National Instruments USB-621 Series, User Manual

Page: 139 / 205

Chapter 9 Isolation and Digital Isolators on USB-6215/6216/6218 Devices
© National Instruments Corporation 9-2 NI USB-621x User Manual
The non-isolated gro u nd is connected to the chassis gro u nd of the PC.
Each isolated gro u nd is not connected to the chassis gro u nd of the PC.
The isolated gro u nd can be at a higher or lower voltage relative to the
non-isolated gro u nd. All analog meas u rements are made relative to the
isolated gro u nd signal.
The isolated gro u nd is an inp u t to the USB-6215/6216/6218 device. The
u ser must connect this gro u nd to the gro u nd of system being meas u red 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 introd u ce any analog error in the meas u rements
taken by the device. The A/D converter, u sed for analog inp u t, is on the
isolated side of the device. The analog inp u ts are digitized before they are
sent across the isolation barrier. Similarly, the D/A converters, u sed for
analog o u tp u t, are on the isolated side of the device.
Benefits of an Isolated DAQ Device
With isolation, engineers can safely meas u re 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
meas u rement system to reject common-mode voltages.
Common-mode voltage is the signal that is present or “common” to
both the positive and negative inp
u t of a meas u rement device, b u t is
not part of the signal to be meas u red.
• Improved Accuracy —Isolation improves meas u rement acc u racy by
physically preventing gro u nd loops. Gro u nd loops, a common so u rce