Current split circuit having a digital to analog converter

Abstract: A current split circuit that includes a multiplying digital to analog converter (DAC) and a controller. The controller establishes a first and second terminal of the DAC at the same potential so that a digital input to the DAC determines the ratio by which a current at a third terminal of the DAC is split between the first and second terminals of the DAC.

Description: Field of the Invention The invention presented herein relates to current split or current division circuits and in particular to precision logic controlled current split circuits using a multiplying digital to analog converter (DAC). Background of the Invention There is a need in electronic measurement and control equipment for a precision logic controlled current split or current division circuit that provides an accurate adjustment of the relative magnitude of two currents. Current split, for example, is used in nulled- bridge type circuits, but manual adjustment is used for the current split. Other prior art circuits provided a current split of fixed magnitudes. Such known current split circuits are also not of a form that would make automatic adjustment of the amount of current split between two circuit paths readily attainable. One example of a current splitter providing a fixed magnitude split of current is GB-A-2,135,846. This reference discloses a current source feeding two resistors, one of which is connected to one output of the circuit while the other is connected to the other output via the emitter-collector path of a transistor. A differential amplifier has one input connected to the first output of the circuit while its other input is connected to the junction of the second resistor and the transistor. In this circuit the current from the current source splits between the resistors in a ratio determined by the values of the resistors and the current values are substantially independent of the output loads and voltages as long as the circuit remains linear. Programmable current source circuits are also known which use a digital to analog converter (DAC) with an operational amplifier and semiconductor switch to provide a precision single output current from a precision input voltage. Such use of a DAC is explained in a publication entitled "CMOS DAC Application Guide", Second Edition, 1984, by Phil Burton, which is available from Analog Devices, Inc. The publication does not, however, contain any current split circuits nor does it teach how any of the circuits disclosed in the publication can be modified to provide a current split circuit using a DAC. The "Linear Databook" 1982, National Semiconductor Corporation, Santa Clara, California, US at pages 8-122 and 8-158 discloses a multiplying digital to analog converter having first, second and third terminals plus a digital input which determines the ratio of a current split between said first and second terminals provided said first and second terminals are at the same potential, the sum of the split currents being present at said third terminal of said digital to analog converter. Summary of the Invention According to the present invention there is provided a current split circuit connectable as a part of two circuit loops for providing a selectable ratio of current split between the two circuit loops, the circuit loops having a common power source and separate loads, comprising:- a multiplying digital to analog converter for receiving a digital input which determines the ratio of the current split, said digital to analog converter having first, second and third terminals plus a digital input, the current at said first and second terminals being in accordance with the ratio of the current split provided said first and second terminals are at the same potential, the sum of the split currents being present at said third terminal of said digital to analog converter, said second terminal providing for connection of said digital to analog converter to one of the loads of the two circuit loops with said third terminal providing for connection of said digital to analog converter with the common power source; and a controller circuit portion for establishing said first and second terminals of said digital to analog converter at the same potential having: (1) an operational amplifier with two input terminals and an output terminal, one of said input terminals operatively connected to said first terminal of said digital to analog converter and the other of said two input terminals connected to said second terminal of said digital to analog converter; and (2) a negative feedback semiconductor linear circuit operatively connected between said output terminal of said operational amplifier and said first terminal of said digital to analog converter, said negative feedback semiconductor linear circuit having a terminal conducting the current at said first terminal of said digital to analog converter, said terminal of said negative feedback semiconductor linear circuit providing for connection of the current split circuit to the other load of the two circuit loops wherein said negative feedback semiconductor linear circuit has a controlled semiconductor linear device and a series connected constant reference voltage source, said constant reference voltage source connected between one electrode of said controlled semiconductor linear device and said one terminal of said digital to analog converter, said controlled semiconductor linear device having a control electrode (G) operatively connected to said output terminal of said operational amplifier and having another electrode (D) connected to said terminal of said negative feedback semiconductor linear circuit. It is possible that the circuit loop connected to the aforementioned terminal of the NFSLC may present a voltage having a polarity that would prevent the NFSLC from conducting. The NFSLC includes a controlled semiconductor linear device (CSLD) plus a series connected constant reference voltage source (CRVS). The CRVS is connected between the CSLD and the first terminal of the DAC. The CRVS being presented in series with the CSLD assures conduction of the CSLD so long as the voltage of the CRVS is not opposed by a larger voltage at the terminal of the CSLD that is connected to the other load of the two circuit loops, thus allowing bipolar voltages to be present at such terminal of the CSLD. Bipolar voltages can appear where the current splitter circuit is used in a null-bridge circuit application. The current split circuit embodying the invention can be configured as a sourcing current splitter, wherein current flow is away from the DAC at its first and second terminals, or can be configured as a sinking current splitter, wherein the current flow is toward the DAC at its first and second terminals. Use of the current split circuit is illustrated by its connection as a part of two circuit loops wherein part of the split current passes via a load in one loop and with the remainder of the total current passing via a load in the other loop with the two loops having a common power source. Brief Description of the Drawings The features of the invention presented herein, which are referred to above and others, will become more apparent to those skilled in the art upon consideration of the following detailed description which refers to the accompanying drawings wherein: Figure 1 is a schematic of a sourcing current split circuit embodying the invention; Figure 2 is a schematic of a sinking current split circuit embodying the invention; Figure 3 is an illustration of the use of the circuit of Figure 1; and Figure 4 is an illustration of the use of the circuit of Figure 2. Detailed Description Referring to the circuits of Figures 1 and 2 of the drawing, which embody the invention presented herein, each includes a digital to analog converter (DAC) 10. Before consideration is given other portions of the circuits, the functioning of the DAC will be considered. DAC's usable in the circuitry of Figures 1 and 2 are multiplying DAC's, which are well known and are commercially available. The DAC used in Figures 1 and 2 is an N-bit CMOS DAC based on an R-2R resistive ladder network. The R-2R ladder divides the current that is present at terminal 13 (generally referred to as the V ref pin of a DAC) into binary weighted currents which are steered by current steering switches relative to terminal 12 (generally referred to as the Out 2 pin of a DAC), which is at DAC power supply ground potential. The digital input to the digital input port 14 of the DAC determines the position of the current steering switches, one switch for each digital input line, with a logic "1" causing the switch to steer current via the terminal 11 and a logic "0" causing the switch to steer current via the terminal 12. The fraction of the current that is steered by a current steering switch is weighted in accordance with the value of the binary input directed to a particular current steering switch. Thus, if the digital input for a 8-bit CMOS DAC was all "0's", all of the current flow would be via terminal 12, while a digital input of "10000000" causes half of the current to flow via terminal 12 and the remainder via terminal 11. Further, if the input is "11111111", then only 1/256 of the current at terminal 13 flows via the grounded terminal 12. The sum of the currents at terminals 11 and 12 is the same for all digital inputs. Such functioning of the CMOS DAC is possible only if the terminals 11 and 12 are at the same potential and furthermore are at zero volts relative to the power supply input voltages supplied to the DAC (not shown). The standard method of holding terminals 11 and 12 at ground is to use an external operational amplifier that is connected as a current to voltage converter providing feedback current to the RFB terminal (not shown) of the DAC. This is not done in the circuitry of Figures 1 and 2. If the RFB terminal of the DAC were used in the usual manner, the accuracy of the current at terminal 11 would not be preserved, but would be converted into a voltage output variable. The DAC, if it is a four quadrant multiplying DAC, is operable for current flow either to or away from terminal 13, allowing the circuitry of the present invention to have a sourcing or sinking current configuration. A sourcing current configuration is shown in Figure 1, wherein the currents flow away from terminals 11 and 12, while Figure 2 shows a sinking current configuration wherein the currents flow toward terminals 11 and 12. Some two quadrant multiplying DACs are usable but only in the sinking current configuration. The remainder of the circuitry shown in Figures 1 and 2, which will be referred to as a controller 15, functions to force a null or virtual ground at terminal 11 with respect to grounded terminal 12. It includes an operational amplifier 17 with a negative feedback semiconductor linear circuit (NFSLC). The controller 15 serves also to preserve the accuracy of the current at terminal 11 as a measurement variable. The controller 15 has a constant reference voltage source (CRVS) 21 as a part of the NFSLC that allows bipolar voltages to be presented at its terminal 16. The controller 15 preserves the accuracy of the current at terminal 11 as a measurement variable by passing this same current on through the constant reference voltage source (CRVS) 21 and a controllable semiconductor linear device (CSLD)20, which is also a part of the NFSLC, such that only minor errors in this split current through the DAC terminal 11 are conducted through the control terminal of CSLD 20. As has been noted, the DAC 10 can operate with either polarity of current while the controller 15 is inherently a unipolar circuit that can be configured for one polarity or the other, which accounts for the differences in the controller 15 in Figures 1 and 2. The NFSLC includes a capacitor 18 and resistor 19 for stabilization of the internal closed loop that includes the operational amplifier 17, the CSLD 20 and the CRVS 21. The capacitor 18 is connected in series with the resistor 19 with such series circuit connected between the inverting input and the output of the operational amplifier 17 with resistor 19 connected to the output of the operational amplifier. A suitable CSLD device 20 which operates as a controllable linear voltage dependent resistor, can be provided, in the case of Figure 1, by a P-channel MOSFET or JFET or a PNP bipolar transistor or PNP Darlington amplifier. In the case of Figure 2, the CSLD 20 can be provided by a N-channel MOSFET or JFET or a NPN bipolar transistor or NPN Darlington amplifier. For example, Figure 1 is shown using a P-channel JFET with its gate connected to the connection common to the resistor 19 and capacitor 18 and its source connected to the positive side of the CRVS 21. The drain of the JFET 20 is connected to terminal 16 of the current splitting circuitry. The inverting input of operational amplifier 17 and the negative side of the CRVS 21 are connected to terminal 11 of DAC 10. The controller 15 of Figure 1 causes current flow away from DAC terminal 12 making the circuit a sourcing version of the current splitting circuit. Referring to Figure 2, the same reference numerals, as are used in Figure 1, are used to identify the same or similar elements in Figure 2. The controller 15 of Figure 2 is shown using an N-channel JFET for the CSLD 20 and the CRVS 21 polarity is reversed with respect to that shown in Figure 1. The controller 15 of Figure 2 causes current flow toward DAC terminal 12 making the circuitry of Figure 2 a sinking version of the current splitting circuit. As mentioned above, it is the function of the controller 15 to force terminal 11 to be at the same potential as terminal 12 permitting the circuit in Figures 1 and 2 to be used as current splitter circuits wherein the digital input at 14 of the DAC 10 determines the amount of current split between the current at terminal 11 and terminal 12. This "forced null" between terminals 11 and 12 is provided by the action of the NFSLC of the controller 15. Explanation of such functioning of the controller 15 will be made in relation to Figure 3 wherein the circuit of Figure 1 is used with loads represented by resistor 25 connected at one end to terminal 12 of DAC 10 and resistor 26 connected to terminal 16. The opposite ends of resistors 25 and 26 are connected to the negative side of a D.C. source 27 which has its positive side connected to terminal 13 of DAC 10 via a resistor 28. For purposes of the explanation to be provided regarding the "forced null" action, the CSLD 20 will be considered to be a P-channel JFET as shown in Figure 3. Other assumptions include the use of a CRVS 21 of 10 volts, a 60 volt D.C. source 27, a 100K ohm resistor for resistor 28, and 300 ohm and 100 ohm resistors for resistors 25 and 26, respectively. The DAC 10 is assumed to be an 8-bit DAC. The supply voltages (not shown) for the operational amplifier 17 are a positive voltage of about +20 volts and a negative voltage of about -5 volts. Assume the output of the operational amplifier 17 in Figure 3 is at zero volts due to a prior condition, when no currents flowed through the DAC 10 and the voltage between terminals 11 and 12 is then zero. When a digital input of 10000000 is then applied to the input 14 of the 8-bit DAC, the DAC internal resistance between terminal 11 and 13 and between 12 and 13 will be the same. Currents flow from terminals 11 and 12 with the JFET 20 conducting at a level such that a "forced null" condition does not exist initially. A negative voltage signal will be presented to the inverting input of operational amplifier 17 which, after a short lag time, causes a positive voltage to be presented at the output of the operational amplifier reducing the source to gate voltage of the JFET 20 causing it to be less conductive. This results in an increase in the source to drain voltage of the JFET 20 to a higher positive value causing the magnitude of the inverting input of the operational amplifier 17 to be reduced, which, after a short lag time, causes an increase in a positive direction of the output of the operational amplifier. The source to gate voltage of the FET 20 is thereby increased to further reduce the level of conduction of the JFET causing the source to drain voltage of the JFET to increase, thereby further reducing the magnitude of the inverting input to the operational amplifier. In this manner, the voltage input to the operation amplifier will be reduced to zero and in this sense, the feedback circuit portion is considered as functioning to produce a "forced null" at the inputs to the operational amplifier 17. As can be seen in Figure 3, the circuitry of Figure 1 is used as a part of two circuit loops wherein the one loop includes the load represented by resistor 25, power source 27, resistor 28 and DAC 10 with the other loop being established by the load represented by the resistor 26, power source 27, resistor 28, DAC 10 and a portion of the controller 15. As described earlier, the digital input at 14 determines the relative magnitude of the current at terminals 11 and 12, wherein the total of these currents remain the same provided the voltages at terminals 11 and 12 are the same. As indicated earlier, if the digital input to an 8-bit DAC was "00000000", then all of the DAC internal switches direct the input current, I₁₃, at terminal 13 to the grounded terminal 12 such that the current at terminal 11, I₁₁, is zero and all current through the DAC passes through terminal 12 as current I₁₂. It was also indicated if the digital input were "11111111", only 1/256 of the current through the DAC passes through the grounded terminal 12. Similarly, a digital input of "10000000" causes an equal split of the current between terminals 11 and 12. Consider the decimal value, D, for the two digital inputs "11111111" and "10000000", D = 255 and 128 respectively. For D = 255, the currents can be expressed mathematically as follows: I₁₁ = 255 256 I₁₃ = 255 256 (I₁₁+ I₁₂) and for D = 128 I₁₁ = 128 256 I₁₃ = 128 256 (I₁₁+ I₁₂) "256" is the decimal representation of 2⁸, where "8" is the number of bits of resolution of the DAC example. Using this information, the above equations for I₁₁ can be expressed in more general terms as follows: I₁₁ = D ₂N (I₁₁ + I₁₂) or D ₂N = I₁₁ I₁₁ + I₁₂ ; where N is the number of bits for the DAC. Accordingly, a desired ratio by which the current through the DAC is split is readily obtained by selection of the digital input to the DAC since the total current through the DAC remains unchanged. The controller 15 then functions to force a null at terminals 11 and 12 which is needed to have the total current remain unchanged independent of the split in the current that is selected by the digital input. An application of the current split circuit in a null-bridge configuration can be shown to permit the determination of an unknown resistance when the value of another circuit resistance is of a known value. Figures 3 or 4 can be used as examples of this type of application wherein either resistors 25 or 26 is of a known value and the other is of an unknown value. For the case where resistor 26 is unknown, its value can be determined by monitoring the voltage at terminals 12 and 16 as the digital input to the DAC 10 is changed in a controlled manner until the same voltages are present at terminal 12 and 16. At such time V₁₂ = V₁₆; I₁₁ = I₁₆ and I₁₂R₂₅ = I₁₁R₂₆. Then, I₁₁ I₁₁+ I₁₂ = R₂₅ R₂₅ + R₂₆ From the earlier explanation given, it is also known that D ₂N = I₁₁ I₁₁ + I₁₂ , so that D ₂N = R₂₅ R₂₅ + R₂₆ . Solving the last equation for R₂₆: Then, With everything known on the right hand side of the last equation, the value for R₂₆ can be calculated. Referring to Figure 4, the circuit of Figure 2 is shown connected for use in a manner similar to the use of Figure 1 in Figure 3. The differences between Figures 1 and 2 have already been noted. Figure 4 is shown using the same resistors 25 and 26 for loads. The D.C. power source 27 and resistor 28 of Figure 2 is also used, but the polarity of the power source 27 is reversed since the circuit of Figure 2 is a sinking current split circuit. In addition, the magnitudes of the D.C. supply voltages (not shown) for the operational amplifier are transposed, i.e., the positive supply voltage must be greater in magnitude than the negative supply voltage since the output of the operational amplifier 17 must provide a gate to source voltage for the N channel type JFET 20 and the CRVS 21 to reduce the drain current of the JFET to zero. The "forced null" operation of the circuitry of Figure 4 can be explained in a similar manner as was done for the circuitry of Figure 3. As can be appreciated from the foregoing description the invention presented herein provides a current split circuit that permits a digital to analog converter (DAC) to be utilized which allows the ratio of the split currents to be readily changed using the digital input to the DAC allowing the current split circuit to be controlled via digital control circuitry such as a microcomputer or computer. The utilization of a DAC in this manner is attained by the use of the controller that has been described which provides the further advantage of allowing the current split circuit to be used without regard to the polarity of a voltage that may be present at the loads that can be connected to the controller of the current split circuit.

Claims: A current split circuit connectable as a part of two circuit loops for providing a selectable ratio of current split between the two circuit loops, the circuit loops having a common power source and separate loads, comprising:- a multiplying digital to analog converter (10) for receiving a digital input which determines the ratio of the current split, said digital to analog converter having first (11), second (12) and third (13) terminals plus a digital input (14), the current at said first (11) and second (12) terminals being in accordance with the ratio of the current split provided said first (11) and second (12) terminals are at the same potential, the sum of the split currents being present at said third terminal (13) of said digital to analog converter, said second terminal (12) providing for connection of said digital to analog converter to one of the loads of the two circuit loops with said third terminal (13) providing for connection of said digital to analog converter with the common power source; and a controller circuit portion (15) for establishing said first (11) and second (12) terminals of said digital to analog converter (10) at the same potential having: (1) an operational amplifier (17) with two input terminals and an output terminal, one of said input terminals operatively connected to said first terminal (11) of said digital to analog converter (10) and the other of said two input terminals connected to said second terminal (12) of said digital to analog converter (10); and (2) a negative feedback semiconductor linear circuit (18-20) operatively connected between said output terminal of said operational amplifier and said first terminal (11) of said digital to analog converter (10), said negative feedback semiconductor linear circuit having a terminal (16) conducting the current at said first terminal (11) of said digital to analog converter (10), said terminal of said negative feedback semiconductor linear circuit providing for connection of the current split circuit to the other load of the two circuit loops wherein said negative feedback semiconductor linear circuit (18-20) has a controlled semiconductor linear device (20) and a series connected constant reference voltage source (21), said constant reference voltage source (21) connected between one electrode of said controlled semiconductor linear device (20) and said one terminal (11) of said digital to analog converter (10), said controlled semiconductor linear device (20) having a control electrode (G) operatively connected to said output terminal of said operational amplifier (17) and having another electrode (D) connected to said terminal (16) of said negative feedback semiconductor linear circuit. A current split circuit according to claim 1 wherein current flows into the digital to analog converter (10) at said third terminal (13) with current flow at said first (11) and second (12) terminals being away from said digital to analog converter (10), said controlled semiconductor linear device (20) providing for the conduction of current at said first terminal (11) of said digital to analog converter from said first terminal to said terminal (16) of said negative feedback semiconductor linear circuit (18-20) and said constant reference voltage source (21) having its negative terminal connected to said first terminal (11) of said digital to analog converter (10). A current split circuit according to claim 1 wherein current flows out of the digital to analog converter (10) provided at said third terminal (13) with current flow at said first (11) and second (12) terminals being into said digital to analog converter (10) and said controlled semiconductor linear device (20) providing for the conduction of current at said first terminal (11) of said digital to analog converter (10) from said constant reference voltage source (21) which is connected to connect its positive terminal to said first terminal (11) of said digital to analog converter. A current split circuit according to claim 1 wherein said constant reference voltage source (21) is connected for current flow in the same direction that current is to flow between said first terminal (11) of said digital to analog converter and said controlled semiconductor linear device (20) when the current split circuit is connected as a part of the two circuit loops whereby said controller circuit portion (15) will operate independent of a voltage that may be present at said terminal (16) of said negative feedback semiconductor linear circuit (18-20) that is of a polarity opposite to and of a magnitude less than the magnitude of said constant reference voltage source (21).

Inventors: REED ERIC L C O MINNESOTA MINI, REED, ERIC L. C/O MINNESOTA MINING AND
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1989
Main classification: G05F3
IPC codes: H03M1, G05F3