Laser optical in-circuit measurement system for immunity applications

Abstract

During immunity testing (ESD, EFT) of a digital circuit, the waveforms of critical signal nets need to be measured for analyzing the failure mechanism. However, it is difficult to measure the induced voltage in the circuit under test due to the unwanted coupling by the non-ideal shielding of the probe cable, especially, if large common mode currents are present. Fiber optical connection avoids this problem. Semiconductor lasers have frequently been used for converting the voltage into an optical signal. The paper shows novel implementations of powering the laser, leading to a small, low cost optical probe.