Calibration Setup for Ultralow-Current Transresistance Amplifiers

We describe a setup for the calibration of the transresistance gain of low-current amplifiers, based on the capacitance-charging method. The calibration can be performed in the current range of typical interest for electron-counting experiments. The setup implementation is simple and rugged, and is suitable to be embedded in larger experiments, where the amplifier is employed. The calibrated transresistance is traceable to the units of capacitance and time. Two different calibration modes were tested: with dc current (obtained using a custom-made piecewise linear ramp generator) and with low-frequency sinewave current (using a commercial generator). The relative base accuracy of the implementation is in the 10^-5 range.