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USB-1808X strange bias when measuring voltage

Francois Ouellette


Hello! I am generating a sine wave with the USB-1808X (10 Hz, 2V amplitude), and measuring it using one of the 8 input channels, by directly connecting output and input. In the future, the input will come from another source (photodetector). 

I made a Labview VI to handle both tasks, on a desktop computer.

However, the sine wave I measure has a strong dc bias on it, about 11 or 12 V, which actually increases or decreases slowly with time. In the end, I can only see it if I put a strong negative bias on it (-7 or -8V), otherwise it's outside the -10 to +10V measurement range. 

BUT, if I use exactly the same software on my laptop, it works perfectly fine. No bias. I just measure exactly what I generate. Only the computer is different. 

I've checked everything, cables, USB cable, different USB ports. If I measure with a scope, there is also no bias. I tried different inputs, same thing, although I also noticed a small, and changing bias on the other empty input ports. 

What's going on???? 


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Hello @Francois Ouellette.

Which version of InstaCal do you have on both systems?

Are the analog input channels configured for single-ended or differential?  Please check the settings in InstaCal on both systems, as well as the wiring between the terminals.

Which terminals are you connecting?

Do not leave any analog input channels open if enabled in software.

Please reply with image of the wiring and your vi example, if not resolved.



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