National Instruments DAQCard-1200 - Manual (Page 29)

Chapter 3 Signal Connections
© 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 not properly
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 your PC with a floating power source,
such as a battery, your system may be floating with respect to earth ground. In this
case, treat all of your 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 considerations for measuring both floating
and ground-referenced signal sources. Table 3-2 summarizes the
recommended input configurations for both types of signal sources.
Differential Connection Considerations (DIFF Configuration)
Differential connections are those in which each DAQCard-1200 analog
input signal has its own reference signal or signal return path. These
connections are available when you configure the DAQCard-1200 in
the DIFF 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 3-2. Recommended Input Configurations 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