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Chapter 2 Hardware Overview of the NI 78x x © National Instruments Corporation 2-7 Mult ifunction Reconfigurable I/O User Manual Connection of AI signals to the NI 783 x /784 x /78 5 x depends on the inpu t mode of the A…
Chapter 2 Hardware Overview of the NI 78xx
Multifunction Reconfigurable I/O User Manual 2-6 ni.com
eight AI channels of the NI 783x/784x/785x. For all input modes, the
AI<0..n>+ signals are connected to the positive input of the
instrumentation amplifier on each channel. The signal connected to the
negative input of the instrumentation amplifier depends on how you
configure the input mode of the device.
In differential input mode, signals connected to AI<0..n>– are routed to the
negative input of the instrumentation amplifier for each channel. In RSE
input mode, the negative input of the instrumentation amplifier for each
channel is internally connected to AIGND. In NRSE input mode, the
AISENSE signal is connected internally to the negative input of the
instrumentation amplifier for each channel. In DIFF and RSE input modes,
AISENSE is not used.
Caution Exceeding the differential and common-mode input ranges distorts the
input signals. Exceeding the maximum input voltage rating can damage the
NI 783x/784x/785x and the computer. NI is not liable for any damage resulting from such
signal connections. The maximum input voltage ratings are listed in Table A-2, NI 78xx
I/O Signal Summary.
AIGND is a common AI signal that is routed directly to the ground tie point
on the NI 783x/784x/785x. You can use this signal for a general analog
ground tie point to the device if necessary.
Chapter 2 Hardware Overview of the NI 78xx
© National Instruments Corporation 2-7 Multifunction Reconfigurable I/O User Manual
Connection of AI signals to the NI 783x/784x/785x depends on the input
mode of the AI channels you are using and the type of input signal source.
With different input modes, you can use the instrumentation amplifier in
different ways. Figure 2-4 shows a diagram of the device instrumentation
amplifier.
Figure 2-4. NI 783x/784x/785x Instrumentation Amplifier
The instrumentation amplifier applies common-mode voltage rejection
and presents high input impedance to the AI signals connected to the
NI 783x/784x/785x. Input multiplexers on the device route signals to the
positive and negative inputs of the instrumentation amplifier. The
instrumentation amplifier converts two input signals to a signal that is the
difference between the two input signals. The amplifier output voltage is
referenced to the device ground. The device ADC measures this output
voltage when it performs A/D conversions.
You must reference all signals to ground either at the source device or at the
NI 783x/784x/785x. If you have a floating source, reference the signal to
ground by using RSE input mode or the DIFF input mode with bias
resistors. Refer to the Differential Connections for Nonreferenced or
Floating Signal Sources section of this chapter for more information about
these input modes. If you have a grounded source, do not reference the
signal to AIGND. You can avoid this reference by using DIFF or NRSE
input modes.
+
+
–
–
V
m
= [V
in+
– V
in–
]
V
in+
V
in–
V
m
Instrumentation
Amplifier
Measured
Voltage
Chapter 2 Hardware Overview of the NI 78xx
Multifunction Reconfigurable I/O User Manual 2-8 ni.com
Types of Signal Sources
When configuring the input channels and making signal connections,
you must first determine whether the signal sources are floating or ground
referenced. The following sections describe these two signal types.
Floating Signal Sources
A floating signal source is not connected to the building ground system but
instead 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 has an isolated output is a floating signal source.
You must connect the ground reference of a floating signal to the
NI 783x/784x/785x AIGND through a bias resistor 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 connected to the building system
ground, so it is already connected to a common ground point with respect
to the NI 783x/784x/785x, assuming that the computer is plugged into the
same power system. Instruments or devices with nonisolated outputs that
plug into the building power system are ground referenced signal sources.
The difference in ground potential between two instruments connected to
the same building power system is typically between 1 and 100 mV. This
difference can be much higher if power distribution circuits are improperly
connected. If a grounded signal source is improperly measured, this
difference might appear as a measurement error. The connection
instructions for grounded signal sources are designed to eliminate this
ground potential difference from the measured signal.
Input Modes
The following sections discuss single-ended and differential measurements
and considerations for measuring both floating and ground-referenced
signal sources.