National Instruments 700 - Manual (Page 29)

Signal Connections Chapter 3
DAQCard-700 User Manual 3-10 © National Instruments Corporation
Differential Connections for Floating Signal Sources
Figure 3-5 shows how to connect a floating signal source to an DAQCard-700 card configured
for DIFF input. Configuration instructions are included in the Input Configurations section
earlier in this chapter.
+
-
+
Floating
Signal
Source
Input Multiplexers
Instrumentation
Amplifier
V
m
Measured
Voltage
-
V
s
-
+
I/O Connector
DAQCard-700 Board in DIFF Configuration
3
5
7
17
4
6
8
18
1, 2
100 k
Ω
Bias
Current
Return
Paths
ACH<8..15>
ACH<0..7>
AIGND
Figure 3-5. Differential Input Connections for Floating Sources
The 100 k
Ω
resistors shown in Figure 3-5 create a return path to ground for the bias currents of
the instrumentation amplifier. If a return path is not provided, the instrumentation amplifier bias
currents charge up stray capacitances, resulting in uncontrollable drift and possible saturation in
the amplifier. Typically, values from 10 to 100 k
Ω
are used.
A resistor from each input to ground, as shown in Figure 3-5, provides bias current return paths
for an AC-coupled input signal. This solution, although necessary for AC-coupled signals,
lowers the input impedance of the analog input channel. In addition, the input offset current of