National Instruments NI USB-621x - Manual (Page 127)

Chapter 11 Isolation and Digital Isolators
© National Instruments Corporation 11-3 NI USB-621x User Manual
Switching voltages through the transformer cause charging and discharging
of the parasitic capacitances and inductances in the switching power
supplies that occur on every switch cycle, resulting in high-speed currents
flowing through the isolated side and returning to the non-isolated side,
which is earth ground.
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 AI GND to the signal source
ground 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 USB 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.
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.