Chapter 11 Isolation and Digital IsolatorsNI USB-621x User Manual 11-2 ni.comThe isolated ground is an input to the USB-6215/6218 device. The usermust connect this ground to the ground of system being measured orcontrolled. Refer to Chapter 5, Connecting AI Signals on theUSB-6210/6211 Devices, Chapter 6, Connecting AI Signals on theUSB-6215/6218 Devices, the Connecting Analog Output Signals section ofChapter 7, Analog Output, the Connecting Digital I/O Signals section ofChapter 8, Digital I/O, and the Connecting PFI Input Signals section ofChapter 10, PFI, for more information.Digital IsolationThe USB-6215/6218 uses digital isolators. Unlike analog isolators, digitalisolators do not introduce any analog error in the measurements taken bythe device. The A/D converter, used for analog input, is on the isolated sideof the device. The analog inputs are digitized before they are sent across theisolation barrier. Similarly, the D/A converters, used for analog output, areon the isolated side of the device.Benefits of an Isolated DAQ DeviceWith isolation, engineers can safely measure a small signal in the presenceof a large common-mode voltage signal. Some advantages of isolation areas follows:• Improved rejection—Isolation increases the ability of themeasurement system to reject common-mode voltages.Common-mode voltage is the signal that is present or “common” toboth the positive and negative input of a measurement device, but is notpart of the signal to be measured.• Improved accuracy—Isolation improves measurement accuracy byphysically preventing ground loops. Ground loops, a common sourceof error and noise, are the result of a measurement system havingmultiple grounds at different potentials.• Improved safety—Isolation creates an insulation barrier so you canmake floating measurements while protecting the USB host computeragainst large transient 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.
