GB1351894A - Method and instrument for the measurement and digital display of direct durrent electric signals - Google Patents

Abstract

1351894 A/D converters LA TELEMECANIQUE ELECTRIQUE 21 March 1972 [25 March 1971] 13227/72 Heading G4H A dual-ramp A/D converter is adapted to perform an initial charge-discharge-charge sequence to compensate for errors present in the unknown voltage presented to the integrator owing to the components of the converter itself. The apparatus comprises a reference input 1 and amplifier 3 and an unknown input 2 which is connected via switch 4 to a second amplifier 6, both inputs being presented to a second switching device 5 which provides the input to a capacitor-type integrator 7 having its output connected with a zero-detector 8 which is connected to two logic circuits 12, 13 which operate on a counter 10, store 14 and display 15. A clock-pulse source 9 is provided for the counter 10 and a third logic circuit 11 is provided to operate the switching devices 4, 5 in response to various zero-count conditions at 10. In operation and with the counter 10 registering zero, the switches 4, 5 are held at positions F2, F3 respectively to connect the integrator 7 to ground, and the counter counts to its maximum during which time the integrator 7 integrates voltages due to the errors in all components 4-7. At the maximum count, the counter 10 resets to zero and the device 11 causes switches 4, 5 to take positions F1 and F4 respectively and so a reference voltage is applied to the integrator 7 to discharge it, the device 8 via device 12 causing the counter 10 to reverse its direction at the zero output point of the integrator to count back to zero, at which time the switch 5 is moved to position F3. At this point, the integrator 7 is charged with the integral over time T of all error voltages affecting the unknown input 2, and with a reverse polarity to the unknown voltage. The last switching procedure connects the unknown voltage, together with any errors present, with the integrator 7 and the integration proceeds for a further time T, the time that the counter requires to count from zero to its maximum. When this resets once more, switch 5 switches back to F4 to cause the discharge of the integrator back to zero. When zero is reached, the device 8 via device 13 causes a store 14 and a display 15 to operate to give an indication of the unknown voltage value. The unknown input may be from a strain gauge bridge.