GB1402636A - Automatic calibration of digital voltmeter - Google Patents

Abstract

1402636 Analogue to digital converters G & E BRADLEY Ltd 7 May 1973 [14 June 1972] 27913/72 Heading H3H An analogue to digital converter comprises a reference source 4, a circuit 1 in which the analogue signal is compared with the reference source and a circuit which compensates variations in the level of the reference source, the compensation circuit including a comparator 7 which compares the output of the converter during a calibration cycle with a level representative of the level of a calibration source 19 and an adjustment circuit 13, 14 responsive to the output of the comparator, the output of the adjustment circuit being added to or subtracted from the reference source 4. The converter is preferably a dual ramp converter in which the integrating circuit 1 controls a gate 6 to enable clock pulses to be accumulated in a display counter 5, the count being a digital representation of the analogue voltage fed to a terminal 22. At intervals determined by a timing circuit 20 a switch 21 is changed over to connect the accurate calibration source 19 to the circuit 1 and the reference input of the comparator 7 is fed with a count equivalent to the voltage of the source 19. If the voltage of the reference source 4 differs from that of the calibration source 19, the comparator 7 causes an exclusive- OR gate 8-10 to hold open a gate 11 for a duration proportional to the magnitude of the difference. The clock pulses fed to a counter 12 energize relays 13 to connect a voltage V via binary weighted resistors 14 to the reference source 4, thereby adjusting the reference source voltage.