National Instruments AT E Series - Manual (Page 69)

Chapter 4 Connecting Signals
AT E Series User Manual 4-22 ni.com
Differential Connections for Nonreferenced or
Floating Signal Sources
Figure 4-7 shows how to connect a floating signal source to an AT E Series
device channel configured in DIFF input mode.
Figure 4-7. Differential Input Connections for Nonreferenced Signals
Figure 4-7 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 saturates, 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
PGIA
Selected Channel in DIFF Configuration
Programmable
Gain
Instrumentation
Amplifier
+
+
–
–
Input Multiplexers
AISENSE
AIGND
I/O Connector
+
–
V
s
ACH+
ACH–
V
m
Bias
Current
Return
Paths
Bias
Resistors
(see text)
Floating
Signal
Source
Measured
Voltage