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Chapter 9 Isolation and Digital Isolators on USB-6215/6216/6218 Devices

The non-isolated ground is connected to the chassis ground of the PC.

Each isolated ground is not connected to the chassis ground of the PC.

The isolated ground can be at a higher or lower voltage relative to the

non-isolated ground. All analog measurements are made relative to the

isolated ground signal.

The isolated ground is an input to the USB-6215/6216/6218 device. The

user must connect this ground to the ground of system being measured or

controlled. For more information, refer to the following:

• The Connecting Analog Input Signals on USB-6215/6216/6218

Devices section of Chapter 4, Analog Input

• The Connecting Analog Output Signals section of Chapter 5, Analog

Output

• The Connecting Digital I/O Signals on USB-6210/6211/6215/6218

Devices or Connecting Digital I/O Signals on USB-6212/6216 Devices

section of Chapter 6, Digital I/O

• The Connecting PFI Input Signals section of Chapter 7, PFI

Digital Isolation

USB-6215/6216/6218 devices u se digital isolators. Unlike analog isolators,

digital isolators do not introduce any analog error in the measurements

taken by the device. The A/D converter, used for analog input, is on the

isolated side of the device. The analog inputs are digitized before they are

sent across the isolation barrier. Similarly, the D/A converters, used for

analog output, are on the isolated side of the device.

Benefits of an Isolated DAQ Device

With isolation, engineers can safely measure a small signal in the presence

of a large common-mode voltage signal. Some advantages of isolation are

as follows:

• Improved Rejection—Isolation increases the ability of the

measurement system to reject common-mode voltages.

Common-mode voltage is the signal that is present or “common” to

both the positive and negative input of a measurement device, but is

not part of the signal to be measured.

• Improved Accuracy—Isolation improves measurement accuracy by

physically preventing ground loops. Ground loops, a common source

National Instruments USB-621 Series User Manual

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