National Instruments NI USB-621x - Manual (Page 58)

Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices
NI USB-621x User Manual 5-8 ni.com
inputs; be aware that there is some gain error from loading down the source,
as shown in Figure 5-4.
Figure 5-4. Differential Connections for AC Coupled Floating Sources
with Balanced Bias Resistors
Using Non-Referenced Single-Ended (NRSE) Connections for Floating
Signal Sources
It is important to connect the negative lead of a floating signals source to
AI GND (either directly or through a resistor). Otherwise the source may
float out of the valid input range of the NI-PGIA and the DAQ device
returns erroneous data.
Figure 5-5 shows a floating source connected to the DAQ device in NRSE
mode.
Figure 5-5. NRSE Connections for Floating Signal Sources
All of the bias resistor configurations discussed in the Using Differential
Connections for Floating Signal Sources section apply to the NRSE bias
resistors as well. Replace AI– with AI SENSE in Figures 5-1, 5-2, 5-3,
and 5-4 for configurations with zero to two bias resistors. The noise
rejection of NRSE mode is better than RSE mode because the AI SENSE
connection is made remotely near the source. However, the noise rejection
of NRSE mode is worse than DIFF mode because the AI SENSE
–
+
AI GND
V
s
AC Coupled
Floating
Signal
Source
AI+
AI–
AI SENSE
AC Coupling
–
+
AI GND
R
AI SENSE
AI
V
s
Floating
Signal
Source