National Instruments 700 - Manual (Page 26)

Chapter 3 Signal Connections
© National Instruments Corporation 3-7 DAQCard-700 User Manual
Recommended Input Configurations
The following sections discuss the use of single-ended and differential measurements and
considerations for measuring both floating and ground-referenced signal sources. Table 3-1
summarizes the recommended input configurations for both types of signal sources.
Table 3-1. Recommended Input Configurations for Ground-Referenced and Floating
Measurement Systems
System Type Type of Signal Recommended Input
Configurations
Ground-referenced system,
such as a desktop computer Ground referenced
(nonisolated outputs, plug-in instruments)
DIFF
Floating
(batteries, thermocouples, isolated outputs)
DIFF with bias resistors
RSE
Floating system, such as a
battery-powered computer Ground referenced
(nonisolated outputs, plug-in instruments)
DIFF with bias resistors
RSE
Floating
(batteries, thermocouples, isolated outputs)
DIFF with bias resistors
RSE
Single-Ended Connection Considerations
Single-ended connections are those in which all DAQCard-700 analog input signals are
referenced to one common ground. The input signals are tied to the positive input of an
operational amplifier that is referenced to their common ground point.
When the DAQCard-700 is configured for single-ended input, 16 analog input channels are
available. You can use single-ended input connections when the following criteria are met by all
input signals.
• Input signals are high level (greater than 1 V).
• Leads connecting the signals to the DAQCard-700 are less than 15 ft.
• All input signals share a common reference signal (at the source).
If any one of the preceding criteria is not met, use the DIFF input configuration.
The DAQCard-700 single-ended configuration is referenced. You can use this mode for
nonreferenced signal sources. In addition, if the computer using the DAQCard-700 is not ground
referenced, you can use this mode for ground-referenced signal sources.
Figure 3-3 shows how to connect a signal source to a DAQCard-700 in single-ended mode.
When you connect grounded signal sources to a DAQCard-700 in a ground-referenced computer,
carefully observe the polarity to avoid shorting the signal source output. A laptop operating from
a battery is not grounded. A laptop powered from an AC/DC adapter may or may not be
grounded.