Measuring by two single ended settings with USB-6211?

[rockarmatte]

I'm trying to measure the frequency of a pulsating square wave with a constant voltage. The signal comes from a wind speed measure sensor, where the frequency is proportional against wind speed. I am using a NI USB-6211  where I measure the difference between the analog port 17 (AI 1) and port 28 (AI GND) with RSE settings (Measurement made with respect to ground). Port 28 has a cable linked to the ground (-) on a transformer which also gives power to the sensors. I'm using LabView 8.6 as software

This works great when I only have one wind sensor. When I attach another sensor to port 19 (AI 2) and select RSE settings between AI 2 and AI GND I get infinitely high frequency from both the sensors.  In contrast, they work well individual.

I wonder, therefore, if I have connected something wrong in any way (maybe the ground cable?) or can you not measure the two analogue signal by referring to the same ground?

Regards Mattias

[Davidek · Accepted solution]

Hi

Mattias contacted NI support and problem was resolved by adding both AI reads as tasks to a single DAQ assistant instead of using two separate assistants.

Best Regards

David

NISW

[Davidek]

(duplicate post due to bad connection....)

Message Edited by Davidek on 10-27-2009 06:54 AM

[rockarmatte]

Yes, thanks to David Ek!