National Instruments NI 4350, User Manual

Page: 25 / 83

Chapter 3 NI 4350 Operation
NI 4350 User Manual 3-4 © National Instruments Corporation
Knowing Your Signal Source
For accurate measurements, you must determine whether your 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 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.
Using Programmable Ground-Referencing
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 M
Ω
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.