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Chapter 11 Isolation and Digital Isolators

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.

National Instruments NI USB-621x User Manual

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