National Instruments USB-621 Series, User Manual

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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 res u lt of a meas u rement system having
m u ltiple gro u nds at different potentials.
• Improved Safety —The isolation barrier allows yo u to make floating
meas u rements while protecting the USB host comp u ter against large
transient voltage spikes.
Reducing Common-Mode Noise
Isolated prod u cts req u ire an isolated power s u pply to deliver power to the
isolated side from the non-isolated side. Isolated power s u pplies work by
switching voltages thro u gh a transformer with high-speed transistors.
Switching voltages thro u gh the transformer ca u se charging and
discharging of the parasitic capacitances and ind u ctances in the switching
power s u pplies that occ u r on every switch cycle, res u lting in high-speed
c u rrents flowing thro u gh the isolated side and ret u rning to the non-isolated
side, which is earth gro u nd.
These parasitic c u rrents interact with parasitic and non-parasitic resistances
ca u sing voltage spikes. These voltage spikes are called common-mode
noise, a noise so u rce that travels in the gro u nd and is therefore common to
both the gro u nd and any signal referenced to the gro u nd, s u ch as AI, AO,
and digital signals. Common-mode noise appears at the harmonics of the
switching power s
u pply freq u ency and can corr u pt meas u rements
depending on the system set u p.
Yo u can perform some tasks to red u ce common-mode noise:
• Better gro u nding from the front connector AI GND to the signal so u rce
gro u nd can red u ce common-mode noise. Use low resistance cabling
and connections and verify that all gro u nd connections are kept short.
Keep the n u mber of connections to a minim u m. If the device’s isolated
gro u nd is being connected back to earth gro u nd, verify that this is done
in the most direct way possible.
• Red u ce so u rce impedances if possible. The parasitic c u rrents 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 adj u stments are made to the meas u rement system.