Keithley 62212182ae digital multimeter dmm keithley 6221 the keithley 6221 is a digital multimeter ac and dc current source that combines ease of use with exceptionally low current noise it is ideal for applications in test environments ranging from rampd to production especially in the semiconductor nanotechnology and superconductor industries the 6221 is the only low current ac source on the market before its introduction researchers and engineers were forced to build their own