Electrical drift correction system

Claim 1

Apparatus for reducing drift in an analog to digital conversion system including a serially connected operational amplifier-integrator and a comparator, said apparatus comprising: means for coupling the signal to be converted, during those times when it is desired to convert said signal, to one input of said operational amplifier-integrator, and, during other times, coupling said one input to a point of ground potential; means for deteting the polarity of the output of said comparator; means for receiving a series of clock pulses; a capacitor; means, effectivE during said other times, for charging said capacitor with a series of positive pulses when said comparator output is of one polarity, and for charging in the opposite direction with a series of negative pulses when said comparator output is of a different polarity; and means coupling one terminal of said capacitor to a second input of said amplifier-integrator; said means effective during said other times including: A. a level shifter circuit coupled to the output of said comparator, and having a pair of output lines therefrom, said circuit being adapted to provide logical levels of 0 and 1, respectively, on said output lines when said comparator output is of positive polarity and to provide logical evels of 1 and 0, respectively, on said output lines when said comparator output is of negative polarity; B. a first set of gating circuits, adapted to be operative during said other times, coupled to receive said series of clock pulses and one of said output lines; C. a second set of gating circuits, adapted to be operative during said other time coupled to receive said series of clock pulses and the other of said output lines, one of said gating circuits including an inverting means so that, during said other times, said first set of gating circuits provides a series of pulses of a fixed polarity when said comparator output is of one polarity, and said second set of gating circuits provides a series of pulses of said fixed polarity when said comparator output is of a different polarity; D. a reference of ground potential, E. means for coupling said reference of group potential to the opposite terminal of said capacitor; F. a first diode means having its anode coupled to said ground potential and having its cathode coupled to a first junction terminal; G. a second diode means having its cathode coupled to said ground potential and having its anode coupled to a second junction terminal; H. a third diode means having its anode coupled to a third junction terminal and having its cathode coupled to said first junction terminal; I. a fourth diode means having its cathode coupled to a fourth junction terminal and having its anode coupled to said second junction terminal; J. a fifth diode means having its anode coupled to said third junction terminal and having its cathode coupled to said one terminal of said capacitor; K. a sixth diode means having its cathode coupled to said fourth junction terminal and having its anode coupled to said one terminal of said capacitor; L. a first and a second resistor for coupling said first and fourth junctions, respectively, to a negative voltage source; M. a third and a fourth resistor for coupling said second and third junctions, respectively, to a positive voltage source; N. first capacitive means for coupling the output of one gating circuit to said first junction; and O. second capacitive means for coupling the output of the other gating circuit to said second junction.