National Instruments Low-Cost Multifunction I/O Board for ISA Lab-PC+ - Manual (Page 40)

Chapter 3 Signal Connections
© National Instruments Corporation 3-9 Lab-PC+ User Manual
+
-
+
Floating
Signal
Source
V
m
Measured
Voltage
-
V
s
-
+
I/O Connector
Lab-PC+ Board in DIFF Configuration
1
3
5
7
2
4
6
8
9
100 k
Ω
Bias
Current
Return
Paths
ACH 0
AGND 11
AISENSE/AIGND
ACH 2
ACH 4
ACH 6
ACH 1
ACH 3
ACH 5
ACH 7
100 k
Ω
(not connected)
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 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 k
Ω
to 100 k
Ω
are used.
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, then only the resistor connecting the negative signal input to
ground is needed. This connection does not lower the input impedance of the analog input
channel.