Differential Connections For Nonreferenced Or Floating Signal Sources; Figure 4-5. Differential Input Connections For Nonreferenced Signals - National Instruments VXI-MIO Series User Manual

Chapter 4 Signal Connections
Floating +
Signal
V
S
Source
-
Bias
Current
Return
Paths
I/O Connector
VXI-MIO Series User Manual
Differential Connections for Nonreferenced or
Floating Signal Sources
Figure 4-5 shows how to connect a floating signal source to a channel
on a VXI-MIO Series module configured in DIFF input mode.
ACH<0..7>
Bias
resistors
(see text)
ACH<8..15>
Other Input Multiplexers
AISENSE
AIGND
Selected Channel in DIFF Configuration

Figure 4-5. Differential Input Connections for Nonreferenced Signals

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, without
4-16
Instrumentation
Amplifier
+
PGIA
+
Measured
- V
m
Voltage
-
National Instruments Corporation