National Instruments NI 435 Series, User Manual, Page: 22 / 77

Chapter 2 Operating the NI 435x Device
NI 435x User Manual 2-4 ni.com
Knowing the Signal Source
For accurate measurements, you must determine whether the signal source
is floating or ground-referenced.
Floating Signal Source
A floating signal source is one that is not connected in any way to
the building ground system but has an isolated ground-reference
point. Examples of floating signal sources are thermocouples with
ungrounded junctions and outputs of transformers, batteries,
battery-powered devices, optical isolators, and isolation amplifiers.
Ground-Referenced Signal Source
A ground-referenced signal source is one that is connected in some way to
the building system ground. Therefore, it is already connected to a common
ground point with respect to the NI 435
x , assuming that the computer is
plugged into the same power system. Examples of ground-referenced
signal sources include the following:
• Thermocouples with grounded or exposed junctions connected to
grounded test points
• The outputs of plug-in devices with nonisolated outputs
• Voltage across RTDs, thermistors, or resistors that you may be
measuring using the built-in current sources of the NI 435
x
Using Programmable Ground-Referencing
The NI 435 x devices have software-programmable ground-referencing on
every channel, which you can use to ground-reference a floating signal
source. This connects CH– to ground through a 10 M
Ω
resistor and
provides a ground-reference for the floating signal source. Even if the
signal source is ground-referenced, this resistance minimizes the effects of
ground-loops, as long as the source impedance and the lead wire resistance
is less than 100
Ω
. Thus, you can take accurate measurements even if you
are uncertain whether the signal source is floating or ground-referenced.
Because you can set ground-referencing on a channel-by-channel basis,
you can have ground-referenced signal sources connected to some channels
and floating signal sources connected to other channels in the same
measurement setup. Table 2-2 summarizes the settings to use for
ground-referencing.