Significant DC current on precision DMM despite not being connected to anything
What is the origin of these currents?: Every cable is an antenna. I think you measure noise (FM radio, cell phone, WIFI, Bluetooth, DECT, ...)
How do I measure currents with nA accuracy? Use shielded cables; shield your DUT; Use a battery as supply for your DUT; Eventually use a shielding enclosure for the whole setup (DUT + DMM); increase the number of samples the DMM uses to build the RMS value;
Maybe this free book helps: &quot;Precision DC Current, Voltage,and Resistance Measurements&quot;
https://download.tek.com/document/LowLevelHandbook_7Ed.pdf