The spec sheet says ±1 fA. The wafer says: maybe — if you've grounded the right things.
On a CheckMate with a triaxial chuck, guarded probes, and a dry enclosure, the analyzer reads stably into the femtoamp range. But getting from a clean sweep on the calibration substrate to a clean sweep on your *actual* device is the part the datasheet hand‑waves through.
Three failures that bite people in the first week, in order of how much they cost: a single ungrounded scope probe sitting in line‑of‑sight of the DUT; humidity over 35 % in the test enclosure; and the BNC cable that someone routed across the floor near the chiller. None of these will show up in a sanity check. All of them will lift the noise floor by an order of magnitude.
Long version with photos and Keysight B1500 screenshots coming next week. If you've hit one of these and want help: service@signatone.com.