National Instruments AT-MIO-64F-5, Руководство пользователя

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Configuration and Installation Chapter 2
AT-MIO-64F-5 User Manual 2-20 © National Instruments Corporation
the two input signals multiplied by the gain setting of the amplifier. The amplifier output voltage
is referenced to the AT-MIO-64F-5 ground. The AT-MIO-64F-5 ADC measures this output
voltage when it performs A/D conversions.
All signals must be referenced to ground, either at the source device or at the AT-MIO-64F-5. If
you have a floating source, the AT-MIO-64F-5 should reference the signal to ground by using
the RSE input mode or the DIFF input configuration with bias resistors (see the
Differential
Connections for Nonreferenced or Floating Signal Sources section later in this chapter). If you
have a grounded source, the AT-MIO-64F-5 should not reference the signal to AI GND. The
AT-MIO-64F-5 board avoids this reference by using the DIFF or NRSE input configurations.
Types of Signal Sources
When configuring the input mode of the AT-MIO-64F-5 and making signal connections, you
must first determine whether the signal source is floating or ground-referenced. These two types
of signals are described in the following sections.
Floating Signal Sources
A floating signal source 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 signal sources are
outputs of transformers, thermocouples, battery-powered devices, optical isolator outputs, and
isolation amplifiers. An instrument or device that provides an isolated output falls into the
floating signal source category. The ground reference of a floating signal must be tied to the
AT-MIO-64F-5 analog input ground in order to establish a local or onboard reference for the
signal. Otherwise, the measured input signal varies as the source floats out of the common-mode
input range.
Ground-Referenced Signal Sources
A ground-referenced signal source is one that is connected in some way to the building system ground
and is therefore already connected to a common ground point with respect to the AT-MIO-64F-5 board,
assuming that the PC AT is plugged into the same power system. Nonisolated outputs of instruments
and devices that plug into the building power system fall into this category.
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. If a grounded signal source is improperly
measured, this difference may show up as an error in the measurement. The following
connection instructions for grounded signal sources are designed to eliminate this ground
potential difference from the measured signal.
Input Configurations
The AT-MIO-64F-5 can be configured for one of three input modesÐNRSE, RSE, 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 2-5
summarizes the recommended input configuration for both types of signal sources.