Knowing Your Signal Source - National Instruments NI 4350 User Manual

Chapter 3 NI 4350 Operation

Knowing Your Signal Source

Floating Signal Source
Ground-Referenced Signal Source
Using Programmable Ground-Referencing
NI 4350 User Manual
For accurate measurements, you must determine whether your signal
source is floating or ground-referenced.
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.
A ground-referenced signal source is one that is connected in some way
to the building system ground and is, therefore, already connected to a
common ground point with respect to the NI 4350 instrument, assuming
that the computer is plugged into the same power system. Examples of
ground-referenced signal sources are thermocouples with grounded or
exposed junctions connected to grounded test points and outputs of
plug-in devices with nonisolated outputs, voltage across RTDs,
thermistors, or resistors you may be measuring using the built-in current
source of the NI 4350.
Your NI 4350 instrument has 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 MΩ resistor and provides a ground-reference for your
floating signal source. Even if your 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
your 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 3-2 summarizes the settings to use for
ground-referencing.
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