Keysight Technologies B1500A Self-Paced Training Manual page 107

Module 3
Low Current Measurement
For ultra low current measurement, use ASU (atto sense/switch unit).
The HRSMU has an innate 1 fA measurement resolution. And the ASU extends it to 0.1 fA.
Note that the ASU must be connected to the HRSMU before turning on the B1500A.
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