  This paper introduces a method for characterizing the current noise of
commercial constant-current sources(CCSs) using a free-induction-decay(FID)
type optically-pumped rubidium atomic magnetometer driven by a
radio-frequency(RF) magnetic field. We convert the sensitivity of the atomic
magnetometer into the current noise of CCS by calibrating the coil constant. At
the same time, the current noise characteristics of six typical commercial
low-noise CCSs are compared. The current noise level of the KeySight Model
B2961A is the lowest among the six tested CCSs, which is 36.233 0.022 nA /
Hz1/2 at 1-25 Hz and 133.905 0.080 nA / Hz1/2 at 1-100 Hz respectively. The
sensitivity of atomic magnetometer is dependent on the current noise level of
the CCS. The CCS with low noise is of great significance for high-sensitivity
atomic magnetometer. The research provides an important reference for promoting
the development of high precision CCS, metrology and basic physics research.
