DAQ scaling

[oil-freemax]

I am using Labview to read voltages input from a USB-6008.  I tell Labview, in the DAQ manager, to scale the voltage linearly, with a slope of 25 and an offset of 0.  I give a maximum and minimum input range of -10 - 10V.  Therefore, I would expect labview to outpu +250 volts when I enter a maximum voltage into the USB-6008.  However, the scaling only works in a range of ~ -1V - 1V.   All voltages outside this range are scaled to the -1V or 1V value.  I have cecked the input voltages and they are, in fact, in between -10V and 10V.  However, if I tell Labview to expect a range of -500V - 500V, the voltages are scaled correctly, but then I am losing resolution.

What have I done wrong?

[Travis G.]

Hello oil-freemax,

I think the problem might be caused by the minimum and maximum you are setting for the input range of your analog input task.  When applying a scale to your acquired data, the minimum and maximum are in terms of a the scaled units.  Therefore, if you are expecting to acquire an actual signal between -10 and 10V, and scale that measurement linearly with a slope of 25, your upper and lower limits should be set to -250 and 250.  If you are not getting the desired resolution in your signal, try narrowing these limits closer to the expected voltage, but realize that the minimum and maximum settings for the input range are in terms of post-scaled units.  Give this a try and let us know if this solves the problem.

Regards,
Travis G.
Applications Engineering
National Instruments
www.ni.com/support

Message Edited by Travis G. on 08-17-2006 12:43 PM