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Chapter 3
to the multiplexer. The following input range and maximum ratings apply to inputs ACH<0..7>:
•
Input signal range
•
Maximum input voltage rating
Warning: Exceeding the input signal range distorts input signals. Exceeding the maximum
input voltage rating may damage the DAQCard-500 card and the computer.
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signal connections.
Analog Input Signal Connection Considerations
When making analog signal connections to the DAQCard-500, you should first determine
whether the signal source and the measurement system are floating or ground referenced. The
two types of signal sources are described as follows, and the types of measurement systems are
described in later sections.
Types of Signal Sources
Ground-Referenced Signal Sources
A ground-referenced signal source is one that is connected in some way to the building system
ground. 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 and 100 mV but can be much higher if power distribution
circuits are not properly connected.
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. The ground reference of a floating signal must be tied to the DAQCard-500
analog input ground to establish a local or onboard reference for the signal. Otherwise, the
measured input signal varies or appears to float. An instrument or device that provides an
isolated output falls into the floating signal source category.
Types of Measurement Systems
In addition to determining the type of signal source, you also need to determine whether your
measurement system is floating or ground referenced. These two types of measurement systems
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3-3
Signal Connections
DAQCard-500 User Manual
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