This article describes a method for testing A/D converters using current testing. This method achieves real-time current testing without any need to analyze the results of testing. We also examine the effectiveness of fault detection by current testing combined with variable power supply voltage testing. The current testing used in this article measures the integral of the power supply current during one clock period when a test vector is applied. The target circuit is a 2-bit CMOS A/D converter and target faults are resistive bridging faults and resistive-capacitive breaking faults. Simulation results show that a step-voltage input stimulus is effective for detecting these faults in the A/D converter because this input stimulus allows the current to be measured at discrete time intervals. Moreover, in a variable power supply voltage environment, fault detection by current testing becomes easier as the power supply voltage becomes higher.