*Article* **Calibration of a Digital Current Transformer Measuring Bridge: Metrological Challenges and Uncertainty Contributions †**

**Guglielmo Frigo \* and Marco Agustoni**

Swiss Federal Institute of Metrology METAS, Lindenweg 50, 3007 Bern-Wabern, Switzerland; marco.agustoni@metas.ch

**\*** Correspondence: guglielmo.frigo@metas.ch

† Any mention of commercial products within this paper is for information only; it does not imply recommendation or endorsement by METAS.

**Abstract:** In this paper, we consider the calibration of measuring bridges for non-conventional instrument transformers with digital output. In this context, the main challenge is represented by the necessity of synchronization between analog and digital outputs. To this end, we propose a measurement setup that allows for monitoring and quantifying the main quantities of interest. A possible laboratory implementation is presented and the main sources of uncertainty are discussed. From a metrological point of view, technical specifications and statistical analysis are employed to draw up a rigorous uncertainty budget of the calibration setup. An experimental validation is also provided through the thorough characterization of the measurement accuracy of a commercial device in use at METAS laboratories. The proposed analysis proves how the calibration of measuring bridges for non-conventional instrument transformers requires ad hoc measurement setups and identifies possible space for improvement, particularly in terms of outputs' synchronization and flexibility of the generation process.

**Keywords:** measuring bridge; calibration; non-conventional instrument transformer; sampled values; digital output; synchronization
