Current-to-voltage converters are used in many photometric and radiometric applications. The calibration of current- to-voltage converters at a few input currents is not always sufficient to understand the linearity and the bias of a device. Many devices have structure deviating from a linear response over the operating range of a gain setting. Measurement services that rely on these devices now have decreased uncertainties to a level that requires quantifying the uncertainties and understanding how they propagate. The National Institute of Standards and Technology has developed a system to calibrate the current-to-voltage conversion factor or "gain" and offset of these devices for direct current photocurrents. The equipment used for the calibration is described here, and the results and uncertainties are discussed.