Strange voltage observed in NI BNC 2090

[Sam]

Hi all, I have a BNC 2090 and AT-MIO-16O-2 DAQ board. I am recently
(starting this morning) experiencing strange voltage readings from the
ACH1, 2,3,4and 5. The voltage input from ACH0 is about -.2V; and range
from -1 to -3 in other channels. These happens even when nothing is
plugged into the BNC other than the its connection to the DAQ in the
computer.

any suggestion on what might be happening?

Roshan.

[JoshuaP]

This is actually very common. When nothing is connected to the pins the voltage will charge up on the leads. Sense the lines are not connected to anything and the voltage has no where to go the lines will normally completely saturate the capacitors on the board.

This is not a bad thing and it should not affect your measurements, if you do not scan the channels that are not connected. If you are worried about the unused floating signals, you can wire the unused inputs to ground.

If you are performing measurements and notice that the inputs are floating you should check your grounding methods. In general use RSE for floating sources, NRSE and DIFF for grounded signal sources. If you want to use NRSE or DIFF on a floating signal you will need to add a 10-
100kOhm bias resistor from the negative lead to AIGND to reference the signal. Refer to this KB for more help.
http://digital.ni.com/public.nsf/websearch/DF70FCE03D397BFE86256A3700780FFA?OpenDocument

I hope this helps,

Joshua