National Instruments NI 6238 用户手册

© National Instruments Corporation

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Improved safety—Isolation creates an insulation barrier so you can 
make measurements at elevated voltages while protecting against large 
transient voltage spikes.

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 causes 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:

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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.

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Reduce source impedances if possible. The parasitic currents react 
with these impedances.