Frequency dependence of gas-dielectric capacitors used in sub-nA reference current generators

We measure the frequency dependence of a sample of commercial air- and gas-dielectric capacitors between 1 kHz and ≈0.01 Hz. We find that the capacitance of sealed-gas capacitors can change by more than 100 μF/F over this range. This has important implications for the use of gas capacitors in reference sub-nA current generators.