Omega OMB-DAQBOARD-500 Series User Manual page 24

Signal Types
Floating Sources
A Floating Source is a signal that has no connection to the building's power ground. Examples of Floating
Sources are thermocouples, batteries and battery powered devices, and signals from optically isolated
devices. When connecting Floating Sources to a data acquisition board, the ground reference of the signal
must be tied to the analog ground (AGND) in order to establish a common reference point.
Ground Referenced Sources
A Ground Referenced Source is one that is connected to the same common ground as the host PC, and
therefore has the same ground as the data acquisition boards. An example is equipment that plugs into the
same building power source as the host PC.
Choosing A/D Input Configuration
Once you have determined what type of input signal source you have, and the voltage level, you then need to select
the proper input configuration on your data acquisition board.
Single-Ended
Applications with a Floating Source are typically wired to a data acquisition board configured for Single-Ended
(SE) configuration. Since only one wire from each input signal is connected to a multiplexed input of the A/D, the
Single-Ended configuration provides a larger number of inputs per board than Differential (see below)
configuration. Grounded Signal Sources can be wired in Single-Ended configuration only when signal leads are less
than 12 feet AND when all signals share a common ground (the signals must be local to one another).
With the Single-Ended configuration, the input signals are tied to the Channel Hi side of an analog input, and all
signal low sides are tied to the SGND ground on the data acquisition board.
Single-Ended configuration should only be used when:
Channel-to-channel isolation is not required
Ground isolation is not required
Signal leads are less than 12 feet
Of the two possible input configurations, Single-Ended offers the least amount of noise rejection. Because of this,
Low Level signals should only be wired in Single-Ended configuration when you are certain that there is little or no
noise being introduced to the signal from the system, or the environment.
The following figure depicts proper wiring for Single-Ended configuration.
2-8 Connections & Pinouts
Due to differences in a building's power system, the Ground Referenced Source and
the data acquisition board's ground may be at different voltage levels. This difference
is referred to as a Common Mode Voltage. Common Mode Voltage can be eliminated
by using Differential (DI) input configurations on the data acquisition board.
Single-Ended Configuration
Not recommended for Low Level Signals
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DaqBoard/500 Series User's Manual