Patent GB-777378-A (GB)
Title: Analog-to-digital converter
IPC: H03M1/00
Filed: 1955-01-20
Abstract: 777,378. Analogue-digital converters. INTERNATIONAL BUSINESS MACHINES CORPORATION. Jan. 20, 1955 [Jan. 27, 1954], No. 1750/55. Class 40 (1). In a system for representing a D.C. input signal (the output voltage of a strain gauge) on a binary scale, the system being of the kind in which the input signal is compared with successive diminishing steps of a reference voltage supplied from a relay controlled register, the steps being additive until the reference exceeds the input when the resultant error signal removes the last applied step ; the error signal is modulated at 3000 c.p.s., amplified and synchronously demodulated before being used to control the register. The register 25, Fig. 5 and Fig. 16 (not shown), is a potential divider including three banks of resistors representing respectively units, tens and hundreds, each bank being divided in the ratio 8, 4, 2, 1. A reference voltage E is connected across the divider and the values of output voltage are selected by electromagnetic relays 310c . . . 310n. There is an additional relay 310b, which reverses the polarity of the reference voltage E if the polarity of the input signal is reversed (see below). The relays 310b . . . 310n are individually controlled via driver amplifiers 302b . . . 302n, Fig. 13 (not shown) by a chain of multivibrators 287b . . . 287n, Fig. 4 and Fig. 8 (not shown), in turn controlled by a cascaded chain of multivibrators 250a . . . 250n, Fig. 6 (not shown). A measuring cycle is started by a switch 360, Fig. 5, being closed (by undisclosed means) this controls three multivibrators 366a, 366b and 287p, and a relay 380 which combine to reset the two multivibrator chains 250a . . . 250n and 287b . . . 287n, Fig. 4, and to temporarily block and then release a free-running multivibrator 200, Fig. 4 and Fig. 12 (not shown) from which is derived timing pulses for the sequence chain 250a . . . 250n. The first timing pulse switches " off" the home trigger 250a. This energizes the sign relay 310b (see below) and also switches " on " the sign trigger 250b. The second timing pulse switches " off " the sign trigger which provides a re-setting pulse for the sign relay 310b (but this pulse is normally blocked by a gating circuit 321b), energizes the " 800 " unit relay 310c and switches " on " the 800 unit trigger 250c and so on down the chain. The output voltage steps from the potential divider 25 and the voltage to be measured are connected in opposition through a carbon-button microphone, Fig. 2 (not shown), which is vibrated at 3000 cps. by a piezo-electric unit so as to modulate the resultant current. This modulated signal is amplified and synchronously demodulated, Fig. 3 (not shown), before being applied to a " multiar " circuit which generates an output pulse in response to a negative input signal, that is when the output from potential divider 25 exceeds the voltage to be measured. This pulse opens the gating circuits, e.g. 321b referred to above, enabling the resetting pulse to remove the step of output from 25 which has caused the total output from 25 to exceed the input signal. The " sign " channel works in an analogous manner, for the sign relay 310b is the first to operate and this reverses both the polarity of the reference voltage E and the phase of the A.C. signal applied to the synchronous demodulator of the error signal. Since none of the other relays have yet been operated, the output from 25 is still zero, and so if the input signal is of negative polarity, the reversal of the demodulator will give a positive signal, hence no re-setting pulse and the sign relay 310b will stay in the reversal position to provide a negative reference voltage. Conversely, a positive input signal will re-set relay 310b to provide a positive reference signal. The position of relays 310c . . . 310n at the end of the measuring cycle from a binary code pattern representing the input signal voltage. These relays, it is stated, also light glow lamps and/or provide an input to " another appropriate system (not shown)."