Expected Voltage Drop on the Digital Output Lines of NI-651x Devices

Issue Details

I am using an NI-651x digital board, one of the 30 V bank isolated industrial devices. These devices require an external power supply for sourcing and sinking output signals. What kind of voltage drop can I expect between Vcc and PX.X for sourcing outputs or PX.X and Ground for sinking outputs?

Solution

The NI-6512/6514/6516/6518 devices offer digital output lines for sourcing current. The following block diagram shows how the output line is switched and how to connect a load.


Notice that the connection between PX.X and Vcc is made by a Darlington Driver. This driver chip will introduce a voltage drop between its connecting terminals. The drop is not constant and is a function of the current being sourced. The voltage drop will not exceed the following specifications:

Output Current	Max Voltage Drop
100 mA	1.8 V
225 mA	1.9 V
350 mA	2.0 V

The NI-6513/6515/6517/6519 devices offer digital output lines for sinking current. The following block diagram shows how the output line is switched and how to connect a load. 

Notice that the connection between PX.X and Ground is made by another Darlington Driver. This is a different chip but will also introduce a voltage drop between its connecting terminals based on the current being accepted. The voltage drop will not exceed the following specifications:
 

Output Current	Max Voltage Drop
100 mA	1.1 V
200 mA	1.3 V
350 mA	1.6 V

Additional Information

Expected voltage drop graphs for sourcing voltage