Analog-to-digital converter

Claim 1

An analog to digital converter for converting a bipolar analog voltage to a unipolar offset digital value, said converter comprising: a. a resistor ladder network; b. an input terminal for supplying an analog voltage to be digitized; c. a reference voltage source communicated with said input terminal for adding thereto a voltage for offsetting the input analog voltage, said reference voltage being of sufficient value to shift the voltage at said input terminal to a value exceeding ground potential; d. differential operational amplifier means connected to receive the resulting voltage at said input terminal as its first input and a fixed reference voltage as its second input, said amplifier means being responsive to be driven in a positive or negative direction from a quiescent condition in forming one bit of of the digitized value of the input analog voltage; e. feedback resistor circuit means connected across the operational amplifier means and communicated with said input terminal for supplying current flow thereto; f. comparator circuit means having a first comparator input terminal with the outpuT of said operational amplifier means and a second comparator input terminal communicated with a standard voltage source for comparing the output voltage from said operational amplifier means therewith, said comparator circuit means forming a positive going pulse to indicate the presence of a positive voltage input to said operational amplifier means; g. output gate means periodically enabled with an enable signal and cooperative with said comparator means for forming the digital bits corresponding to the digitized value of the input analog voltage; h. a source of negative voltage adapted to be coupled to the resistors of said ladder network; and i. switching means responsive to said digital bits for controllably coupling said source of negative voltage to said input terminal through selective resistors of said resistor ladder network.