An improved measurement system may include a source measure unit (SMU) capable of performing accurate low-level current measurements. Based on an SMU design that provides a controlled DC voltage source with precision current limiting and a controlled 0V (zero Volt) DC at the measurement terminal, an AC design may be implemented to establish the same (or very similar) conditions over a specified frequency range. Instead of controlling each digital-to-analog converter (DAC) at respective source terminals of the SMU as a respective DC output, each DAC may be controlled as a respective function generator with programmable frequency and continuously variable phase and amplitude. Off-the-shelf pipelined analog-to-digital converters (ADCs) may be used to monitor voltage, current and the voltage at the measurement terminal, and a Fourier transform may be used to obtain both the amplitude and relative phase measurements to be provided to respective control loops.