Current Comparator Using Ferromagnetic Cores and Its Application to Analog-to-Digital Converters

A current comparator using ferromagnetic cores with a rectangular hysteresis loop is described. Design criteria for this comparator are derived from the B-H characteristics of the core. To verify the performance of the current comparator, basic experiments and experimental A-D converters with these comparators have been made. By application of the current comparator, it would be possible to realize an A-D converter that could convert a current between a few milliamperes and a few amperes into 9- to 10-bit binary forms within an order of microseconds. The merits of an A-D converter of this type are 1) its very low input impedance, 2) its high impedance between the balanced input terminal pair and the ground, and 3) its ease of conversion of the sum/difference of two currents. Because of these features, the A-D converters can measure currents in circuits having high potential to ground without giving disturbance. The input impedance, measured at 500 kHz, of the experimental A-D converter is shown as a series connection of 0.3-ohm resistance and 0.24-¿H inductance, with a stray capacitance of 7.5 pF between input terminals and ground.