High performance analog to digital converter for integrated circuits

An analog to digital converter for converting an unknown analog input voltage into a digital equivalent thereof including a calibration means comprising:

• a. a storage means,
• b. means for applying said input voltage to said storage means,
• c. a voltage controlled constant current source for constantly drawing a current of a magnitude dependent upon a voltage applied at a control input thereof,
• d. means for altering the charge held by said storage means by discrete equal amounts in response to control pulses applied thereto by connecting said voltage controlled constant current source to said storage means during the presence of each control pulse to cause said constant current to flow through said storage means and alter the charge held by said storage means said discrete amount which is dependent upon the magnitude of said constant current,
• e. means for applying control pulses to said means for altering,
• f. threshold means connected to said storage means for detecting when the charge on said storage means reaches a predetermined threshold value,
• g. accumulator means responsive to the said control pulses for accumulating a number corresponding to the voltage applied to said storage means,
• h. means for applying a reference voltage to said storage means, the resulting charge held by said storage means again being altered in discrete equal amounts by said means for altering, and
• i. means, responsive to said threshold means and connected to said means for altering, for comparing the time of detection of said predetermined threshold by said threshold means with a known time corresponding to a number of control pulses nominally required for varying the charge held by said storage means from its initial value when said reference voltage is applied thereto to a value corresponding to said predetermined threshold value, and for varying said discrete amount in accordance with the difference between said time of detection and said known time.