Chapter 3
Connecting Signals
3-4
ni.com
Caution
Exceeding the input signal range distorts input signals. Exceeding the maximum
input voltage rating may damage the DAQCard-700 card and the computer. NI is
not
liable
for any damage resulting from such signal connections.
Types of Signal Sources
When configuring the input mode of the DAQCard-700 and making
signal connections, first determine whether the signal source and the
measurement system are floating or ground referenced. The two signal
source types are described as follows, and the types of measurement
systems are described in later sections.
Ground-Referenced Signal Sources
A ground-referenced signal source is one that is connected to the building
system ground. 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 improperly connected.
The connection instructions described later in this chapter for grounded
signal sources are designed to eliminate this ground potential difference
from the measured signal.
Floating Signal Sources
A floating, or nonreferenced, signal source is one that is not connected in
any way to the building ground system and has instead 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. The ground reference of a
floating signal must be tied to the DAQCard-700 AI ground to establish
a local or onboard reference for the signal. Otherwise, the measured input
signal varies or appears to float. An instrument or device that provides
an isolated output falls into the floating signal source category.
Measurement System Types
In addition to determining the signal source type, you must also determine
whether the measurement system is ground-referenced or floating. These
two measurement system types are described below. Depending on
the power connection, a portable computer can represent either a
ground-referenced or floating measurement system. If a portable computer
is entirely battery powered, it is a floating system. If it is operated from an