New source measure unit architecture for control loop configurability

Source-measure units (SMUs) can be extremely susceptible to the electrical characteristics of the unit under test (UUT) that is acting as a load, occasionally leading to marginal stability, sub-optimal settling times, dangerous overshoots, and even sustained oscillation. Surprisingly, very little has been done to address this issue, and most vendors are content to “specify around it”: effectively telling users that if a certain load causes the device to misbehave, they shouldn´t be using that load. In a precision power controller, the load is part of the closed control loop. The impedance of the load then directly affects the overall transfer function and response of the system. If we want to be able to reliably source power into arbitrary loads, we need to have a flexible control loop that can be programmatically reconfigured to match the unit under test. This implies a fundamental re-imagining of the architecture of the SMU. Instead of the traditional analog power control loop with a fixed compensation, we´ve implemented a high-throughput digital power control loop with a configurable transfer function. This is made possible by leveraging compelling trends in field programmable gate arrays (FPGA), converter technologies, and productive software. We will provide an architectural overview of our solution to this problem including an analysis of its key technologies and demonstration of its performance. We will show that not only does this instrument architecture address a problem that has been ignored for years by instrument manufacturers, but does so without sacrificing traditional SMU figures of merit such as density, accuracy, precision, and speed.