National Instruments Isolated Current Output Module SCC-CO20 User Manual page 5

If your load is grounded and you are using the internal supply, connect the load
Note
ground to the current sink terminal at pin 2 on the SCC-CO20. Do not connect the load
ground to the isolated ground terminal because doing so could defeat the module isolation.
Driving Higher-Resistance Loads with the SCC-CO20
© National Instruments Corporation
In order to calculate the supply voltage your application requires, multiply
the load resistance by the current you need to sink, then add 2.5 V for the
voltage drop across the protection diode and the current-sink MOSFET.
If you need a higher voltage than the 15 V provided by the internal loop
supply, you can either add a second SCC-CO20 module or provide an
external loop supply.
Adding a Second SCC-CO20 Module
You can add another SCC-CO20 loop supply to create a supply of 30 V,
as shown in Figure 3. Do not use more than two SCC-CO20 modules on
a single circuit, because the resulting voltage can cause overheating and
damage the modules and other equipment.
5 SCC-CO20 Isolated Current Output Module User Guide