Patent GB-812432-A (GB)
Title: Analogue-to-digital converters
Inventor: STURGEON JOHN ROYAL
IPC: H03M1/00
Filed: 1956-06-05
Abstract: 812,432. Analogue to digital converters. NATIONAL RESEARCH DEVELOPMENT CORPORATION. May 29, 1957 [June 5, 1956], No. 17399/56. Class 40 (1). A converter for providing a digital representation of an analogue voltage includes means for determining in which sub-division of a first range the voltage lies, means for subtracting the lower limit of this sub-division from the voltage and determining in which sub-division of a second range the remainder lies, this process being repeated with further ranges until the desired accuracy is obtained. According to the invention each succeeding range overlaps the lower sub-division of the previous range so that a certain error in the operation of the voltage level responsive means is tolerable. In the arrangement shown in Fig. 1 the voltage V to be digitized is applied at I and is a low negative voltage measured in microvolts. Terminal I is connected to a resistor R11 to the other side X of which is connected relay contacts associated with resistors R5-R10 connected to a +ve 1- volt supply. These resistors are so arranged that each when its contact is closed causes a predetermined voltage to be effectually subtracted from the voltage V at point X. This point is connected through switch arm S1 to one of four high-gain amplifiers A1, A2, A3, A4. These have negative feedback loops with inserted resistances R1, R2, R3, R4 of different values so that the gain increases successively from Al to A4. The amplifiers in effect subtract from the voltage at X a proportion of the output voltage through the feedback loop to reduce the gain, the reduction being determined by the feedback resistance. The output from the amplifier A1 is applied to a voltage-level discriminator BC which is adapted to energize neither, one or the other or both of a pair of relays to close corresponding relay contacts B1, C1 according to the voltage applied to it. Similarly amplifier A2 supplies a discriminator EF working in a smaller voltage range as determined by feedback resistor R2 and controlling contacts E1, F1 and amplifier A3 supplies a discriminator HJ working in a still smaller voltage range controlling contacts H1, J1. Amplifier A4 supplies a last discriminator LM working in the smallest range. Each discriminator provides an output on two lines indicating the sub-division of a range of voltage in which the applied voltage lies the different discriminators corresponding to the different ranges of voltage. The voltage range for discriminator BC may be sub-divided into four steps 0 to 30, 30 to 60, 60 to 90 and above 90 volts. A certain tolerance or error is allowed in the operation of the discriminator whereby for example if the voltage lies in the region 30 to 40 volts the signal may indicate sub-division 0-30 or 30-60 but not both. This error is compensated for by the fact that the next range overlaps the lower sub-division so that if a voltage of 35 volts is indicated as being in the division 0-30, neither of contacts B1 or C1 is closed so that no voltage is effectively subtracted from the 35 volts which is applied to the next range when the switch arm S1 moves to contact b. The discriminator EF determines in which sub-division of the second range the applied voltage lies and closes one or both of contacts E1, F1 to subtract the lower value of that sub-division and the difference is applied via switch arm S1 and contact C to the next discriminator HJ which indicates in which sub-division of the third range the difference lies. This closes neither one or both of contacts H1, J1 to subtract the lowermost value of that sub-division from, the difference, the final remainder being applied to discriminator LM working in the smallest range and indication of the appropriate division of that range being passed together with all previous indications to networks of relay contacts Q, P in which the lower values of the indicated sub-divisions are added together carry means being provided from network P to network Q and from network Q to a " hundreds " indicator. Decoders V and T indicate the tens and units values of the voltage in terms of microvolts. In a modification, Fig. 2, the amplifiers A1-A4 are replaced by a single amplifying arrangement using a mirror galvanometer T to which the voltage to be digitized is applied and a photomultiplier tube UC upon which a light spot from the mirror is directed. The face of the photo-multiplier tube is halfmasked and its output is determined by the extent to which the spot overlaps the edge of the mask. A small deflection therefore increases this output from zero to maximum. The output is connected through one of four different resistors R1-R4 to oppose the applied voltage, this being equivalent to the negative feedback loops on amplifiers A1-A4. The resistors are selected by the arm of a switch S2. The output from the photomultiplier, i.e. the amplified applied voltage, is passed to a discriminator D the range of discrimination of which is determined by the switch S2 connecting one or other of the feedback resistors R1-R4 in circuit. The indications are passed to relays B and C, E and F, H and J or L and M by switches S3 and S4 ganged to switch S2 so that in each step appropriate subtractions are made from the applied voltage by contacts B1-J1 and resistors R5 to R10 operating as before. The form of the invention may be applied to digitize the measurement of a strain gauge. A dummy gauge and the strain gauge are connected in a bridge, and the galvanometer is connected to measure out-of-balance voltage. This is amplified by the photomultiplier as before and the relay contacts B1-J1 are controlled as before to switch in resistances which tend to correct the balance of the bridge.