GB1220962A - Improvements to ternary coders - Google Patents

Abstract

1,220,962. Analogue to digital converters. INTERNATIONAL STANDARD ELECTRIC CORP. 22 April, 1968, No. 18873/68. Heading G4H. An automatic calibration arrangement for centring the response curve of a feedback comparison ternary scale analogue-to-digital converter comprises means periodically to couple a calibration voltage to the converter, which should produce a predetermined ternary output when the converter is properly centred, and means detecting a deviation from this output which adjusts the centring to compensate for the deviation. A number of input voltages ec1 &c. and the calibration voltage eo are gated in turn to the input circuit NP. The calibration voltage is half the full scale voltage of the circuit. The maximum signal in a 5-digit ternary coder produces an output of 22222, and the half fullscale calibration voltage should produce an output of 11111. Each ternary digit is represented by two binary bits stored in register RG. The logical control unit LU sets the most significant digit in register RG to 2 (binary bits 1, 0) and comparator CM detects whether the input is exceeded or not by the output from decoder DC. This may be as described in Specification 1,219,148. If the input is exceeded, the first binary bit is reset to 0, giving a ternary digit of 1. If this is still too much the second binary bit is set to 1 giving binary 01, ternary 0. In the same way the pairs of binary bits are set in turn for succeeding ternary digits. These digits appear on flip-flop BD and are transmitted to an output A and serve to control the logical control unit LU. During the time calibration voltage eo is applied by gating signal Vs, the centering correction circuit CC is made operative. The pairs of two binary bits representing the successively lower ternary digits are examined and the first to have a value other than 1 are used to control the increase or decrease of the charge on a capacitor providing the correction voltage U. For a properly calibrated circuit every digit should be 1. If a digit is found valued 0, the charge decreases and if a digit is found valued 2, the charge increases.