Thanks Harold,
the default open circuit state is high.  I was thinking a pull down resistor
might be necessary as well. thanks
ben

On Thu, May 28, 2009 at 11:59 PM, Harold Hallikainen <harold@hallikainen.org
> wrote:

>
> > Hi all,
> > this is probably something silly i'm overlooking btu I have a custom
> built
> > optical sensor. When I measure across it with an oscilloscope, I get a
> > range
> > from 0 to 5V.  If I then hook it up to a Ni DAQ card, the computer always
> > reads it as high... measuring in parallel the range is now only 2 V to 5
> > V... any idea why it's unable to drive to ground now? thanks a lot
> > Ben
>
> I wonder what the input of the DAQ card looks like. It may have a negative
> bias current that causes a positive input voltage when driven by a high
> resistance. Does the DAQ read zero if the input is open circuited?
> Depending on what you're driving this with, you may be able to add a
> pull-down resistor to fix the problem.
>
> Harold
>
>
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