Measurement Computing DataShuttle - Manual (Page 25)

DataShuttle and DynaRes 11-13-01 DataShuttle - Technical Notes 2-7
Current Outputs
Most current outputs connect to current loop devices, such as controllers. If you select the
“Current Output” option in the software then, the output current loop will be in the 4 to 20 mA range.
A 4 to 20 mA current device has the advantage of running the power and signal over the same pair of wires.
Also, current signals are much less susceptible to noise pickup than are voltage signals.
Note: The current loop connection on the Data Shuttle must be powered by a + 12V source. Since no such
source is available on the Data Shuttle itself, an external power supply must be used
Ungrounded Load
Several loads can be powered from a single supply, as long as the negative side of the supply is connected
to the Data Shuttle ground. The circuit in Figure 9b is the most common way to connect a current loop to
the Data Shuttle. In this case, the load is not grounded but is floating above ground level.
If there are multiple loads, then the negative terminal of each load must be connected to a
separate Analog Output (AO) terminal on the DataShuttle.
Figure 9b. Current Output, Ungrounded Load
For the current output option to work properly, the voltage from each AO terminal to ground must be at
least 2.6 volts. Therefore, since the voltage source may vary by +/- 1 V, the maximum load resistance that
can be used in this circuit is 420 ohms.
Grounded Load
If one side of the load must be grounded for some reason, use the circuit shown in Figure 9c. The
disadvantage of this circuit is that each load requires its own power supply. The same maximum load
resistance applies, i.e., 420 ohms.
Figure 9c. Current Output, Grounded Load
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Inadvertent damage to the Data Shuttle’s Analog Output is possible! Some
situations can occur in which the software could start up in a default voltage
mode [instead of the selected current mode]. To avoid this possibility,
physically disconnect the current loops from the Data Shuttle prior to
shutting down or disabling the software, or prior to rebooting the PC.