ADLINK Technology USB-2401, User Manual

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28 Operation
3.2.2 Ground-Referenced Signal Source
Connected in some way to the existing ground system, to a
common ground point with respect to the USB DAQ, when the
computer is connected to the same power system. Non-isolated
output of instruments and devices connected to the existing
power systems are ground-referenced signal sources.
3.3 Signal Connection
Each analog input channel can be configured to different input
modes by the software API. Details of signal connection in
different input modes follow.
3.3.1 Voltage Input Mode
The properties of the signal to be measured must be considered.
The differential input mode provides two inputs that respond to
signal voltage difference between them. If the signal source is
ground-referenced, the differential mode can be used for the
common-mode noise rejection.
Connection of ground-referenced signal sources under differential
input mode is as shown.
Figure 3-2: Ground-Referenced Source and Differential Input
For floating signal sources, addition of a resistor at each channel
provides a bias return path. The resistor value should be about
AI-
AI+
Vcm
Common mode noise
& ground potential
Ground reference
signal source
GND
Amplifier
Amplifier
USB-2401