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
VmMeasured
Voltage
-
Vs
-
+
I/O Connector
DAQCard-700 Board in DIFF Configuration
3
5
7
17
4
6
8
18
1, 2
100 kΩ
Bias
Current
Return
Paths
ACH<8..15>
ACH<0..7>
AIGND
Figure 3-5. Differential Input Connections for Floating Sources
The 100 kΩ 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 kΩ 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