Chapter 4
Signal Connections
PCI-4451/4452/4453/4454 User Manual
4-14
www.ni.com
Types of Signal Sources
Before configuring the input channels and making signal connections,
determine whether the signal sources are floating or ground-referenced.
The following sections describe these two types of signals.
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.
Ground-Referenced 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 PCI-445X, assuming that you plug the computer
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 between two instruments connected to
the same building power system is typically between 1 and 100 mV, but can
be much higher if power distribution circuits are not properly connected.
For this reason, National Instruments does not recommend connecting
AIGND or AOGND to the source signal ground system, since the
difference between the grounds can induce currents in the PCI-445X
ground system.
Analog Output Signal Connections
♦
PCI-4451
The analog output signals for the PCI-4451 are +DAC0OUT,
−
DAC0OUT,
+DAC1OUT, –DAC1OUT, and AOGND. +DAC0OUT and
−
DAC0OUT
are the plus and minus voltage output signals for analog output channel 0.
+DAC1OUT and
−
DAC1OUT are the plus and minus voltage output signal
for analog output channel 1.
AOGND is a ground-reference signal for both analog output channels. It is
connected directly to the ground system on the PCI-4451. You can use this
signal for a general analog ground tie point to your PCI-4451 if necessary,
