Figure 2-6 – National Instruments NI PXI-7831R User Manual
Page 31
Chapter 2
Hardware Overview of the NI PXI-7831R
2-10
ni.com
Figure 2-6. Differential Input Connections for Nonreferenced Signals
Figure 2-6 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 instrumentation amplifier, and the
instrumentation amplifier will saturate, causing erroneous readings. You
must reference the source to AIGND, which you can do by connecting the
positive side of the signal to the positive input of the instrumentation
amplifier and connecting the negative side of the signal to AIGND and to
the negative input of the instrumentation amplifier, without any resistors at
all. This connection works well for DC-coupled sources with low source
impedance (less than 100
Ω).
However, for larger source impedances, this connection leaves the
differential signal path significantly out of balance. Noise that couples
electrostatically onto the positive line does not couple onto the negative
line because it is connected to ground. Hence, this noise appears as a
differential-mode signal instead of a common-mode signal, and the
instrumentation amplifier does not reject it. In this case, instead of directly
connecting the negative line to AIGND, connect it to AIGND through a
resistor that is about 100 times the equivalent source impedance. The
resistor puts the signal path nearly in balance, so about the same amount
of noise couples onto both connections, which yields better rejection of
electrostatically coupled noise. Also, this input mode does not load down
Bias
Resistors
(see text)
DIFF Input Mode Selected
–
+
–
+
AISENSE
AIGND
V
m
Measured
Voltage
Instrumentation
Amplifier
x8 Channels
AI+
AI–
–
+
Floating
Signal
Source
Bias
Current
Return
Paths
V
s
I/O Connector