Realization of the farad from the dc quantum Hall effect with digitally-assisted impedance bridges

A traceability chain for the derivation of the farad from dc quantum Hall effect has been recently implemented at INRIM. The main components of the chain are two coaxial transformer bridges: a resistance ratio bridge and a quadrature bridge, both operating at 1541 Hz. Both bridges are energized and controlled by a polyphase sinewave direct-digital-synthesizer; the automated control of relative amplitudes and phases of the generator voltage outputs permits principal and auxiliary balances of each bridge to be achieved. Such a technique, which may be called digital assistance, is implemented for the first time in an ohm-farad traceability chain. The relative standard uncertainty in the realization of the farad, at the level of 1000 pF, is estimated to be 64 x 10(-9). A first verification of the realization is given by a comparison with the maintained INRIM capacitance standard, where an agreement within a relative combined standard uncertainty of 420 x 10(-9) is obtained.