In recent years, the electrical substations are experiencing a rapid transition towards a fully digital measurement infrastructure, in terms of data acquisition, storage and processing. The IEC Std 61850-9-2 defines the data format, also known as Sampled Value (SV), to be employed by instrument transformers, merging units, and controller devices. In this scenario, it is reasonable to expect that also Phasor Measurement Units (PMUs) will be directly connected with digital instrument transformers and will be required to process directly the SV data stream. In this paper, we present the design and development of a stand-alone PMU based on digital inputs to be used as reference instrument in the metrological characterization of Intelligent Electrical Devices (IEDs) operating both as PMU and Stand Alone Merging Units (SAMUs). The prototype has been characterized in terms of estimation accuracy and computational requirements, by means of a dedicated IEC Std 61850-9-2 calibration system. The preliminary results prove the potential of SV-based PMUs, but also reveal some implementation bottlenecks that have to be properly addressed before an effective deployment on the field.