National Instruments DAQCard-1200 - Manual (Page 32)

Chapter 3 Signal Connections
DAQCard-1200 User Manual 3-10 © National Instruments Corporation
Figure 3-4. Differential Input Connections for Floating Sources
The 100 k
Ω
resistors shown in Figure 3-4 create a return path to ground for
the bias currents of the instrumentation amplifier. If there is no return path,
the instrumentation amplifier bias currents charge stray capacitances,
resulting in uncontrollable drift and possible saturation in the amplifier.
A resistor from each input to ground, as shown in Figure 3-4, provides bias
current return paths for an AC-coupled input signal.
If the input signal is DC-coupled, you need only the resistor that connects
the negative signal input to ground. This connection does not lower the
input impedance of the analog input channel.
+
–
+
Floating
Signal
Source
Vm
Measured
Voltage
–
Vs
–
+
Front I/O Connector
DAQCard-1200 in DIFF Configuration
1
3
5
7
2
4
6
8
100 k
Ω
Bias
Current
Return
Paths
ACH 0
AGND 11
ACH 2
ACH 4
ACH 6
ACH 1
ACH 3
ACH 5
ACH 7
100 k Ω