Chapter 9 Isolation and Digital Isolators on USB-6215/6216/6218 DevicesNI USB-621x User Manual 9-3 ni.comof error and noise, are the result of a measurement system havingmultiple grounds at different potentials.• Improved safety—The isolation barrier allows you to make floatingmeasurements while protecting the USB host computer against largetransient voltage spikes.Reducing Common-Mode NoiseIsolated products require an isolated power supply to deliver power to theisolated side from the non-isolated side. Isolated power supplies work byswitching voltages through a transformer with high-speed transistors.Switching voltages through the transformer cause charging anddischarging of the parasitic capacitances and inductances in the switchingpower supplies that occur on every switch cycle, resulting in high-speedcurrents flowing through the isolated side and returning to the non-isolatedside, which is earth ground.These parasitic currents interact with parasitic and non-parasitic resistancescausing voltage spikes. These voltage spikes are called common-modenoise, a noise source that travels in the ground and is therefore common toboth the ground and any signal referenced to the ground, such as AI, AO,and digital signals. Common-mode noise appears at the harmonics of theswitching power supply frequency and can corrupt measurementsdepending 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 sourceground can reduce common-mode noise. Use low resistance cablingand connections and verify that all ground connections are kept short.Keep the number of connections to a minimum. If the device’s isolatedground is being connected back to earth ground, verify that this is donein the most direct way possible.• Reduce source impedances if possible. The parasitic currents reactwith these impedances.Creating an AC Return PathCaution Adding a capacitor will degrade the USB-6215/6216/6218 device withstandvoltage and isolation specifications. Withstand voltage must be retested by an approvedtesting facility after adjustments are made to the measurement system.
