In the revised International System of Units (SI), the unit of electrical capacitance, the farad, is defined from the exact value of the von Klitzing constant, RK = h/e2 = 25812.8074593045 Ω, and from the unit of time. This opens the possibility of realizing the farad directly from the quantum Hall effect by means of suitable impedance bridges. We present here the design, the implementation and a preliminary validation against the Italian national capacitance scale of a four-terminal-pair fully-digital impedance bridge optimized for the direct comparison of an 8 nF standard capacitor with a quantum Hall resistance standard at a frequency of about 1541 Hz, where the impedance magnitude ratio is 1 : 1. The uncertainty of the 1 : 1 comparison, according to a preliminary evaluation, is at the level of 10–7. We also present here a possible new traceability chain for the farad based on a fully-digital impedance bridge and on a graphene quantized Hall resistance in the AC regime.
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