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Analog input floats high

I have a usb 6211.  I am monitoring a voltage that is supplied through the 5 v output of the DAQ card itself.  I have a switch that will cut off the voltage to that input.   The problem is that the voltage level does not drop to 0v when the input voltage is turned off right away.  The voltage level slowly decreases to near 0 volts.  This is a problem because I am checking contact bounce on a mechanical rotary switch at 200 KS/s, so this is very critical to my application. How do I solve this problem?
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It sounds like a ground reference is missing from the signal.  So that the card slowly sees it turn into 0 V as the signal bleeds off.  If you ground reference is however (through bias resistors or something like that) than the signal should more quickly bleed out the voltage.  Check out the field wiring diagram linked below for more information.

 

http://zone.ni.com/devzone/cda/tut/p/id/3344

Doug Farrell
Solutions Marketing - Automotive
National Instruments

National Instruments Automotive Solutions
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