GB1271856A - Analog to digital converter - Google Patents

Abstract

1,271,856. Analogue/digital converters. INTERNATIONAL STANDARD ELECTRIC CORP. 23 July, 1969 [26 July, 1968], No. 36999/69. Heading G4H. [Also in Division G1] After a first capacitor C1 (Fig. 1) has been charged to a value dependent on an analogue input signal across terminal el, e2 a second capacitor C2 is connected in series with it so that the first capacitor C1 discharges a fixed quantity of electricity U.C2 into the second capacitor C2, capacitor C2 then being discharged. This process is repeated and the number of increments of electricity transferred to the second capacitor counted to give a digital indication of the input signal. The count may be displayed on Nixie tubes together with a polarity signal. Alternatively a standard signal may be applied across input terminals e1, e2 and the capacitance of capacitor C1 measured. As described during time T1 a switch P1 is closed so that with a positive input signal capacitor Cl charges to a positive level. At the start of time interval T2 switch P1 is opened and switch P4 is closed to connect capacitor C1 in series with capacitor C2. Each time the junction E of the capacitors is at zero potential a comparator CO delivers (1) a counting pulse to counter C p and (2) a pulse to control circuit CD to open switch P4 and close switch P5 to short circuit capacitor C2 until the potential of junction E drops to -U when the switch P4 is reclosed to recharge capacitor C2, this process being repeated until capacitor C1 is discharged. At the start of time interval Tx1 switch P2 is closed to charge capacitor C1 to a negative level but since the potential of junction E remains negative the comparator CO does not operate. Light POS is illuminated to indicate a positive input since the comparator output c exists during time T2. With a negative input, during time T2 the junction E remains negative capacitor C1 being charged to a positive level during time interval T<SP>x</SP>1 and incrementally discharged during time T<SP>x</SP>2. A lamp Neg is then lit since the comparator output exists during time T<SP>x</SP>2.