DI–720/DI–722/DI–730 Series User Manual
Accessories
43
Two signal measurement conditions govern your configuration of a differential channel input: The signal to be mea-
sured is ground-referenced; or the signal to be measured is isolated from ground.
Ground-referenced Signal Sources
—A ground-referenced signal source is one with a local ground that may not be
(and probably is not) at the same potential as the computer's ground. This potential difference between signal ground
and computer ground is referred to as a common mode voltage and is caused by a number of different factors.
The most common of these is different physical locations of the computer and signal ground points. Since wire is not
a perfect conductor (exhibiting zero resistance regardless of length) a voltage drop, however small, will always be
present. The differential amplifier is unique in its ability to measure signals originating from sources with different
ground potentials relative to the computer
provided it is connected properly
.
V
Signal
Source
Computer
Ground
Ground Potential Difference
(Common Mode Voltage)
+
Signal
Source
Differential
Amplifier
-
gnd
Signal
Lead
Shield
Ground loop caused by circulating currents.
DI-205 terminal strip
WRONG APPROACH
Circulating currents in signal shield
induce noise on signal of interest.
Summary of Contents for DI-725/E
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