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Chapter 4 Signal Connections
PCI-4451/4452 User Manual 4-10
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National Instruments Corporation
amplifier output voltage is referenced to the ground for the device. Your
PCI-4451/4452 device A/D converter (ADC) measures this output voltage
when it performs A/D conversions.
Connection of analog input signals to your PCI-4451/4452 device depends
on the configuration of the input signal sources. For most signals, you use
a DIFF configuration and simply connect the signal to +ACHx (where x is
the PCI-4451/4452 channel) and the signal ground (or signal minus, as
appropriate) to −ACHx. However, if a signal has a high output impedance
(greater than 1 kΩ) and is floating, you may find it useful to use an SE
configuration and tether the signal minus to AIGND to reduce
common-mode interference. You can make the DIFF and SE connections
through the BNC-2140 accessory.
Types of Signal Sources
When configuring the input channels and making signal connections, first
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-4451/4452 device, 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 to the source signal ground system, since the difference between
the grounds can induce currents in the PCI-4451/4452 ground system.
User.book Page 10 Tuesday, April 14, 1998 10:20 AM