2-8
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ni.com
Chapter 2
Analog Input and Temperature Sensor Measurements
When to Use Referenced Single-Ended (RSE) Connections with
Floating Signal Sources
Only use RSE input connections if the input signal meets the following conditions:
•
The input signal can share a common reference point, AI GND, with other signals that use
RSE.
•
The input signal is high-level (greater than 1 V).
•
The leads connecting the signal to the device are less than 3 m (10 ft).
Differential input connections are recommended for greater signal integrity for any input signal
that does not meet the preceding conditions.
In the single-ended modes, more electrostatic and magnetic noise couples into the signal
connections than in differential configurations. The coupling is the result of differences in the
signal path. Magnetic coupling is proportional to the area between the two signal conductors.
Electrical coupling is a function of how much the electric field differs between the
two conductors.
With this type of connection, the NI-PGIA rejects both the common-mode noise in the signal and
the ground potential difference between the signal source and the device ground.
Refer to the documentation for your DAQ device for more information about RSE connections.
Using Differential Connections for Floating Signal Sources
It is important to connect the negative lead of a floating source to AI GND (either directly or
through a bias resistor). Otherwise, the source can float out of the maximum working voltage
range of the NI-PGIA and the DAQ device returns erroneous data.
The easiest way to reference the source to AI GND is to connect the positive side of the signal
to AI+ and connect the negative side of the signal to AI GND as well as to AI– without using
resistors. This connection works well for DC-coupled sources with low source impedance
(<100
Ω
).
Figure 2-4.
Differential Connections for Floating Signal Sources without Bias Resistors
–
+
Inped
a
nce
<100
Ω
AI GND
AI+
AI–
AI
S
EN
S
E
V
s
Flo
a
ting
S
ign
a
l
S
o
u
rce
DAQ Device
