National Instruments DAQ M Series - Manual (Page 109)

Chapter 9 Isolation and Digital Isolators
© National Instruments Corporation 9-3 NI 6236 User Manual
These parasitic currents interact with parasitic and non-parasitic resistances
causing voltage spikes. These voltage spikes are called common-mode
noise, a noise source that travels in the ground and is therefore common to
both the ground and any signal referenced to the ground, such as AI, AO,
and digital signals. Common-mode noise appears at the harmonics of the
switching power supply frequency and can corrupt measurements
depending on the system setup.
To reduce common-mode noise:
• Better grounding from the front connector can reduce common-mode
noise. Use low resistance cabling and connections and verify that all
ground connections are kept short. Keep the number of connections to
a minimum. If the device’s isolated ground is being connected back to
earth ground, verify that this is done in the most direct way possible.
• Reduce source impedances if possible. The parasitic currents react
with these impedances.
Creating an AC Return Path
Caution Adding a capacitor will degrade the M Series device withstand voltage and
isolation specifications. Withstand voltage must be retested by an approved testing facility
after adjustments are made to the measurement system.
Non-Isolated Systems
A non-isolated measurement system is one where the device’s isolated
front end connects to earth ground.
In this case, further noise reduction can be achieved by adding 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.
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.