Chapter 9 Isolation and Digital IsolatorsNI 6232/6233 User Manual 9-2 ni.comThe non-isolated ground is connected to the chassis ground of the PC orchassis where the device is installed.The isolated ground is not connected to the chassis ground of the PC orchassis. The isolated ground can be at a higher or lower voltage relative tothe non-isolated ground. All analog measurements are made relative to theisolated ground signal.The isolated ground is an input to the NI 6232/6233 device. The user mustconnect this ground to the ground of system being measured or controlled.Refer to the Connecting Analog Voltage Input Signals section of Chapter 4,Analog Input, the Connecting Analog Voltage Output Signals section ofChapter 5, Analog Output, the Connecting Digital I/O Signals section ofChapter 6, Digital Input and Output, and the Connecting PFI Input Signalssection of Chapter 8, PFI, for more information.Digital IsolationThe NI 6232/6233 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 voltage in the presenceof a large common-mode signal. Some advantages of isolation are asfollows:• 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 against large transientvoltage spikes.
