  We describe a calibration setup for the transresistance 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
calibration is traceable to units of capacitance and of time. The base relative
accuracy of the implementation is in the 10^-5 range.
