signal where none should appear

[Sy 714]

I am using a PXI-6031E board (connected to two TBX-68 screw terminal blocks) and a PXI-1000B chassis. When the inputs of the TBS-68 are not connected to anything, the system reads a signal of 0-(-9) V, which changes over time. Different channels show slightly different signals, but all within about 2 V of one another. The channels in question use differential input mode.

What causes this? Is there also an offset when data is being sent through those channels?

[Filipe A.]

Hello;

In case the analog input channels are not connected to a valid signal source, that means they are floating. That will make the ADC inputs to float to, since the ADC doesn't know that is not a "valid" data source, the analog to digital convertion will happen on whatever voltage signal is inthe input. That's why you see a voltage even though you don't have a valid source connected to the board.

In case you don't want that to happen, you will need to ground the AI channels, then you will see zero volts.

Hope this helps.
Filipe A.
Applications Engineer
National Instruments