Post by thread:Optical Sensor

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

> 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 -- FCC Rules Updated Daily at http://www.hallikainen.com - Advertising opportunities available!

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 <spam_OUTharoldTakeThisOuThallikainen.org {Quote hidden}> 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 > > > -- > FCC Rules Updated Daily at http://www.hallikainen.com - Advertising > opportunities available! > -

On Thu, 2009-05-28 at 23:10 -0400, Benjamin Grant 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 What's the output impedance of your sensor? We've used NIDAQ cards before and have never seen anything like that. That said, we always drive the NIDAQ cards with "normal" drivers. You may have to add an op-amp in unity gain mode to get it to work properly, it sounds like the card has a weak pull up that your sensor can't pull down. TTYL

Thanks Herbert, it turned out to be an intrinsic problem with the sensor - there was an indicator LED that was causing the signal to jump to 2 V. Not exactly clear why this wasn't evident on the oscilloscope. Regardless, ended up utilizing a transistor and created a logic gate and all is well. Thanks Ben On Mon, Jun 1, 2009 at 4:59 PM, Herbert Graf <.....hkgrafKILLspam@spam@gmail.com> wrote: {Quote hidden}> On Thu, 2009-05-28 at 23:10 -0400, Benjamin Grant 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 > > What's the output impedance of your sensor? We've used NIDAQ cards > before and have never seen anything like that. That said, we always > drive the NIDAQ cards with "normal" drivers. You may have to add an > op-amp in unity gain mode to get it to work properly, it sounds like the > card has a weak pull up that your sensor can't pull down. > > TTYL > > -