National Instruments Data Acquisition Device NI USB-621x - Manual (Page 133)

Chapter 9 Isolation and Digital Isolators on USB-6215/6216/6218 Devices
NI USB-621x User Manual 9-3 ni.com
of error and noise, are the result of a measurement system having
multiple grounds at different potentials.
• Improved safety—The isolation barrier allows you to make floating
measurements while protecting the USB host computer against large
transient voltage spikes.
Reducing Common-Mode Noise
Isolated products require an isolated power supply to deliver power to the
isolated side from the non-isolated side. Isolated power supplies work by
switching voltages through a transformer with high-speed transistors.
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.
You can perform some tasks 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-6215/6216/6218 device withstand
voltage and isolation specifications. Withstand voltage must be retested by an approved
testing facility after adjustments are made to the measurement system.