Chapter 2
Using the NI 447X
2-8
ni.com
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
AI 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 447
X
, you tie the signal ground to the AI ground
simply by attaching the signal cable to any of the AI channel
SMB connectors. Hence, all floating signals fed to the NI 447
X
are
automatically ground referenced.
Note
Attach the NI 447
X
to the chassis with screws to provide a reliable ground
connection. If you are using an NI PCI-4472/4474, keep the small screw that held the PCI
slot cover to the computer chassis. Reinsert this screw to securely attach the NI 447
X
. If
you are using an NI PXI-4472, be sure to tighten the small screws at the top and bottom of
the front face of the module.
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 447
X
, assuming the PCI-bus computer or PXI
or CompactPCI chassis and controller are plugged into the same power
system. Non-isolated 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 50
Ω
resistor on the signal ground is usually sufficient to reduce this current to
negligible levels, but results can vary depending on the system setup.
It is best to use the NI 447
X
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 447
X
.
