Chapter 3
Connecting the Signals
©
National Instruments Corporation
3-7
DAQCard-1200 User Manual
The difference in ground potential between two instruments connected to
the same building power system is typically between 1 mV and 100 mV but
can be much higher if power distribution circuits are improperly connected.
The connection instructions that follow for grounded signal sources
eliminate this ground potential difference from the measured signal.
Note
If you power both the DAQCard-1200 and the PC with a floating power source, such
as a battery, the system may be floating with respect to earth ground. In this case, treat all
the signal sources as floating sources.
Input Configurations
You can configure the DAQCard-1200 for one of three input modes—RSE,
NRSE, or DIFF. The following sections discuss the use of single-ended and
differential measurements and give considerations for measuring both
floating and ground-referenced signal sources. Table 3-3 summarizes the
recommended input modes for both types of signal sources.
Differential Connection Considerations (DIFF Input Mode)
Differential connections are those in which each DAQCard-1200 AI signal
has its own reference signal or signal return path. These connections are
available when you configure the DAQCard-1200 in the DIFF input mode.
Each input signal is tied to the positive input of the instrumentation
amplifier, and its reference signal, or return, is tied to the negative input of
the instrumentation amplifier.
Table 3-3.
Recommended Input Modes for Ground-Referenced
and Floating Signal Sources
Type of Signal
Recommended Input
Configuration
Ground-Referenced
(nonisolated outputs, plug-in instruments)
DIFF
NRSE
Floating
(batteries, thermocouples, isolated outputs)
DIFF with bias resistors
RSE
