Connecting Analog Input Signals; Types Of Signal Sources - National Instruments PXIe-6355 User Manual

Connecting Analog Input Signals

Table 4-7 summarizes the recommended input configuration for different types of signal sources
for Simultaneous MIO X Series devices.
Table 4-7. Simultaneous MIO X Series Analog Input Signal Configuration
Examples:
•
•
Input
•
Differential
(DIFF)
Refer to the Analog Input Terminal Configuration

Types of Signal Sources

When configuring the input channels and making signal connections, first determine whether the
signal sources are floating or ground-referenced:
• Floating Signal Sources—A floating signal source is not connected in any way to the
building ground system, and instead has an isolated ground-reference point. Some
examples of floating signal sources are outputs of transformers, thermocouples,
battery-powered devices, optical isolators, and isolation amplifiers. An instrument or
device that has an isolated output is a floating signal source. You must connect the ground
reference of a floating signal to the AI ground of the device to establish a local or onboard
reference for the signal. Otherwise, the measured input signal varies as the source floats
outside the common-mode input range.
• Ground-Referenced Signal Sources—A ground-referenced signal source is connected in
some way to the building system ground and is, therefore, already connected to a common
ground point with respect to the device, assuming that the computer is plugged into the
same power system as the source. Non-isolated outputs of instruments and devices that plug
into the building power system fall into this category.
The difference in ground potential between two instruments connected to the same building
power system is typically between 1 mV and 100 mV, but the difference can be much
Floating Signal Sources
(Not Connected to
Earth Ground)
Ungrounded thermocouples
Signal conditioning with isolated
outputs
Battery devices
AI 0 +
+
V
AI 0 –
1
–
R
R
Ground-Referenced Signal
Example:
•
+
+
–
–
AI GND
section for descriptions of the input modes.
X Series User Manual
Sources
Plug-in instruments with
non-isolated outputs
AI 0 +
V
AI 0 –
1
© National Instruments | 4-41
+
–
AI GND