DI–710 Series and DI-715B Series Hardware Manual
Accessories
46
Connecting a Differential Amplifier
A differential channel configuration is a special case approach to signal measurement that should be used with all
low-level measurements (less than 1v) or when the signal to be measured is located far away from the DI-205 (greater
than 15 feet). When in a differential operating mode, the instrument sees the difference in voltage between the high
(+) and low (-) inputs. Differential amplifiers have the advantage of reducing common mode voltages (those appear-
ing simultaneously and in phase on both inputs). This capability may be leveraged to greatly reduce noise riding on a
signal of interest.
Two signal measurement conditions govern the 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 poten-
tial 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
V
Signal
Source
Computer
Ground
Ground Potential Difference
(Common Mode Voltage)
