Abstract:
A translation circuit provides an output referenced to ground in response to either an ECL input or a TTL input. ECL or TTL supply voltages are selectively applied to first, second, third and fourth supply voltage terminals in accordance with the type of input signal received.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to a translation circuit that may be operated with either a single supply voltage or dual supply voltages for providing an output signal referenced to a specific voltage regardless of an input signal voltage reference and more particularly to circuitry within a system such as a video digital-to-analog converter for driving high-resolution monitors, wherein either single or dual supply voltages may be selectively applied to the system for receiving ECL and/or TTL inputs. 
     BACKGROUND OF THE INVENTION 
     Integrated circuits fabricated with bipolar technology may comprise a PNP transistor coupled to a +5.0 volts supply for providing an output signal referenced to ground. However, for special purpose circuits such as video digital-to-analog converters driving high resolution monitors, monolithic fabrication technology has generally not provided an adequately fast PNP transistor; therefore, these DACs typically comprise NPN output current sinks coupled to a resistor referenced to the positive supply. In order to interface properly with specific systems, such as a video monitor, it is desired to place the positive supply at ground potential. If the input signals are standard ECL, then a single supply circuit will suffice. However, where TTL inputs or above ground ECL inputs are encountered, having the positive supply at ground will be less than desirable. 
     The variety of supply options and logic thresholds could be covered by a variety of devices, each one fitting a particular application. However, it would be more desirable to have as many options as practical in a single device. 
     Thus, what is needed is a translation circuit that selectably operates on either a single supply voltage or dual supply voltages for providing an output signal referenced to a specific voltage regardless of an input signal voltage reference. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an improved translation circuit for providing an output signal referenced to a specific voltage regardless of an input signal voltage reference. 
     Another object of the present invention is to provide a circuit wherein either single or dual supply voltages may be selectively applied to the system for receiving ECL or TTL inputs and providing an output referenced to ground. 
     In carrying out the above and other objects of the invention in one form, there is provided a circuit comprising a first, second, third and fourth supply voltage terminals. An output circuit is coupled between the third and fourth supply voltage terminals and is coupled to an output terminal for providing an output signal at the output terminal. A translation circuit is coupled between the first, second and fourth supply voltage terminals and is coupled to the input terminal and the output circuit for biasing the output circuit in order to provide an output at the output terminal that is referenced to a second supply voltage when one of a first or a second condition occur, said first condition being when a first supply voltage is applied to the first and third supply voltage terminals and a second supply voltage is applied to the second and fourth supply voltage terminals and the input signal is referenced below the third supply voltage terminal (most positive supply voltage, which may or may not be ground), the second condition being when the first supply voltage is applied to the first supply voltage terminal, the second supply voltage is applied to the second and third supply voltage terminals, and a third supply voltage is applied to the fourth supply voltage terminal and the input signal is referenced above said third voltage terminal (typically ground), the first, second and third supply voltages being successively smaller in value. 
     The above and other objects, features, and advantages of the present invention will be better understood from the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a partial schematic of the present invention. 
     FIG. 2 is a schematic of a bias generator for the circuit of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, the translator circuit comprises supply voltage terminals V CC1 , V CC2 , V EE1  and V EE2 , input terminals 11 and 12, and output terminals 13 and 14. Diodes 17 and 18 are coupled between input terminals 11, 12 and nodes 19, 20, respectively, for transferring the proper voltage level of input signals applied to input terminals 11 and 12 to nodes 19 and 20. 
     Transistors 21 and 22 have their emitters connected to supply voltage V CC1 , their collectors connected to nodes 20 and 19, respectively, and their bases connected to bias generator 23 by connector 24. Differentially connected NPN transistors 25 and 26 have their emitters connected to the collector of current source transistor 27, their bases connected to nodes 19 and 20, respectively, and their collectors both coupled to supply voltage V CC2  by resistors 28 and 29, respectively, and to output terminals 13 and 14, respectively, for translating the signals on nodes 19 and 20 to output terminals 13 and 14. Current source transistor 27 has a base connected to voltage reference V R  and an emitter coupled to supply voltage V EE2  by resistor 31. Current source transistors 32 and 33 have their collectors connected to nodes 20 and 19, respectively, their bases connected to receive voltage reference V R , and their emitters coupled to node 34 by resistors 35 and 36, respectively. Transistor 37 has a collector connected to node 34, an emitter connected to supply voltage V EE2 , and a base coupled to supply voltage V EE1  by resistor 38. 
     The translator circuit of FIG. 1 may be operated with either a single supply voltage or a dual supply voltage in order to provide an output signal referenced to a specific voltage regardless of an input signal voltage reference. For the single supply option, V CC1  and V CC2  are tied together and V EE1  and V EE2  are tied together. V CC1  and V CC2  are connected to +5.0 volts or ground, and V EE1  and V EE2  are connected to ground or -5.0 volts, respectively. When the supply voltages are applied in this single supply option, transistors 21 and 22 are turned on for reasons to be discussed hereinafter relating to FIG. 2. Since V EE1  is at the same potential as V EE2 , transistor 37 is off, causing transistors 32 and 33 to be biased off. Therefore, diodes 17 and 18 are biased on, transmitting the input signals on input terminals 11 and 12, shifted up by one diode drop, to nodes 19 and 20. 
     For the dual supply option, V CC1  and V EE1  are connected to +5.0 volts and ground, respectively, and V CC2  and V EE2  are connected to ground and -5.0  volts, respectively. When the supply voltages are applied in this manner, transistors 21 and 22 are turned off for reasons to be discussed hereinafter relating to FIG. 2. Since V EE1  is at a higher potential than V EE2 , transistor 37 will be biased on, causing transistors 35 and 36 to be biased on. Therefore, diodes 17 and 18 become zener diodes which shift the input signals below ground to interface with the differentially connected transistors 25 and 26. 
     Referring to FIG. 2, bias generator 23 comprises PNP transistor 41 having an emitter connected to supply voltage V CC1 , a base connected to connector 24, and a collector connected to node 42. Transistor 43 has a collector connected to supply voltage V CC1 , a base connected to node 42, and an emitter connected to node 44. Connector 24 is coupled to supply voltage V CC1  by resistor 45 and to node 42 by capacitor 46. Transistors 47 and 48 have their collectors connected to nodes 42 and 44, respectively, their emitters coupled to node 49 by resistors 51 and 52, respectively, and their bases coupled both to node 49 by resistor 53 and for receiving reference voltage V R . Transistor 54 has a collector connected to node 49, an emitter connected to V EE1 , and a base coupled to supply voltage V CC2  by resistor 55. Transistor 56 has a collector connected to node 57, a base connected to node 44 and an emitter both coupled to V EE1  by resistor 58 and connected to the anode of diode 59. Transistor 61 has a collector connected to connector 24, a base connected to node 57, and an emitter coupled both to node 57 by diode 62 and to node 63 by diode 64. Node 57 is coupled to supply voltage V CC2  by resistor 65. Node 63 is connected to the cathode of diode 59 and is coupled to V EE1  by diode 66. 
     When V CC1  and V CC2  both have the same voltage level (single voltage supply option) and are 5.0 volts higher than V EE1 , transistor 61 will be on, pulling conductor 24 low, turning on transistors 21 and 22. The feedback path comprising transistors 43 and 56 insure that transistor 61 is appropriately biased to force transistor 41 to source the same magnitude of current that reference transistor 47 sinks. Transistors 21 and 22 are designed to match transistor 41 so their respective collector currents also source the same value of current. When V CC1  is 5.0 volts higher than V CC2 , and V CC2  and V EE1  both have the same voltage level (dual voltage supply option), transistors 47, 54 and 61 will be off, causing conductor 24 to go high, turning off transistors 21 and 22. 
     By now it should be appreciated that there has been provided circuitry within a system such as a video digital-to-analog converter for driving high-resolution monitors, wherein either single or dual supply voltages may be selectively applied to the system for receiving ECL or TTL inputs and providing an output referenced to ground.