Patent GB-939021-A (GB)
Title: Improvements in or relating to electrical multi-channel selection devices, especially for analogue-to-digital converters
IPC: H03M1/00
Filed: 1959-10-23
Abstract: 939,021. Electric selective signalling. GENERAL PRECISION Inc. Oct. 23, 1959 [Oct. 29, 1958], No. 36004/59. Class 40 (1). A circuit adapted to convert to digital form one of a number of analogue voltages each on a different lead has a comparator for each lead, the balance voltage from the'converter being fed to all the comparators in common and one of the difference voltages being selected to control the converter by means of an electronic switching circuit. In Fig. 1 the input voltages are on terminals Ii-In and the balancing voltage of the converter 20 is subtracted from all of them. The input selector causes one of the difference voltages to pass on lead 12 to a decision unit 14 which gives a " 1 " output if the difference is positive and a " 0 " if negative. The " 1 " causes a counter in converter 20 to increase its count until the analogue output produced thereby, when amplified at 22, balances the input voltage and reduces the difference to zero. As shown in Fig. 4 the input voltage on terminal I1 is applied via resistor R1 to point 50. The opposite balance voltage from the amplifier 22 is applied via resistor R<1>1 to point 50 so that the voltage at this point is the difference. All the other inputs I2-In are similarly connected with the balance voltage and the difference voltages are connected to switching triodes VIA, VIIA &c. The cathodes of these triodes are connected with the cathodes of associated triodes VIB, VIIB &c. and the common points 60, 62 &c. are connected to a negative source through resistors R4, R6 &c. The cathodes are normally held positive to prevent conduction by connection through diode XI, X2 &c. and resistors R7, R8, &c. to a positive source. Triodes VI, VII &c. connected to leads 64, 66 act as switches and are caused to conduct by signals on selector terminals S1, S2 &c. When a pulse is applied to terminal SI, for example, triode V<1>I conducts and the potential of lead 64 falls and the positive bias on point 60 is removed. Triodes VIA and VIB therefore conduct under the control of the difference voltage at point 50 to give a balanced output on leads 56 and 58. This is amplified at 70 before application to the decision circuit. The decision circuit produces a " 1 " if the balance voltage is too low and " 0 " if it is too high, these signals serving to control a counter which increases or decreases its count until a digital-to-analogue converter controlled thereby produces an exactly balancing voltage. In another arrangement the signal from the decision circuit is used to gate pulses into a counter, a staircase voltage generator being used to produce the balance voltage. Pulses are entered into the counter for the time the generator takes to exactly balance the input voltage. The amplifier 22 acts as a low impedance source so that the voltage at point 55 is unaffected by variations of the voltages at inputs 11, 12 &c. Terminal A, Fig. 1, may be used as a low impedance source having the same voltage as a selected one of the inputs I1, 12 &c. The circuit then acts as an analogue switch.