USB-6009 low current analog input

[JimSchwendeman]

Hi All,

I'm running into an interesting problem, and thought I'd see if anyone has dealt with anything similar.  I'm using a USB-6009, and attempting to use with a high-impedance (>1MOhm for each leg) voltage divider, with a low excitation voltage (1V).  I'm then comparing the voltage output to a threshold to verify that the line is good, or not good.  I have noticed for some lines that I scope out, they measure fine, and so the resulting voltage should be low, but when I connect the USB-6009, the measured voltage is greater than my excitation voltage.  This makes me wonder if the voltage divider INSIDE the 6009 analog circuit is throwing fits.

Thoughts or previous experiences?

Thanks!

Jim

[Brandon_B]

Hello Jim,

The 6009 isn't really made to measure across such large impedances.  Take a look at the 6009 analog input circuitry diagram to see the resistance values used for the internal voltage divider.  I'm assuming the voltage you are seeing is around 1.4 V.  It is because of this that I recommend not measuring across such a large impedance.  Please let me know if you have any additional questions.

Regards,