Chapter 2
Using Your NI 4472
©
National Instruments Corporation
2-7
NI 4472 User Manual
Floating Signal Sources
A floating signal source does not connect in any way to the building ground
system but instead has an isolated ground-reference point. Some examples
of floating signal sources are outputs of transformers, thermocouples,
battery-powered devices, optical isolator outputs, and isolation amplifiers.
An instrument or device that has an isolated output is a floating signal
source.
It is important to tie the ground reference of a floating signal to the analog
input ground to establish a local reference for the signal. Otherwise, the
measured input signal varies as the source floats out of the common-mode
input range. With the NI 4472, you tie the signal ground to the analog input
ground simply by attaching the signal cable to any of the analog input
channel SMB connectors. Hence, all floating signals fed to the NI 4472 are
automatically ground referenced.
Note
To ensure a good ground connection, securely fasten the front panel of the NI 4472
to the chassis with the screw that held the slot cover (NI 4472 for PCI) or two screws
attached for that purpose (NI 4472 for PXI/CompactPCI).
Grounded Signal Sources
A ground-referenced signal source connects in some way to the building
system ground and is, therefore, already connected to a common-ground
point with respect to the NI 4472, assuming the PCI-bus computer or PXI
or CompactPCI chassis and controller are plugged into the same power
system. Nonisolated outputs of instruments and devices that plug into the
building power system fall into this category.
The difference in ground potential, or common-mode voltage, between
two instruments connected to the same building power system is typically
between 1 and 100 mV, but the common-mode voltage can be much higher
if power distribution circuits are not properly connected. This difference in
ground potential induces currents in the ground system that can cause
errors in your measurement. For low common-mode voltages, the resistor
on the signal ground has a resistor value of 50
Ω
and is usually sufficient to
reduce this current to negligible levels, but your results can vary depending
on the system setup.
It is best to use the NI 4472 to acquire data from floating signal sources, but
you can measure signals from grounded sources if the ground reference of
the source does not vary by more than ±2.5 V from the ground reference of
the NI 4472.