National Instruments E Series - Manual (Page 52)

Chapter 4 Signal Connections
DAQCard E Series User Manual 4-16  National Instruments Corporation
Differential Connections for Nonreferenced or
Floating Signal Sources
Figure 4-5 shows how to connect a floating signal source to a channel
on a DAQCard configured in DIFF input mode.
Figure 4-5 shows two bias resistors connected in parallel with the
signal leads of a floating signal source. If you do not use the resistors
and the source is truly floating, the source is not likely to remain within
the common-mode signal range of the PGIA, and the PGIA will
saturate, causing erroneous readings. You must reference the source to
AIGND. The easiest way is simply to connect the positive side of the
signal to the positive input of the PGIA and connect the negative side
of the signal to AIGND as well as to the negative input of the PGIA,
Figure 4-5. Differential Input Connections for Nonreferenced Signals
+
-
+
Floating
Signal
Source
Input Multiplexers
Instrumentation
Amplifier
V
m
Measured
Voltage
-
V
S
-
+
I/O Connector
AIGND
Bias
Current
Return
Paths
Bias
Resistors
(see text)
ACH<8..15>
ACH<0..7>
AISENSE
Selected Channel in DIFF Configuration
PGIA