USB DAQ-6009 not measuring electric pulses's voltage accurately

[lakshithadinusha]

Hi!

I have a USB DAQ-6009 and I tried to take voltage measurements of electric pulse which was obtained from a Geiger Muller Tube. Even-though the oscilloscope measure the pulse accurately (Vpk-pk~9.4V and the screenshot is attached), USB DAQ-6009 recorded the pulses with less peak-peak to voltage (Vpk-pk~2.5V and the plot of electric pulses from 6009 is attached) and the baseline is lifted to around 1.4V. I connected the output of GM Tube to AI1(AI 1+) and the GND terminal of the GM Tube to AI5(AI 1-) according to 6009 manual as following the "Ground-referenced signal source with differential input mode".

When I am setting up the DAQ Assistant, I chose the "Sample Rate" as the maximum value (40kHz) since the pulse duration is less than a millisecond. But my input pulses to DAQ is in the range of -10V to +10V .Other options I selected are Terminal Configuration: Differential, Acquisition Mode: Continuous Samples, Samples to Read 10k. Here I have attached the LabVIEW Program screenshot and its VI for your reference. The final output of the LabVIEW program is a txt file with Time and Voltage data. Did I miss any addition of external circuit to the DAQ to measure correct voltage values or did I use wrong options in the DAQ Assistant?

I would appreciate any advice. 

Thank you very much in advance for your help!

Best Regards;

Lakshitha

[crossrulz]

Seems to me that you should wire up the signal ground to AIGND.  You then use a Referenced Single Ended configuration to read the signal.

Even then, I'm a little worried about the acquisition rate being able to capture that pulse.  I think you should be able to catch it, but I would want something trying to get into the 500kHz to 1MHz sample rate.

[AnalogKid2DigitalMan]

What is the tube's output drive current or minimum ohmic load?

I bring that up since the 6009's analog input impedance is just 144kOhms, whereas your scope is probably 1Mohm.

It could be that the lower impedance of the 6009 is attenuating the signal.

-AK2DM