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.
Keywords: SI traceable photocurrent; calibration; current amplifier; current-to-voltage converter; low DC current; reference current meter; uncertainty.