This paper develops a self-balancing AC and DC current comparator for meeting the growing demand of wideband current measurement. The AC and DC current comparator is built up on the basis of magnetic modulation-based direct current comparator and a closed-loop alternative measurement channel has been added. The new design can not only expand the measuring band width, but also make the current comparator self-balanced. The development process has been described in detail and an automatic measuring system based on virtual instrument control has been set up to test measurement performance of the current comparator. The measuring results show that its DC linear error is below 2×10-4, AC measurement bandwidth is no less than 100kHz, and standard measurement uncertainty is 2×10-5.