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Chapter 9
Isolation and Digital Isolators on USB-6215/6216/6218 Devices
©
National Instruments Corporation
9-4
Isolated Systems
A f
u
lly isolated meas
u
rement system is one where the device’s isolated
front end is not connected back to earth gro
u
nd.
Create an AC path back to earth gro
u
nd from the device’s isolated gro
u
nd
by connecting a high voltage capacitor between the isolated board gro
u
nd
and earth gro
u
nd. The voltage rating of the capacitor m
u
st be larger than
the voltage drop between the isolated gro
u
nd and earth gro
u
nd.
Non-Isolated Systems
A non-isolated meas
u
rement system is one where the device’s isolated
front end connects to earth gro
u
nd.
Add an AC ret
u
rn path from the device isolated gro
u
nd to earth gro
u
nd. For
non-isolated systems, an AC ret
u
rn path is only needed for high or so
u
rce
impedances. An AC ret
u
rn path can be created by connecting a capacitor
between the device’s isolated gro
u
nd and earth gro
u
nd.