NI USB 6251 - Signal Express - Input Ports display idential signals

[AlfredoT]

Hello,

I connected a DET50B (phototedector) to the Ai0 input port in my NI USB-6251 and through LabView SingalExpress 3.0.0 took a look at the signals from the Ai0 and Ai1 by clicking on the following:

Acquire Signal -> DAQmx Acquire -> Analog Input -> Voltage

I was expecting the Ai1 signal to be at 0 Volts, because there is nothing connected to it, and the Ai0 signal to vary with respect to the light I was shinning on the photodetector. Instead, both Ai0 and Ai1 amplitudes were identical and corresponded to the light I was shinning.

Why is it that these ports seem to be connected? The same thing happened when I opened the other ports for viewing through SignalExpress except the Ai8; this one seemed to be at zero (probably because it does not physically exist on the NI USB 6251 which has Ai0-Ai7).

Message Edited by AlfredoT on 09-30-2009 02:01 PM

[Dennis_Knutson · Accepted solution]

No, you will not read 0 volts from an unconnected channel. Do a search for 'ghosting' for a detailed explanation. If you want 0 voltas on a channel, you need to gnd it.

p.s. You do not have a Signal Generator board. New questions should be posted to the Multifunction DAQ or Signal Express boards