Dataq DI-725/E, User Manual

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DI–720/DI–722/DI–730 Series User Manual
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The most common error made in connecting differential amplifiers is the tendency to ground both ends of a signal
shield. This causes current to flow through the shield and induces noise on the signal to be measured. Ensuring that
only one ground exists on the signal circuit eliminates this problem.
Isolated Signal Sources —There are two considerations when connecting a differential amplifier to an isolated signal
source: The elimination of ground loops as described above, and the use of bias resistors to provide bias current for
the differential amplifier's positive and negative inputs.
The bias resistors should be chosen to be as low as possible, but the value you use is dependent upon the output
impedance of your signal source. Generally, the value of the bias resistor should be 10 times the output impedance of
your signal source or higher. Bias resistors greater than 1M
Ω
are not recommended.
Signal Cable and the Differential Amplifier
Quick rule-of-thumb: Any application requiring a differential amplifier also defines a need for quality signal cable.
Four elements combine to ensure adequate quality signal cable: a twisted signal pair with low resistance stranded
+
Signal
Source
Differential
Amplifier
-
gnd
Signal
Lead
Shield
Potential difference
(no path for current to circulate)
DI-205 terminal strip
RIGHT APPROACH
One ground on the signal circuit eliminates
noise-inducing ground loops.
+
Floating
Signal
Source
Differential
Amplifier
-
gnd
Signal
Lead
Shield
DI-205 terminal strip
Locate bias resistors as close
to amplifier as possible