National Instruments Low-Cost Multifunction I/O Board for ISA Lab-PC+ - page 40

Chapter 3 Signal Connections
© National Instruments Corporation 3-9 Lab-PC+ User Manual
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Floating
Signal
Source
V
m
Measured
Voltage
-
V
s
-
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I/O Connector Lab-PC+ Board in DIFF Configuration
1
3
5
7
2
4
6
8
9
100 kΩ
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 kΩ
(not connected)
Figure 3-4. Differential Input Connections for Floating Sources
The 100 kΩ 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 kΩ to 100 kΩ 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.