39;log converter which is typical of the prior art. In this converter a ladder network of three binary-weighted resistorsi- R, 2 R and 4R, is connected at one end via a summing resistor R, to ground. Switches 5,, S and S represent electromechanical switching devices which can-selectively connect the free end of any of the binary-weightedresistors either to ground or to&#39;a com mon &#34;reference voltage E,.,. The switches S through 8;, areshown in their normal, or open, position and in that position the switches connect ground to each of the binary-weighted resistors; When operated, however, each of the switches S, through S applies the potential +E to the associated binary-weighted resistor causing current flow therethrough. The output voltage, E developedacross the summing resistor R, may be calculated by taking the ratio of resistor R,- and all binary-weighted resistors in parallel. with it to the sum of that quantity just calculated and the parallel combination of all binary-weighted resistors connected through operatedswitches to the reference source. Because the binary-weighted resistors. have resistance values which increase according to the powers of 2, the output voltage E will increase linearly when switches 8, through S are selectively closed in binary order.  
  FIG. lb illustrates the operation of the.converter in FIG. la and shows the condition of switches S,,.through S for various combinations of input signals as. well as the output voltage E developed across the summing resistor R for each combination. This table assumes that R R: If desired, the circuitry can be arranged so that switchesS, through 5;, switch the binary-weighted resistors between E,,., and +E,.,., rather than ground and +E however, the linear relationship between the analog output E and the binary input will not be affected. In this lattercas e&#39; the graph of E, versus the binary input would be shifted downward and would pass throughthe&#39;E 0 point between the binary input numbers 01 l and rather than at the normal 000 point. This type of arrangement is used, for example, if the analog signal is to be used to control the deflection of an electron beam and it is desired that the rest position of the beam be at the center of thecathode ray screen.  
  By increasing the number of binary-weighted resistors in FIG. la, it ispossible to increase the resolution of such a. converter. However, as discussed previously, if more than 12 or 13 binary-weighted resistors are connected tothe converter, the change in output voltage E across summing resistor R,- as the last switch, corresponding to the least significant digit of the binary number to be converted, is operated, is so small that the reference voltage source E must be exceedingly well regulated. This is necessary so that no voltage perturbations will occur in E which even approach in magnitude the voltage change across R,- due to a change in the least significant digit of the binary number.  
  The cost and difficulty of regulating a power supply for use with a digital-to-analog converter having more than 13 bits has already been discussed. The contact resistance of electromechanical switches S, through S,, must also be less than the change in resistance which occurs as the last binary-weighted resistor is switched in and out of the circuit. This, too, limits the resolution of the digital-to-analog converter shown in FIG. la.  
  As previously discussed, this problem can be alleviated somewhat by the use of semiconductor switches. FIG. 2 is a schematic diagram of a portion of a commercially available 13-bit digital-to-analog converter which uses semiconductor switching elements. Referring to FIG. 2, when transistor T is turned .On by the application ofa binary one signal to the base of transistor T the voltage from source E,,; is applied to the binary-weighted resistor 2R, as shown. Similarly, the application ofa binary zero signal to the base of transistor T, turns transistor T Off&#34; and transistor T On thus connecting a ground to binary-weighted resistor 2R. The alternate operation of transistors T and T is analogous to the operation of one of the electrome-.  
 chanical switches in FIG. la. Obviously, the use of transistors in lieu of electromechanical switches eliminates the contact resistance problem but introduces yet another problem in its place. As is well known, when a semiconductor switching device is turned heavily On, it simulates a short circuit in that the resistance of the emitter-collector path is very low. There is, nevertheless, a finite voltage drop across a conducting transistor and this voltage drop must be taken into account in a high resolution digital-to-analog converter. At this time, it is still not possible to manufacture transistors accurately enough so that the collector-emitter voltage drop inthe Qn condition is uniform from sample to sample. Thus, difficulty is. experienced in calibrating the various. stages of va digitalvto-analog converter which uses. transistorized switching and if a transistor fails and must be, replaced, the entire converter must be recalibrated, as each stageinteracts with every other stage. Thus, while the use of transistorized switching circuits, particularly field-effect transistors, may increase the resolution of prior art converters by one or possibly two bits, the limitation imposed by the inability to accurately regulate the reference voltage source after switching occurs are still limiting.  
  The instant invention successfully eliminates the aforementioned problems and provides a digital-toanalog converter which is significantly superior to anything heretofore attainable. This superiority is attained by the novel concept of regulating an unregulated voltage source after it has been switched to generate current flow in a binary-weighted resistor and by using a unique parallel-current voltage regulating circuit to maintain this regulation to a very high degree of accuracy.  
  FIG. 3a depicts one embodiment of the parallelcurrent voltage regulating circuit utilized in the instant digital-to-analog converter. Referring to the drawing, a circuit 11 suppliesa reference voltage from an external voltage source 12 which may advantageously comprise a standard cell having an open-circuit voltage E, and an 7 internal impedance R}. The standard cell, whose voltage must be accurately known, for example, to at least one part per million, is maintained at a constant temperature by the use of an oven. The circuit 11 connects the reference voltage source 12 tothe input 13 of an operational amplifier 14 via a resistor R The other input of operational amplifier 14 is grounded. In the experimental digital-to-analog converter actually constructed and operated, a commercially available Analog Devices Model 211 operational amplifier was used. This amplifier has a gain ofA an input resistance of 0.5 megohms, an output resistance of essentially 0 ohms, and a noise factor of IO microvolts r.m.s. Obviously one skilled in the art could substitute other types of operational amplifiers for the one used in the experimental embodiment provided that the characteristics are similar. The output 15 of operational amplifier 14 is connected by a feedback loop 16 to the input 13 thereof. An output resistor R and a resistor R, are connected in the feedback loop. R,,, the load resistor to be energized, is connected between ground and the juncture of resistors R and R in feedback loop 16, A second circuit 18 connects an unregulated voltage from an external voltage source 19 to the juncture 17 of resistors R and R via a resistor R The unregulated voltage source I9 advantageously comprises any suitable source having a voltage E, and an internal impedance R,..  
  In operation, the unregulated voltage source 19 supplies a current through R, and R,, to ground, and the voltage E,- which is developed across load resistor R at juncture 17 is regulated so that it equals the open circuit voltage E, of reference voltage source 12 to a high degree of accuracy, regardless of wide variations in the magnitude of the voltage from unregulated voltage source 19. If, as in the illustrative embodiment, the potential of reference source 12 is known to within 1 part per million then resistors R R and R, must also be accurate to within one part per million-or better.  
  Assume that unregulated voltage source 19 is supplying a current i to load resistor R and that the voltage developed across juncture l7 is E The potential of unregulated voltage source 19 isfseen. by the input of operational amplifier 14 through resistors&#39;R, and R R being in feedback loop 16, Input 13 of operational arn plifier 14 also sees the potential from reference voltage source 12 through resistor R In order that the operational amplifier will regulate overthe ,desired rang e, the magnitude of resistors R and R are selectedsuch that R is approximately equivalent to the series comb ination of R and the internal resistance R, of reference voltage source 12. Typically, R, is negligibly smalland, thus in practice, R will equal R,. If the potential of unregulated voltage source 19 should vary for any reason, the magnitude of voltage E will also tend to vary. This variation is sensed at the input of operational amplifier 14 and an output currentjs generated in feedback loop 16. A fraction, i of this current will flow through load resistor R Since-i is in parallel with i,,-th e currentsupplied from feedback loop 16 will tend to alter the magnitude of E in an offsetting manner, and maintain E, equal to the voltage of reference voltagesource 1 2, For this to occur, it is necessary that the magnitude of resistor R, be selected so that its resis&#39;tance,together with the internal impedance- Riof unregulated voltage source 19, approximately equals the resistance of load resistor R Load resistor R will, of course, vary in resistance for each stage of the digital-to-analog converter. 1  
  A mathematical anal ysis&#39; o&#39;f this circuit is set forth below and-this analysis proves that theparallel-current voltage regulating circuit shown in FIG. 3 does indeed maintain the voltage E, developed across load resistor R equal to the voltage of reference voltage source 12.  
  FIG. 4a shows the circuit of FIG. 3a redrawn in simplified form. The internal -resistance R,- of reference voltage source 12 and the internal resistance R,. of unregulated voltage source 19.,hav e been ignored, as typically they are very s mall compared to R and R respectively. If they are not small compared to R and R the following mathematical analysis is still valid provided that R and R are substituted proof where v FIG. 4a itself may be further simplified, as shown in Assume that currents i i and 1}, flow asshown in FIG. 4b. No current is shown flowinginto the operational amplifier as&#39;it normally.requires no input current.  
  Now the currents i i and i may be found from the following equations.  
 in the following Therefore, from Equations 5, 6, 7 and l l r s)/( l 2) n sV O rl/ r and from Equations 8 and I0 E0 &#39;A R1/(R1+ R2) r A R2/(R1 R2). s  
 From Equations 12 and 13, we obtain Therefore E. E. (RORT A Ran/1m&#34; R1)(R| R2) RORT A RIRT] Now from Equation 3 RT R4 L/( -l 1.)  
 and  
  4 mpiml z 300 ohms RI. luplr&#39;ul ohms Thus RT luplru! z X 0 )/800 200 ohms Further, in a typical digital-to-analog converter stage R0 lmm&#39;ul z 500 ohms R1 uun&#39;cul z l0,000 ohms 2 umlrul z l0,000 ohms and (Ill  typical a: 10 Substituting these values into Equation 18, we get EV[(500)(200) (l0 )(l0,000)(200)] I ET(20,O00)(5O0) E, 10 200 10 1 E (l00) s 1400 +10 200 10 140 1+ 20 +10 Therefore E..- [E,(10** /(1 1 Er/(I z (ET/10) Ignoring this latter term, which is exceedingly small, we get E,- -E, and the effect of E (and thus E,.) upon E, may be ignored.  
  Another important feature of the parallel-current voltage regulating circuit shown in FIG. 3a is its excellent response to transient switching pulses. An experimental regulator, constructed in accordance with this invention, was found to have a settling time of less than one microsecond. This figure is suitable for virtually all applications which require a digital-to-analog converter of this degree of resolution.  
 FIG. 3b shows a modified version of the circuit dis closed in FIG. 3a. In FIG. 3b a diode 21 is connected between output lead 15 and input 13 and poled so that if the output voltage E of operational amplifier 14 goes positive, diode 21 conducts shorting out the operational amplifier 14. Obviously, diode 21 could be poled in the opposite direction if it is desired to shortout operational amplifier 14 when the output voltage goes negative.  
  It is possible to improve still further the parallelcurrent voltage regulating circuit shown in FIG. 3a. As shown in FIG. 5, this is accomplished by substituting for the external reference voltage source 12 in FIG. 3a, an unregulated voltage source which itself has been regulated by a parallel-current voltage regulating circuit of the type disclosed in FIG. 3a. This process may be repeated over and over again although practical considerations make the use of more than two parallelcurrent voltage regulating circuits in tandem of doubtful value. Referring now to FIG. 5, a circuit 11 connects a reference voltage source 12 to a resistor R thence to the input 13 of an operational amplifier 14 A feedback loop 16 which includes an output resistor R and a resistor R connects the output 15 of operational amplifier 14 to the input 16&#39;. A load resis- The regulated voltage E1, developed across resistorhr R is next connected via circuit 11 and resistor R to the input of a second operational amplifier 14 and the remainder of its circ&#39;uit and the operation thereof; is  
 identical to that describedwith reference to FIG. 3a. The circuit of FIG: 5 accomplishes the substitution of 1 via the slider arm thereof to input 122a. The other end E the voltage developed&#39;across resistorR K-for the reference voltage-source 12 normally use&#39;d. Unlike a standard cell, however. the circuit arrangement of FIG. 5 can supply significant current from the new reference source E, which increases the range and accuracy of overall regulation. 1  
 of trimmer resistor 13011 .is connected via a resistor 125a to an externalreference voltage E,,.,. The output 123a of operational amplifier 121a is also connected FIG. 6 shows apreferred embodiment ofa digital-toanalog converter according to the invention. With the exception of minor changes in the resistance of some components caused by a corresponding change in the resistance of the associated binary-weighted resistor, each stage of the converter&#39;i s identical, thus only the first is illustrated and described in detail.  
  As discussed in connection with FIG. 2, in the preferred embodiment, the potential applied to the various binary-weighted resistors in the ladder network is switched&#39;between +E,. and E,.. However, it-will be appreciated that the circuitrydisclosed is equally suitable for switching between iE,- and ground, should this be desired. lf this were done, it would, of course, be necessary to regulate the ground potential, as well as the supply source, if a high degree of resolution is required.  
  In FlG.-6 a network 101 comprises a plurality of binaryeweighted resistors R, 2R, 4R. .&#34;2&#34;R-connected at one end. Network 101-is connected by a lead 102, to an external load 103 which, in the preferred embodiment shown, is the deflection coil of an electron-beam deflection device. The series combination of load 103 and one of the binary-Weighted resistors corresponds to load resistor Rjjin FIG. 3a. It will be appreciated, however, that load 103 maybe any external load capable of working with the digital-to-analog converter disclosed. i a  
  Input 104, is connected by leads 106a and 10612 to essentially identical upper and lower halves of&#39;the first stage of the converter. Again, for simplicity,&#39;only the upper half of the first stage will be discussed in detail, although both are illustrated. The circuitry and operation of the lower half is entirely analogous to the operation of the upper half, provided, of course, that appropriate corrections are made for the change in polarity of the supply source. l  
  Returning now to the input 104,, lead 106a connects the input via a resistor 107a and a capacitor 108a to the base of transistor Ola Resistor 109a connects biasing potential from source E,- to the base of transistor Ola. The emitter of transistor Ola is connected to a second biasingpotential -E,, via a lead 111a. The collector of transistor Ola is connected viaa resistor 112a and a ca-.  
 pacitor 113a to the baseof transistor 02a. A biasing resistor 114a connects the base-of transistor 02a to its emitter and thence via a lead ll-6a&#39;to an unregulated voltage source +E,.. The collectorv of transistor 02a is connected by a resistor. ll7ato a feedback loop 1118a,  
 accomplish this regulation, after switching, an operational amplifier 121a having an input 122a and anoutvia a pairof diodes l31aand 132a to the input 122a thereof. A resistor 133a connects the midpoint of diodes 131a and 132a to ground.  
  As previously discussed, the lower half of the first converter stage is essentially identical to the upper half just described. However, it will be noted that the polarity of transistors Qla and Olb and O2a and 02b are reversed. Similarly, lead lllb connects the emitter of transistor Qlb to ground rather than to the biasing voltage -E,; and resistor 10912 connects the base of transistor Olb to the biasing potential +E,. rather than E,.. Similarly, lead 116b lconnects the emitter of transistor O2b to E, rather than +E,. and the polarity of diodes 126b, 131b, 132k and Zener diode l27b are also reversed.  
  :ln operation, assume that a very low potential, illustratively zero volts, is applied to input 104, and is indicative of a binary one&#34; input to the first stage of the converter. Because the emitter of transistor Olb is grounded, when zerovolts is applied to the base thereof there will be, of course, zero volts on the base-emitter junction of transistor Qlb. Thus, the large positive potential +E,. which is applied to the base of transistor Q11; via biasing resistor 10912 maintains transistor Qlb positively turned Off. This is turn keeps transistor O2b Off which inhibits the application of the -&#39;-E,. voltage present on lead 11611 to &#39;thejunc&#39; tion point 119, and binary-weighted resistor R.  
  Resistors 107a and 10712 prevent transistors Ola and Qlb from loading down the input signal on input 104 and capacitors 108a and l08b are provided to speed-up the transient response of transistors Ola and Qlb. The zero potentialwhichis applied,&#39;-vi&#39;a lead 106a, to the base of transistor Ola turns transistor Ola heavily On, by virtue of the negative potential E,, connected to the emitter thereof by lead 111a. The resistor 109a which connects thebase of transistor Ola to&#39;biasing source E,- plays no part inlthe operation of thecircuit at this time. This connection is provided to ensure that transistor Qla is positively turned Off when the input on lead l04 -corresponds to a binary zero&#34; and tran sistors Qlb and O2b are conducting.  
  When transistor Ola is turned On the potential at the collector thereof becomes approximately equal to the negative potential -E,, which is present on its emitter. The base of&#39;transistor O2a on the other hand, is maintained positive because of the positive potential +E,. present on its emitter via lead 116a. Current therefore flows down through current-limiting resistor 112a into the nowconducting transistor Ola and this downward current flowdraws current from the base of transistor 02a and .turns transistor OZa heavily On permitting currentzt o flow-from source +E,. through resistor 117a to junction ll9 and the first binary-weighted resistor R, thence via summing circuit 102 to the load 103.  
  The value of resistor 112a is selected to limit the current flow through transistor Ola to a reasonable figure and capacitor 11311 is provided to speed-up the transient response of transistors Ola and O2a.  
  As previously discussed with reference to FIG. 3a, the potential at junction 129a is regulated by means of operational amplifier 121a and feedback loop 118a so that the voltage at junction 129a is maintained equal to the potential of reference voltage E, Trimmer resistor 130a, which is typically a 2 O-turn wire wound potentiometer of approximately l ohm resistance. is-provide d .for alignment purposes so that the potential at input 122a to operational amplifier 121a is maintained at zero volts. I  
  Also as previously discussed in connection with FIG. 30, if for some reason the potential at junction 1290 should vary from the reference potential E,,.,, a compensating, parallel current is supplied to junction 119, andbinary resistor R from feedback loop 118a to maintain the voltage at junction 129a and hence junction ,typically volts, the remaining 5 volts being dropped across resistor 1 24a. Diode 126a is provided to block leakage current from operational amplifier 121a when current is being supplied to resistor R by the lower half of the circuit, which, of course, only occurs when a binary zerosignal is present an input 104,. Diodes 131 a and l32a-andresistor 1330 are connected across operational amplifier 121a to short it out in this latter condition when operational amplifier 12111 is functioning. I  
  The operationof the circuit when-a binary zero signal is applied to input 104 is entirely analogous. The potential applied to input 104, in thatcase is typically 3 volts, thus the biasing potential ,E,. applied to the base of transistor Ola will turn Ola Off which, in turn, will turn transistor Q2a Off disconnecting the positive source +E,. from junction 119, and binary-weighted resistor R. The -3 volt potential applied at input l04, is passed to the base oftransistor Qlb via lead I06b and, by virtue of the ground present on the emitter of transistor Qlb from lead .lllb, turns transistor Qlb On. This, in turn, turns Q2b On passing the E,. potential on lead 1161; to junction I19, and binary-weighted resistor R. thence via summing circuit 102 to the load 103 as beforeOperationahamplifier 12112 and feedback loop 1181; act &#39;to maintain the potential at junction 12% equal to the potential +E applied to the input of operational amplifier 121b, in amanner entirely analogous to the operation of the upper half of the circuit.  
  As previously discussed, under this condition, operational amplifier 121a will be shorted by diodes 132a and 131a and diode 126a will prevent any leakage to sentially identical except for minor changes in resis.-: tance values, etc. Thus, the application of input signals and the opera-&#34; to input circuits 104 through 104 tion of switching circuits through 30 and 30 through 30 as well as parallel-current voltage regulatingcircuits 40 through 40 and 40 through 40 need not be discussed in detail.  
  ..,It is essential, in the first stage of the converter, that resistors a. 125b, 128a, 128b as well as binaryweighted resistorR-be asaccurate as the potential of the reference voltages &#39;+E;,. and E;,.;. In the illustrative embodiment of the invention actually constructed, these potentials were known to within; 1 part per million. In succeeding less-significantstages of the converter, these tolerances may be reduced, for example, to 2 parts per million, 4 parts per million, 8 parts per million, etc. As. previously discussed, the reference voltages +E,.,., and E,,., are advantageously obtained from a standard cell. The standard cell is advantageously maintained in a controlled environment in a temperature controlled oven. It may also be advantageous, under some circumstances, to maintain resistors 1250, 125b, 128a and 12812 and the binary-weighted resistors in a temperature controlled oven, although this has not been found necessary in the experimental 18- bit digital-to-analog converter actually constructed. One advantage of switching the binary-weighted resistors between +E,. and -E,. rather than Hi, and ground is that there is always a current in the binary-weighted resistors, regardless of the input signals to the various stages. Thus, the binary-weighted resistors are subject to constant FR heating and, therefore, less likely to alter their resistance value, which would, of course, affect the accuracy of the digital-to-analog conversion.  
  FIG. 7 shows an alternate embodiment of the digitalto-analog converter, according to this invention, in which difference amplifiers are substituted for operational amplifiers 121a and 121b, shown in FIG. 6, and in which certain changes are made in the switching circuits. The 6-bit converter shown in FIG. 7 encompasses a number of components which may be fabricated on a single monolithic circuit chip, using presently available technology. However, it will be apparent to the practitioner that converters with more than six input bits could be built on a plurality of monolithic chips, or on a single larger chip, if fabrication technology permits. Conveters with less than 6 input bits may also be constructed.  
  In FIG. 7, certain elements are numbered identically to like elements in FIG. 6, as the overall operation of the. circuit in FIG. 7 is similar to the operation of the circuit in FIG. 6. However, switching circuits 30 and parallel-current voltage regulation circuits 40 in the schematic of FIG. 7 are different from like elements in the circuit of FIG. 6. These differences will become evi-.  
 dent from the description of FIG. 7 which follows.  
  Referring now to FIG. 7, simplifications have been made in switching circuits 30 to eliminate capacitors and to reduce the number of resistors. Switching circuits 30 are now shown for operation with an input signal having either a positive voltage level, typically +3 volts, or ground as its two input states. Such an input signal is compatible with the positive signal levels currently used in.systems using monolithic circuits. However, the modifications necessary to permit switching circuits 30 tooperate with negative-going or bipolar input signals would be apparent to the skilled practitioner. L  
  As in the previous description, only the upper half of the most significant stage of the converter will be described in detail. Input 104,, is connected by lead 1060 to the base &#39;oftransistorQla. The emitter of transistor Ola is connected through resistor 140a to ground. The collector of transistor Ola is connected by lead 141a to the baseof transistor 02a. Biasing resistor 114a connects the base of transistor O to its emitter, which is also connected to unregulated voltage source +E,.. The collector of transistor 02a is connected through resistor 1170 to leads 142a and 143a. Lead 142a connects to point 119,, and binary-weighted resistor R which further connects to summing circuit 102, and load 103. Lead 143a connects through resistor 144a to the base of transistor Q 3a,and through diode 146a to the collector of transistor O3a. The base of transistor 03a is also connected to ground through resistor 145a. Diode 147a is connected between the emitter and base of transistor 03a. The emitters of transistors O3a and O4a are connected together, and to ground through resistor 148a. The base of transistor 04a is connected to reference voltage +E and the collector of transistor-O4a is connected to unregulated voltage source +E,..  
  As previously stated, the lower half of the first stage is essentially identical to the upper half just described. However, it will be noted that transistors Qlb, O21), 03b and O4!) in the lower half areof opposite polarity compared to their counterparts Ola, O2a, 03a, and  
 04a in the upper half. Also. the emitter of transistor 02b and the collector of transistor O4b connect to an unregulated voltage source -E rather than +E,.. Similarly, the base of transistor 04b is connected to reference voltage E,,.; rather than +E,.,.,. The emitter of transistor Qlb is connected through resistor l40b to biasing voltage source +E,, rather than ground. The magnitude of +E,, is chosen so that during the application ofa positive voltage level on input .104 transistor 0111 will be turned Off, and during the application of a ground or zero voltage level on input 104, transistor Qlb will be turned On. Diodes 14612 and l47b in the lower half are reversed with respect to their counterparts in the upper half.  
  In operation, assume that a positive voltage level appears on input 104,, which is indicative of a binary one input to the first stage of the converter. Transistor Olb will be turned Off by this positive voltage level. This in turn maintains transistor O2b Off which inhibits the application of voltage -E,. to junction point 119, and binary-weighted resistor The positive voltage level on input 104 is applied via lead 106a to the base of transistorOla. Since the emitter of transistor Ola is connected through resistor 140a to ground, a current will flow through the emitter-base junction of transistor Ola. This emitter-base current turns transistor Ola On.  
  When transistor Ola is turned On, current flows from source +E,. through resistor 11411, the collector of Ola, and resistor 140a to ground. Resistor 140a limits the magnitude of this current. The resulting voltage drop across resistor 114a causes current to flow through the emitter-base junction of transistor Q2a, turning transistor O2a heavily On, permitting current to flow from source +E,. through resistor 117a to junction 119 and binary-weighted resistor R to summing&#39;circuit 102 and load 103.  
  &#34;The potential at junction 1290 is regulated by means of the difference amplifier comprising parallel-current voltage regulation circuit (40 )21 so that the voltage at junction 192a is maintained constant with respect to reference voltage +E Typically, +E will be half the magnitude of the voltage to be maintainedat junction 12911.. Resistors 144a. and 145a are chosen so that the voltage appearing on the base of transistor 03a will also be approximately half the voltage at junction 12%.  
 &#39;The voltage onthe emitters of transistors 03a and 04a will be maintained at slightly less than +E, by transistor O4a. If the voltage at junction 129a increases for any reason, the voltage on the base of transistor q3a will also increase momentarily, causing increased current to flow through the emitter-base junction of 03a. This increased emitter-base current in transistor 03a causes a corresponding increase in collector current, which flows from junction 129a through diode 1460. This increased collector current must be drawn through resistor 117a, causing a larger voltage drop thereacross, decreasing the voltage at junction 129a. The converse action results if the volt-age at junction 129a decreases for any reason.-lt can be seen, therefore, that the difference amplifier comprising regulation circuit (40,)a regulates the voltageat point 129a by varying the current flowing in lead 143a, resulting in parallel-current voltage regulation.  
  Diodes 146a. and 147a are provided to protect transistor O3a when junction 129 is at the negative potential which results when a binary zero appears at input 104. Diode 146a protects the collector-base junction of 03a, and diode 147a protects the emitter-base junction of transistor O3u when junction 129a becomes negative. u  
  The operation of the circuit when a binary zero signal is applied to input 104, is analogous to the operation described above for a binary one. The potential applied to input 104, to represent a binary zero is typically close to zero volts, or-ground potential, which thereby turns transistor Ola Off and transistor Olh On. Transistor O2 is thereupon turned On, passing the E,. potential through resistor 117b to point 119,, resistor R, summing circuit 102, and load 103. Parallel-current voltage regulation circuit (40,)b maintains the voltage at junction 12% at the correct negative potential in a manner entirely analogous to the action of regulation circuit(40,)a described above.  
  The operation of the other stages in the converter shown in FIG. 7 is identical to the operation of the most significant stage described above, and need not be. described in detail. 7  
  It can beseen that the operation of the alternate embodiment digital-to-analog converter,v using difference amplifiers to perform parallel-current voltage regulation, is similar to the operation of the preferred embodiment using.operational amplifiers for regulation. However, the alternate embodiment, typically cannot provide the same degree of precision as the preferred embodiment. As previously mentioned, the preferred embodiment is capable of operation ,in configurations of up to 18 bits having a conversion accuracy of4 parts per million. The alternate embodiment converter is intended for use in applications needing fewer bits, typically 6; and lower conversion accuracies, typically 1 part per thousand. The advantage of the alternate embodiment converter, however, which distinguishes it from prior art converters, is. its faster conversion time.  
  While there has been shown and described the novel features of the invention according to a preferred embodiment and an alternate embodiment, it willbe understood that various omissions and substitutions in the circuitry illustrated and inits operation may be made by those skilled in the art without departing from the spirit of the invention. l  
 What is claimed is;  
  1. In a digital-to-analog Converter of the type including a plurality of binary-weighted resistors each having a first terminal connected to acommon load and a sec ond terminal selectively connect&#39;able by either a first switching circuitto a source of positive energizing voltage or a second switching circuit to a source of negative energizing voltage, &#39;the improvement comprising for each binary-weighted resistor:  
 first means for comparing the voltage at the second terminal with voltage from a first external reference source; I  
 means for supplying a first current to the second terminal and, responsive to the first comparing means,  
 for varying the first current to maintain the voltage at the&#39;second terminal-substantially constant regardless&#39;of variations in&#39;t&#39;he voltage of the positive energizing source;  
 second means for comparing the voltage at the second terminal with a voltage from a second external reference source; and  
 means for supplying a second current to the second terminal and, responsive to the second comparing means, for varying the second current to maintain the voltage at the second terminal substantially constant regardless of variations in the voltage of the negative energizing source; wherein the first external reference source is positive;  
  the second external reference source is negative;  
 the combination of the first comparing means and the first supplying and varying means comprises:  
 a first difference amplifier having a sensing input, a reference input&#39;connected to the first external reference source, and an output;  
 a first circuit corinected betwee&#39;n the sensing input of the first&#39;difference amplifier and the second terminal of the binary-weighted resistor; and  
 a second circuit connected between the output of the first difference amplifier and the second terminal of the binary-weighted resistor; and  
 &#34;the combination of the second comparing means and the second supplying and varying means comprises:  
 a second difference amplifier having a&#39;sensing input, a reference input connected to the second external reference source, and an output;  
 a third circuit connected between the sensing input of the second difference amplifier and the second terminal of the binary-weighted resistor; and  
 a second circuit connected between the output of the second difference amplifier and the secondterminal of the binary-weighted resistor.  
  2. Adigital-to-analog converter according to claim 1, wherein said first and second switching circuits each include a resistor having a value chosen so that the sum of the resistance of each switching circuit and the internal resistance of each corresponding energizing voltage source substantially equals the resistance of said binary-weighted resistor.  
  3. In a digital-to-analog&#39;converter of the&#39;type including a plurality of binary-weighted resistors each having a first terminal connected to a common load and a second terminal selectively connectable by either a first switching circuit to a source of positive energizing voltage or a second switching circuit to asource of negative 16 energizing voltage,&#39;the improvement comprising, for each binary-weighted resistor:  
 first means for comparing the voltage at the second terminal with voltage from a first external reference source;  
 means for supplying a first current to the second terminal and, responsive to the first comparing means, for varying the first current to maintain the voltage at the second terminal substantially constant regardless of variations in the voltage of the positive energizing source;  
 second meansfor comparing the voltage at the second&#39;terminal with a voltage from a second external reference source; and  
 means for supplying a second current to the second terminal and, responsive to the second comparing means, for varying the second current to maintain the voltage at the second terminal substantially constant regardless of variations in the voltage of the negative energizing source; wherein the first external reference source is negative;  
 the second external reference source is positive;  
 the first current supplying and varying means comprises:  
 a first operational amplifier having an input and an output, and  
 a first circuit connecting the output of the first op- 1 erational amplifier to the second terminal of the I binary-weighted resistor;  
 the first comparing means. comprises:  
 a second circuit for connecting the input of the first operational amplifier to a negative reference source, v  
 a third circuit connecting the input of the first operational amplifier to the second terminal of the binary-weighted resistor, and  
 means for disabling the first operational amplifier when the second terminal is connected by the second switching circuit to a negative energizing source;  
 the second current supplying and varying means comprises:  
 a second operational amplifier having an input and 7 anoutput, and a fourth circuit connecting the output of the second operational amplifier to the second terminal I of the binary-weighted resistor; and  
 the second comparing means comprises:  
 a fifth circuit for connecting the output of the second operational amplifier to the positive reference source,  
 a sixth circuit connecting the input of the second operational amplifier to the second terminal of the binary-weighted resistor, and  
 means for disabling the second operational amplifier when the second terminal is connected by the first switching circuit to a positive energizing source &#39;4. A digital-to-analog converter according to claim 3 wherein the second circuit is a first resistor, the third circuit is a second resistor substantially equal in resistance to the first resistor, the fifth circuit is a third resistor, and the sixth circuit is a fourth resistor substantially equal in resistance to the third resistor.  
  5. A digital-to-analog converter according to claim 4 wherein said first and second switching circuits each include a series-connected resistor whose resistance is selected such that the sum of the resistance of the corresponding switching circuitand the internal impedance of the energizing voltage source connected thereto is substantially equal to the resistance of the associated binary-weighted resistor.  
  UNITED STATES PATENT OFFICE CERTIFICIATE OF CORRECTIO Patent No. 3 77, Dated April 8, 1975 O lnventor(s) JAAN RAAMOT It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:  
 In the specification, Column 6, equation L),  
 &#34;E Il /(B B E should read -E [R /(B R )].E  
  Column 7, equation (10), O  
  R /(R R E 30 [R /(R $1 E should read [B /(R 1R )].E [H /(R 3 .E  
  equation (13), Q  
 &#34;E -A R /(R R E -A R /(R +3 E should read ----E -[A R /(R R E [A R /(R 11 E equation (15), that portion of the equation reading O E l/R l/R [l/(R 3 A R /R (R R should read o ES A R2)-3 equation (16), that portion of the equation reading E /R l[(R R should read [E /R l[ (R R .--3 and that portion of the equation reading /R R l A R /R should read &#34;&#39;oon G EIgncd and Scaled this [SEAL] tenth D3) 0f February 1976 Attest.  
 RUTH c. MASON C. u  
  UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 5.877.021 Dated April 8, 1Q76 Inventor( J aan Raamot It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:  
 Column 7, equation (10),  
 &#34; n /(n R2). E 3o [RE/(R1 3 E should read [B /(R $1 E [RE/(R1 3 ES&#34;--;  
 equation (15), that portion of the equation reading ES l/R i/R [l/(R 11 A R2/RO(R1 R2)&#34; should read ES [l/R I/R l/(R R A la /R m R Column 8, line 5, that portion of the equation reading &#34;EV (500) (200)&#34; should read --E (500) (200)--. Column 10,  
 line 35, &#34;This is turn&#34; should read --This in turn--. Column 11, line 31, &#34;an input 10 4 should read --at input lO L line 60, &#34;in the most&#34; should read --is the most--; line 65, &#34;lO L through lO L should read --lO L through lO L Column 12, line 23, rather than +E should read --rather than i&#39;E line #2, &#34;Conveters&#34; should read --Converters--. Column l L, line 7, &#34;q3a&#34; should read Signed and Scaled this sixteenth Day Of December 1975 [SEAL] Arrest.  
 RUTH C. MASON C, MARSHALL DANN Atleszmg Officer Commissioner of Patents and Trademarks