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Chapter 3 Connecting the Signals
DAQCard-1200 User Manual 3-6 ni.com
The instrumentation amplifier applies gain, common-mode voltage
rejection, and high-input impedance to the AI signals connected to the
DAQCard-1200. Signals are routed to the positive and negative inputs of
the instrumentation amplifier through input multiplexers on the
DAQCard-1200. The instrumentation amplifier converts two input signals
to a signal that is the difference between the two input signals multiplied by
the gain setting of the amplifier. The amplifier output voltage is referenced
to the DAQCard-1200 ground. The DAQCard-1200 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 DAQCard-1200. If you have a floating source, you must use a
ground-referenced input connection at the DAQCard-1200. If you have a
grounded source, you must use a nonreferenced input connection at the
DAQCard-1200.
Types of Signal Sources
When configuring the input mode of the DAQCard-1200 and making
signal connections, you must first determine whether the signal source is
floating or ground referenced. This section describes these two types of
signals.
Floating Signal Sources
A floating signal source is not connected in any way to the building ground
system but 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.
You must tie the ground reference of a floating signal to the
DAQCard-1200 AI 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 supplies an isolated output falls into the floating
signal source category.
Ground-Referenced Signal Sources
A ground-referenced signal source is connected in some way to the
building system ground and is therefore already connected to a common
ground point with respect to the DAQCard-1200, assuming that the
computer is plugged into the same power system. Nonisolated outputs of
instruments and devices that plug into the building power system fall into
this category.