Advantech PCI-1715U, Руководство пользователя

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PCI-1715U User Manual 10
Differential Channel Connections
The differential input configuration has two signal wires for each chan-
nel, and each input responds only to voltage differences between High
and Low inputs. On the PCI-1715U card, when all channels are config-
ured to differential input, up to 16 analog channels are available.
If one side of the signal source is connected to a local ground, the signal
source is ground-referenced. The ground of the signal and the ground of
the PCI-1715U will not be the same voltage, as they are connected
through the ground return of the equipment and building wiring. The dif-
ference between the voltages forms a common-mode voltage (V
cm
).
To avoid the ground loop noise effect caused by common-mode voltages,
you can connect the signal ground to the Low input. Figure 3-3 shows a
differential channel connection between a ground-referenced signal
source and an input channel on the PCI-1715U card. With this connec-
tion, the PGIA rejects a common-mode voltage V
cm
between the signal
source and the PCI-1715U ground, shown as V
cm
in Figure 3-3.
If a floating signal source is connected to the differential input channel,
the signal source may exceed the common-mode signal range of the
PGIA, and the PGIA will be saturated with erroneous voltage-readings.
You must therefore reference the signal source to the AIGND.
Figure 3-4 shows a differential channel connection between a floating
signal source and an input channel on the PCI-1715U card. In this figure,
each side of the floating signal source is connected through a resistor to
the AIGND. This connection can reject the common-mode voltage
between the signal source and the PCI-1715U card ground.
Figure 3.3: Differential Input-Grounded Signal Source
External Internal
Ground-
Referenced
Signal
Source Multiplexer
Multiplexer
PGIA
AI30
AI31
Measured
Voltage
_
+
AI0
AI2
AI1
AI3
GND
I/O Connector