NI myDAQ User Guide and Specifications
12
ni.com
Note
Most laptop computers have isolated power supplies, and are consequently not
connected to the building ground system. In these cases, treat the analog input signal as
floating with respect to NI myDAQ.
The difference in ground potential between two instruments connected to
the same building power system is typically between 1 and 100 mV. This
difference can be much higher if power distribution circuits are improperly
connected. If a grounded signal source is improperly measured, this
difference might appear as a measurement error. Connect the differential
analog inputs across the signal source and do not connect the NI myDAQ
AGND pin to the grounded source.
Figure 6.
Ground-Referenced Differential Connection
Floating Signal Sources
A floating signal source is not connected to the same ground reference as
NI myDAQ, but instead has an isolated reference point. Some examples of
floating signal sources are battery-powered devices, outputs of
transformers, thermocouples, optical isolator outputs, and isolation
amplifiers. An instrument or device that has an isolated output is a floating
signal source. You must connect the ground reference of a floating signal
to an NI myDAQ AGND pin through a bias resistor or jumper wire to
establish a local or onboard reference for the signal. Otherwise, the
measured input signal varies as the source floats out of the common-mode
input range.
The easiest way to reference the source to AGND is to connect the positive
side of the signal to AI+ and connect the negative side of the signal to
AGND as well as to AI– without using resistors. This connection works
well for DC-coupled sources with low source impedance (less than 100
).
+
–
+
–
AI+
AGND
Sign
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AI–
