Low-current calibration service is not available from most NMIs even if the lowest possible current measurement uncertainty is needed in optical radiometery, especially in radiation thermometry. In order to solve this problem at NIST, reference current-to-voltage converters were designed and constructed. A new calibration procedure was developed for the reference converters. The converter design and calibration issues and the uncertainties are discussed here. The new converters were compared to a previous generation converter and also to a commercial current source. Now, DC current measurements between 10 pA and 10 mA can be performed with uncertainties lower than 0.006 % (k=2) upto a signal-gain of 1010 V/A. The uncertainty of the calibration was improved by one order of magnitude for a high-performance commercial current-source at currents lower than 100 nA. Based on the described procedure, low DC-current routine calibrations are suggested to NMIs. A calibrated travelling current-source can be used for gain calibrations of test current-to-voltage converters.
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