National Instruments Network Device DAQ S - Manual (Page 37)

Chapter 4 Analog Input
NI 6124/6154 User Manual 4-10 ni.com
Figure 4-5 shows a bias resistor connected between AI 0 – and the floating
signal source ground. This resistor provides a return path for the bias
current. A value of 10 k
Ω to 100 kΩ is usually sufficient. If you do not use
the resistor and the source is truly floating, the source is not likely to remain
within the common-mode signal range of the instrumentation amplifier, so
the instrumentation amplifier saturates, causing erroneous readings. You
must reference the source to the respective channel ground.
Figure 4-6 shows how to connect a floating signal source to a channel on
an isolated S Series device.
Figure 4-6. Differential Connection for Non-Referenced Signals on Isolated Devices
DC-Coupled
You can connect low source impedance and high source impedance
DC-coupled sources:
• Low Source Impedance—You must reference the source to AI GND.
The easiest way to make this reference is to connect the positive side
of the signal to the positive input of the instrumentation amplifier and
connect the negative side of the signal to AI GND as well as to the
negative input of the instrumentation amplifier, without using resistors.
This connection works well for DC-coupled sources with low source
impedance (less than 100
Ω).
+
+
Floating
Signal
Source
Instrumentation
Amplifier
V
m
Measured
Voltage
AI 0–
AI 0+
AI 0 Connections Shown
V
s
–
+
–
–
Isolated S Series Device
Digital
Isolators
Isolation
Barrier