NI USB-6259 BNC bridge with load cells

[meryy19]

Hi all

I'm having some problems trying to measure a load cell in a Wheatstone bridge configuration with NI USB-6259 BNC board.

The excitacion voltage is +-5V and I'm providing this to the bridge from the Analog Outputs of the board.

Load cell is 2mV/V, so at full load (+-50N) the expected value is +-10mV.

When I try to measure the output of the bridge, I connect each output to one Analog Input ( AI0 and AI1 for example, with a BNC with 2 wires, one to the input and the other to GND because I select the Ground Source) and with DAQ Assistant I select Differential configuration. But with this configuration, the DAQ M series User Manuel specify that the DIFF mode AI0 is the positive input and AI8 is the negative although in the board there is no BNC connection for AI8 (it goes from AI0-AI7 and jump to AI16-AI23).

By now I'm substracting the AI of each channel in DIFF (with signal input rate +-5V) and GS mode but I'm not obtaining the value that I want to see with no load (multimeter measure around -3mV ).

I also tried the same with NRSE and GS mode without success.

My main doubts are:

- Is it possible the DIFF mode with this board? As I don't know how to find the AI8.

- With DIFF mode and if I could have the AI8 measure of the negative output of the bridge, should I change the Signal Input Rate? So I expect to have a mV measure, and the lower level this board has is +-100mV. Is this correct or I should also change the Signal Input Rate with my actual configuration?

- Is it correct the DIFF and GS mode or it should be better NRSE, RSE with GS or Floating Source? I've been searching in the forum and reading the user manual but Is not really clarified.

- A minor question...load cells are 350 ohms. Is it possible that the Analog Outputs on board provides insufficient current to the bridge? (I could solve this with external source, main problem is how to obtain with the board the same measure of the multimeter).

I'm aware that maybe is not the best board to measure a bridge but I think I could achieve the measures with the correct configurations in DAQmx.

Thanks in advance for any response if someone has some experience with this board and DAQmx modes.

Here is DAQ M series User Manual

http://www.ni.com/pdf/manuals/371022k.pdf

USB-6259 BNC datasheet

http://www.ni.com/datasheet/pdf/en/ds-21

M.