Appendix A Device-Specific Information
NI 6124/6154 User Manual A-12 ni.com
The non-isolated ground is connected to the chassis ground of the PC or
chassis where the device is installed.
The isolated ground is not connected to the chassis ground of the PC or
chassis. 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 NI 6154 device. The user must
connect this ground to the ground of the system being measured or
controlled. For more information, refer to the following:
• The Connecting Analog Input Signals section of Chapter4, Analog
Input
• Figure4-2, Isolated S Series Analog Input Block Diagram
• The Connecting Analog Output Signals section of Chapter5, Analog
Output
• Figure5-2, Isolated S Series Device Analog Output Block Diagram
• The Connecting Digital I/O Signals on Isolated Devices section of
Chapter6, D igital I/O
• Figure6-5, Isolated S Series Devices Digital I/O Block Diagram
• Chapter8, Programmable Function Interfaces (PFI)
Digital Isolation
The NI 6154 uses 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 voltage in the presence
of a large common-mode 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.