Chapter 3
Configuring Your SCXI Hardware and Software
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National Instruments Corporation
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Each analog module includes jumpers that configure the output referencing
mode of the module. Configure these jumpers according to the analog input
mode of the DAQ device. The recommended analog input mode of your
DAQ device is as follows:
•
E Series AI and MIO devices default to differential (DIFF) mode.
National Instruments recommends that you leave your AI or MIO
device configured for DIFF inputs.
•
The Lab/1200 devices default to referenced single-ended (RSE) mode.
Change its configuration to non-referenced single-ended (NRSE)
mode for optimal noise elimination. Because of the cabling, you
cannot use the Lab and 1200 devices in DIFF mode with SCXI.
Some analog SCXI modules include jumpers to configure module
parameters such as filter cut off frequency, gain, analog input referencing,
and output referencing. You must select the analog input referencing of a
module according to the type of signal you are measuring. This section
begins with a description of the main types of signals.
Ground-Referenced and Floating Signal Sources
Signal sources are either ground-referenced or floating.
A ground-referenced source is connected to the building ground and thus
shares a common ground with the DAQ device, assuming that your
computer is connected to the building power supply as well. Examples of
ground-referenced signals are the non-isolated outputs of any devices that
plug into the building power supply, such as signal generators and power
supplies. Isolated outputs are not ground-referenced sources; you should
treat them as floating sources.
A floating source has an isolated ground-reference point that you cannot
assume to be at the same potential as any other ground reference. Examples
of this type of source include transformers, insulated thermocouples,
optical isolators, isolation amplifiers, and battery-powered devices.
In general, you should reference an input signal to ground at only one point.
Therefore, do not reference the input of a differential amplifier module to
ground if the signal source is already ground-referenced. If you are
measuring a floating source, on the other hand, reference the input signal to
ground.
