Isolated Systems; Non-Isolated Systems - National Instruments Data Acquisition Device NI USB-621x User Manual

Chapter 9 Isolation and Digital Isolators on USB-6215/6216/6218 Devices

Isolated Systems

A fully isolated measurement system is one where the device's isolated
front end is not connected back to earth ground.
Create an AC path back to earth ground from the device's isolated ground
by connecting a high voltage capacitor between the isolated board ground
and earth ground. The voltage rating of the capacitor must be larger than
the voltage drop between the isolated ground and earth ground.

Non-Isolated Systems

A non-isolated measurement system is one where the device's isolated
front end connects to earth ground.
Add an AC return path from the device isolated ground to earth ground. For
non-isolated systems, an AC return path is only needed for high or source
impedances. An AC return path can be created by connecting a capacitor
between the device's isolated ground and earth ground.
© National Instruments Corporation 9-4 NI USB-621x User Manual