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Signal Connections
are described below. Depending on the power connection, a portable computer can represent
either a floating or ground-referenced measurement system. If a portable computer is entirely
battery powered, it is a floating system. If it is operated from an AC/DC wall adapter, it may or
may not be ground referenced, depending on the connection. You should determine from your
computer documentation whether any of the power connections are tied to the building power
system ground.
Ground-Referenced Measurement System
A ground-referenced measurement system is one that is connected in some way to the building
system ground. Instruments that plug into the building power system fall into this category.
Floating Measurement System
A floating measurement system is one that is not connected in any way to the building ground
system but rather has an isolated ground-reference point. Some examples of floating
measurement systems are battery-powered instruments, instruments powered with a nonground-
referenced power adapter, and instruments with differential inputs. A floating measurement
system will float to the level of the signals being measured.
Analog Input Connections
The analog input connections on the DAQCard-500 are single ended. They are measured with
respect to one common ground. The input signals are tied to the positive input of an operational
amplifier that is referenced to this common ground. Figure 3-2 shows how to connect single-
ended analog input signals to the DAQCard-500. Table 3-1 summarizes this information.
Table 3-1. Recommended Input Configurations for Ground-Referenced and Floating
System Type
Floating system such as
a battery-powered
computer
Ground-referenced
system such as a
desktop computer
DAQCard-500 User Manual
Measurement Systems
Type of Signal
Floating, not referenced (batteries,
thermocouples, isolated outputs)
Grounded, referenced (nonisolated
outputs, plug-in instruments)
Floating, not referenced (batteries,
thermocouples, isolated outputs)
Grounded, referenced (nonisolated
outputs, plug-in instruments)
3-4
Considerations
Signal ground (negative side of
signal) should be connected to
AIGND of the DAQCard-500.
Signal ground (negative side of
signal) should be connected to
AIGND of the DAQCard-500.
Signal ground (negative side of
signal) should be connected to
AIGND of the DAQCard-500.
Not recommended due to noise
caused by ground currents
© National Instruments Corporation
Chapter 3
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