National Instruments BNC -2140 - Manual (Page 21)

Chapter 4 Theory of Operation
BNC-2140 User Manual 4-4 © National Instruments Corporation
Analog Output
The BNC-2140 has two analog output channels. The BNC-2140 can also
select between DIFF and SE outputs.
In DIFF mode, one line connects to the positive output of the channel and
the other connects to the negative output of that same channel. You can
connect the differential output to either floating or ground-referenced
signals.