Abstract:
A four quadrant analog by digital multiplier includes an amplifier gain network in which the digital sign bit controls the polarity of the gain of an amplifier to determine the product of the digital sign and the analog value, and in which the digital magnitude bits control a multiple of resistor steps of a resistance network to determine the product of the digital magnitude and analog value as previously multiplied by the digital sign bit in the amplifier gain network.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to analog by digital multipliers capable of operating in all four quadrants of analog and digital sign combinations. 
     2. Description of the Prior Art 
     In situations which require the multiplication of an analog value by a digital value, as, for example when real-time analog data is to be modified according to stored digital data, there is a requirement for an analog by digital multiplier. Analog by digital multipliers of the prior art have routinely performed such multiplications for a particular combination of digital and analog signs. For example, the multiplication has been performed provided that the digital and analog values are both positive. However, there are four possible combinations of analog and digital signs: both may be positive; both may be negative; the digital may be positive while the analog is negative; or the analog may be positive while the digital is negative. These four possibilities are commonly referred to as the four quadrants of analog by digital multiplication. 
     Analog by digital multipliers which are operable in all four quadrants have been developed in the prior art. Some prior art multiplier devices have utilized an operational amplifier for the purpose of multiplying the two values. These devices generally achieve multiplication by the control of a resistance located at one of the inputs to the operational amplifier. Another approach has been to carry a particular sign bit along with the digital information and to supply the binary data in its complement form for multiplication. Some four quadrant multipliers involve the use of two quadrant multipliers which are made to behave as four quadrant devices as by adding a constant voltage to a variable that is not permitted to change signs. In other prior art four quadrant multipliers, the sign bit of the digital value is used for switching purposes at the output of an operational amplifier. 
     SUMMARY OF THE INVENTION 
     The disclosed invention provides apparatus for multiplying an analog value by a digital value for any combination of analog and digital signs. This apparatus includes an amplifier gain network for controlling the analog value in relation to the sign of the digital value and a resistance network for controlling the magnitude of the analog value in relation to the magnitude of the digital value. The amplifier gain network is comprised of an amplifier having inverting and non-inverting inputs, first, second, third, and fourth input impedances and a feedback impedance of predetermined values, and a switch which is controlled in relation to the sign of the digital value. The resistance network includes a multiple of resistance steps connected in parallel relation and which are comprised of step impedance, switch and ground impedance combinations. 
     The purposes of the disclosed invention include providing an analog by digital multiplier which utilizes the sign plus magnitude binary format; providing an analog by digital multiplier with accuracy comparable to the accuracy to a digital to analog converter; and providing an analog by digital multiplier which is operable at relatively high speeds. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The FIGURE shows a symbolic representation of the preferred embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in the FIGURE, the disclosed analog by digital multiplier expresses the product of an analog value, represented by an analog voltage, and a digital value, represented by a digital binary word comprised of a digital sign bit and at least one digital magnitude bit. An amplifier gain network 10 controls the sign of the analog voltage in relation to the digital binary representing the sign of the digital value and a resistance network 12 controls the magnitude of the analog voltage in relation to the digital binary bits representing the magnitude of the digital value. The output of resistance network 12 is a current whose magnitude and direction represent the magnitude and sign product of the analog and digital values. 
     The multiplication process can be numerically expressed as: ##EQU1## where 
     I o  is the output current representing the product of the analog and digital values; 
     S is the sign of the digital value; 
     V i  is the analog voltage representing the analog value; 
     R 1  is the resistance of the branch input impedance included in the branch of the resistance network associated with the most significant digital magnitude bit; 
     n is an integer representing the significance of a particular digital magnitude bit; and 
     B n  is the binary value of the nth magnitude bit of the digital value. 
     Referring to the right-hand side of equation (1), unity gain amplifying network 10 determines the quantity SV i  ; the product of the sign of the digital value S, is provided to digital sign bit input terminal 14, and the analog value V i  is provided to analog input terminal 16. Amplifier gain network 10 includes an amplifier 18, which is provided with an output terminal 20, an inverting input terminal 22, and a non-inverting input terminal 24. Inverting input terminal 22 is electrically connected to analog input terminal 16 through a first input impedance 26, and a second input impedance 28. Non-inverting input terminal 24 is electrically connected to analog input terminal 16, through a third input impedance 30 and to ground contact 32 through a fourth input impedance 34. Inverting input terminal 22 is connected to output terminal 20 through feedback impedance 36. When the digital number has a positive sign, the binary value of the digital sign bit is applied to terminal 14 causing a switch which, for example, may be comprised of field effect transistor 38, to close so that the junction of first input impedance 26 and second input impedance 28 is selectively coupled to ground contact 32. When the digital number has a negative sign, the binary value of the digital sign bit causes field effect transistor 38 to open so that the junction of input impedances 26 and 28 is electrically isolated from ground contact 32. 
     Basic operational amplifier gain equations will be used to demonstrate that amplifier gain network 10 will provide either positive or negative gain depending upon the condition of field effect transistor 38 and the resistive magnitudes of impedances 26, 28, 30, 34 and 36. First input impedance 26 and second input impedance 28 have equal resistive magnitudes which are represented as R a  ; Fourth input impedance 34 has a resistive magnitude which may be represented as R b  ; third input impedance 30 has a resistive magnitude which is represented as αR b  and feedback impedance 36 has a resistance magnitude which is represented as αR a . When the field effect transistor 38 is conducting, the gain (G) is positive and may be expressed by the equation: ##EQU2## 
     When field effect transistor 38 is non-conducting, the gain (G) is negative and may be expressed by the equation: ##EQU3## If the output amplifier gain network 10 at output terminal 20 represents a value whose magnitude is the magnitude of the analog number and whose sign is the product of the signs of the analog and digital numbers, the gain of gain network 10 must be positive unity when the sign of the digital number is positive and negative unity when the sign of the digital number is negative. This may be expressed mathematically as: ##EQU4## Using the quadratic equation, the simultaneous solution for α has the result that: 
     
         α = 1 + √ 5                                   (6) 
    
     Equation (6), therefore, gives the unique solution of α for the preferred embodiment of FIG. 1. Therefore, when the sign of the digital number is positive and the binary value provided to digital sign bit input terminal 14 causes field effect transistor 38 to close, the gain of amplifier network 10 is positive unity. In the same fashion, when the sign of the digital number is negative and the binary value provided to digital sign bit input terminal 14 causes field effect transistor 38 to open, the gain of amplifier network 10 is negative unity. 
     Referring again to equation (1), the output of resistance network 12 provides ##EQU5## the product of the output of the gain network 10 [SV i  ]  by the magnitude of the digital number ##EQU6## which is equal to I o  the product of the analog and digital values. In the preferred embodiment, resistance network 12 is a modified form of a digital-to-analog converter in which the binary values of the magnitude bits of the digital number control respective switches and the output of gain amplifier 10 replaces the stable voltage reference which is usually provided. The resistance network 12 is comprised of networks 42-49 which include switches comprised of field effect transistors 50-57; branch input impedances 60-67 and branch ground impedances 70-76. The conduction of field effect transistors 50 through 57 is controlled by the binary values of the digital magnitude bits which are provided to digital magnitude terminals 80 through 87 respectively. Switches 50 through 57 are connected to input impedances 60 through 67 respectively as illustrated in the FIGURE. The resistive magnitude of impedances 60 through 67 increase progressively according to the relation: 
     
         R.sub.n = 2.sup.n.sup.-1.  R.sub.1                         (7) 
    
     where: 
     n is an integer representing the significance of a particular digital magnitude bit; 
     R 1  is the resistance of the branch input impedance included in the branch of the resistor network associated with the most significant digital magnitude bit; 
     R n  is the resistance of the branch input impedance for the nth branch of resistor network 12. 
     In the FIGURE the resistance network 12 will accommodate a digital value whose magnitude is represented by eight digital bits. If R 1  the resistance of the branch impedance 67, associated with the most significant digital bit, is 200 ohms; the resistance branch impedance 66, associated with the second most significant bit, is 400 ohms. Similarly, the resistance of branch impedance 65 associated with the third most significant bit, is 800 ohms; the resistance of branch impedance 64 is associated with the fourth most significant bit is 1600 ohms; and the resistance of branch impedance 63 associated with the fifth most significant bit is 3200 ohms. The resistances of branch impedances 62, 61 and 60 would increase in a similar manner. 
     The junction of field effect transistors 50 through 56 with respective branch impedances 60 through 67 are connected to ground potential 32 through respective branch ground impedances 70 through 76. The resistive magnitude of ground impedances 70 through 76 decreases progressively according to the relation: ##EQU7## where: 
     n is an integer representing the significance of a particular digital magnitude bit; 
     R 1  is the resistance of the branch input impedance included in the branch of the resistor network associated with the most significant digital magnitude bit; and 
     R gn  is the value of the branch ground impedance of the nth resistor step. If R 1  the resistance of the branch impedance 67, associated with the most significant digital bit, is 200 ohms, the resistance of branch ground impedance 76, associated with the second most significant bit, is 400 ohms. Similarly, the resistance of branch impedance 75 associated with the third most significant bit is 800/3 ohms; the resistance of branch impedance 74 associated with the fourth most significant bit is 1600/7 ohms; and the resistance of branch impedance 73 associated with the fifth most significant bit is 3200/15 ohms. The resistances of branch impedances 72, 71 and 70 would decrease in a similar manner. Since the solution to equation (8) is infinity when n equals one, it is unnecessary to provide a branch ground impedance to the branch 49 which is responsive to the most significant digital magnitude bit. 
     When field effect transistors 50-57 are non-conducting, respective branches 42-49 provide no current to the output current I o . When field effect transistors 50-57 are conducting, branches 42-49 respectively contribute to the output current I o  by an amount ##EQU8## When the binary value of a digital magnitude bit is zero, the field effect transistor of the network branch associated with that digital magnitude bit is made non-conductive by the signal provided to the respective digital magnitude terminal 80-87. When the binary value of a digital magnitude bit is one, the signal provided to the respective digital magnitude terminal makes the associated field effect transistor conductive. In this manner the current provided by the branches 42-49 in which field effect transistors 50-57 are conducting provide the output current I o  which represents the product of the analog and digital signals. 
     Assuming negligible impedance in amplifier 18, when field effect transistors 50-57 are conducting, the effective resistance at each branch 42-49 of resistance network 12 is R 1 . This is demonstrated by the following equations: ##EQU9## where: 
     R n (eff) is the effective resistance of the nth branch. Substituting equations (7) and (8). ##EQU10## Since the impedances of resistor network 12 provide a constant resistance to field effect transistors 50 through 57, an error caused by the finite resistance of the field effect transistors can be compensated for by matching the resistances of field effect transistors 50 through 57 and reducing the value of the branch input impedances 60 through 67 and branch ground impedances 70 through 76 by an amount substantially equal to the resistance of each transistor 50-57. Therefore, the output of resistance network 12 is a current provided to multiplier output terminal 90 which represents the product of the analog voltage provided to terminal 16 and the digital value provided to terminal 14 and terminals 80 through 86.