©
National Instruments Corporation
5
BNC-9920 Operating Instructions and Specifications
Complete the following steps to measure a differential (DIFF) analog input signal.
1.
Use a BNC cable to connect a differential analog input signal to one of the AI <0..7> BNC
connectors on the front panel of the BNC-9920.
2.
If you are using AI 0, move the BNC/Temp. Ref. switch to the BNC position.
If you are using AI 1, move the BNC/Thermocouple switch to the BNC position.
If you are using AI 3, move the RES/BNC switch to the BNC position.
3.
Configure your software to measure the channel differentially.
4.
Move the FS/GS switch to the applicable position, depending on whether you are measuring a
floating source (FS) or ground-referenced source (GS) analog input signal. Refer to the
Measuring Ground-Referenced Signals
section for more
information about these signal sources.
Measuring Floating Signals
To measure floating signal sources, move the corresponding switch below the BNC connector to the FS
position. In the floating source switch position, the negative terminal of the NI 9205 AI channel connects
to ground through a 4.99 k
Ω
resistor, as shown in Figure 2. Refer to the
NI 9205 Operating Instructions
and Specifications
for more information about measuring floating signals.
Figure 2.
Measuring a Floating Signal Source
Measuring Ground-Referenced Signals
To measure ground-referenced signals, move the switch below the BNC connector for the AI channel
you are using to the GS position. Using the ground-referenced source switch position avoids ground
loops, as shown in Figure 3. Refer to the
NI 9205 Operating Instructions and Specifications
for more
information about measuring ground-referenced signals.
Figure 3.
Measuring a Ground-Referenced Signal Source
+
–
S
ign
a
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S
o
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rce
BNC-9920
NI 9205
(Differenti
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t Mode)
AI GND
+
–
4.99 k
Ω
F
S
F
S
Mode
0.1
μ
F
+
–
S
ign
a
l
S
o
u
rce
BNC-9920
NI 9205
(Differenti
a
l Inp
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t Mode)
AI GND
+
–
4.99 k
Ω
G
S
G
S
Mode
0.1
μ
F
