Analog to digital converter

Claim 1

An electric circuit for providing an output which is a digitised form of the continuous time integral of data represented by a variable electric current, which circuit includes a capacitor to which may be applied a variable direct current proportional to the instantaneous value of a variable quantity, a buffer amplifier having an input connected to the capacitor a reference stage connected to an output of the buffer amplifier operable to define upper and lower voltage levels and responsive to the voltage across the capacitor to deliver a first control signal whenever the said voltage reaches the upper voltage level and a second control signal whenever the said voltage reaches the lower voltage level, charging means responsive to either one of the first and second control signals to apply to the capacitor a pulse of known constant charge in such a sense that the voltage across the capacitor changes to a value near to the voltage level giving rise to the other control signal, counting means for counting the number and polarity of the charge pulses applied to the capacitor by the charging means, an analogue-to-digital converter connected to the output of the buffer amplifier operable to sample the voltage across the capacitor at a rate such that the time interval between successive samples is greater than the duration of a charge pulse, and an adder for combining the output of the counting means with the instantaneous output of the analogue-to-digital converter.