Calibration of Sensitive Ammeters Using a Noiseless Electron Pump

We compare an electron pump and an ultrastable low-noise current amplifier (ULCA) as reference current sources for calibrating a high-gain current preamplifier at pA-level currents. Using the electron pump on a quantized current plateau, the current noise is dominated by the device under test, whereas using the ULCA, the larger ULCA noise is the biggest contribution. In a 20-second measurement at a nominal current of 3.2 pA, the preamplifier gain is resolved with a relative type A uncertainty of $5\times 10^{-5}$ using the pump, and $2\times 10^{-4}$ using the ULCA.