Using HV application with DAQ

[perrin.mr]

I recently built a portable high voltage, low current adjustable power supply for a project I have been working on the last couple months. The device simply takes an incoming dc voltage, uses a EMCO DC to HV DC converter, and outputs the high voltage. Also, the high voltage is also reduced back by 1000 using a EMCO voltage divider, so the user can safely put a voltmeter to measure the output as they vary it for different applications.

I am also putting together a vi that will allow them to automatically log voltage and current and allow them to store the data in a spreadsheet and view a graph. This part is done, but I am having one problem interfacing with DAQ and need some help or advice on how to make it work, if possible.

For example: If i measure the output of the voltage divider (+volts and neg common), the output would be 5Vdc (which translates to a 5KVdc output on the power supply using a Fluke multimeter). However, when I attach my USB 6008 DAQ I/O to it, then the voltage will drop to 1.1Vdc. I am using a differential config in MAX because it may range to 12V on some models I built. My question is why is DAQ dropping the total output voltage?

Thanks in advance for any help or advice.

[AnalogKid2DigitalMan]

The input impedance of the analog input of the 6008 is 144Kohms. I suspect that it is loading down or changing the ratio of the EMCO voltage divider. You may need to add a unity gain buffer between the EMCO divider and the 6008 so as to not load it down. A Fluke meter typically has in input inpedance of 10Mohm or greater, so it will not load the divider doen.

-AK2DM

[perrin.mr]

Makes sense.....I will give it a try and let you know if it works or not. thanks!

[z2830than]

I had posted as perrin.mr:

to the person who suggested impedance was the issue: You were correct. Thanks!

I ended up using a 6210 DAQ (input impedance of 10Gohms) instead of making a unity gain buffer to use with the 6008. After some minor scaling in the program my reading are about .03 tolerance. That is accurate enough for my application.

Thanks again

[AnalogKid2DigitalMan]

Very good, I don't know why NI did not put a better front end on the 600x's, probably a cost saving measure.

-AK2DM