ANALOG TO DIGITAL CONVERTER

1276490 Detecting signals between two limits; detecting largest signal INTERNATIONAL BUSINESS MACHINES CORP 19 April 1971 [7 April 1970] 26038/71 Heading H3T [Also in Division G4] In an analogue to digital converter, the analogue current is compared simultaneously with a number of reference currents in a series of stages. A chain of diodes D, Fig. 2, is fed with reference currents I a . This sets up an electrical scale, with the current flowing through each stage, in the absence of an analogue current, increasing down the length of the chain. When an analogue current Ia is drawn from the up stage end of the chain, the current flow in the stages is reversed -down to that stage whose current is less than one step greater than the analogue current. While the -currents in the various stages are flowing towards earth the transistors T 1 associated with each stage are switched off. When the current is reversed, the transistor associated with the highest reverse current-stage conducts, since this transistor-has the highest base voltage. This supplies a signal to diode matrix 10 which causes a binary or gray code output to be produced on lines 1, 2 or 4. In a second embodiment Fig. 3, a second transistor T 2 is associated with each stage, giving regenerative feedback so that if T 1 conducts, T 2 is biased to give extra current driving T 1 hard on. This eliminates the possibility of two adjacent stages being actuated together. In a third embodiment, Fig. 4, the individual stages are supplied with currents I 0 , 2I 0 , 3I 0 &c. Two transistors T3 are associated with each stage. In the absence of an analogue signal the left hand transistor of each stage carries a current I 0 . When the current flows through a stage is-reversed however the right hand transistor carries a current I 0 . Thus the common collector output associated with a pair of stages has zero output if the lower stage has its current reversed and the upper stage does not. This signal is again fed to a diode matrix. Modification.-Carrier storage effects may be minimized by use of Schottky diodes and Schottky field-effect transistors.