National Instruments NI PCI-6110 - Manual (Page 39)

Chapter 4 Connecting Signals
© National Instruments Corporation 4-9 NI PCI-6110/6111 User Manual
The PGIA applies gain and common-mode voltage rejection and presents
high input impedance to the AI signals connected to the NI PCI-6110/6111.
Signals are routed to the positive and negative inputs of the PGIA. The
PGIA converts two input signals to a signal that is the difference between
the two input signals multiplied by the gain setting of the amplifier. The
amplifier output voltage is referenced to the device ground. The ADC
measures this output voltage when it performs A/D conversions.
Types of Signal Sources
When making signal connections, first determine whether the signal
sources are floating or ground-referenced. The following sections describe
these two signal types.
Floating Signal Sources
A floating signal source is not connected in any way to the building ground
system but, rather, 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. You must tie the ground reference of a floating signal to the
NI PCI-6110/6111 AI ground to establish a local or onboard reference for
the signal. Otherwise, the measured input signal varies as the source floats
out of the common-mode input range.
Ground-Referenced Signal Sources
A ground-referenced signal source 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 PCI-6110/6111, assuming that the
computer is 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 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.
If a grounded signal source is improperly measured, this difference may
appear as a measurement error. The connection instructions for grounded
signal sources are designed to eliminate this ground potential difference
from the measured signal.