National Instruments Low-Cost Multifunction I/O Board for ISA Lab-PC+ - Manual (Page 42)

Chapter 3 Signal Connections
© National Instruments Corporation 3-11 Lab-PC+ User Manual
V
s
+
+
+
-
-
-
V
m
Measured
Voltage
Floating
Signal
Source
ACH 0
AISENSE/AIGND
AGND
I/O Connector
1
2
3
8
9
Lab-PC+ Board in RSE Configuration
11
ACH 1
ACH 2
ACH 7
Figure 3-5. Single-Ended Input Connections for Floating Signal Sources
Single-Ended Connections for Grounded Signal Sources
(NRSE Configuration)
If a grounded signal source is to be measured with a single-ended configuration, then the
Lab-PC+ must be configured in the NRSE input configuration. The signal is connected to the
positive input of the Lab-PC+ instrumentation amplifier and the signal local ground reference is
connected to the negative input of the Lab-PC+ instrumentation amplifier. The ground point of
the signal should therefore be connected to the AISENSE pin. Any potential difference between
the Lab-PC+ ground and the signal ground appears as a common mode signal at both the positive
and negative inputs of the instrumentation amplifier and is therefore rejected by the amplifier.
On the other hand, if the input circuitry of the Lab-PC+ is referenced to ground, such as in the
RSE configuration, this difference in ground potentials appears as an error in the measured
voltage.
Figure 3-6 shows how to connect a grounded signal source to a Lab-PC+ board configured in the
NRSE configuration. Configuration instructions are included under Analog Input Configuration
in Chapter 2, Configuration and Installation.