Filter
Top Products
Chapter 3 Signal Connections
© National Instruments Corporation 3-5 DAQCard-700 User Manual
isolation amplifiers. The ground reference of a floating signal must be tied to the DAQCard-700
analog input 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.
Types of Measurement Systems
In addition to determining the type of signal source, you also need to determine whether your
measurement system is floating or ground referenced. These two types of measurement systems
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.
Input Configurations
To measure the different types of input signals, you can configure the DAQCard-700 for one of
two input modes–DIFF or RSE. These two modes, differential and referenced single-ended, can
be implemented by changing the configuration of the instrumentation amplifier onboard the
DAQCard-700. Figure 3-2 shows a diagram of the DAQCard-700 instrumentation amplifier.