NI 9237 Bridge Offset

[IsaacC]

Hi,

I'm currently using the NI 9237 with a Full Bridge but not a strain gauge. One resistor in each leg is constant while the other is varied. My problem is the bridges aren't always perfectly matched so there is a (sometimes large) offset voltage. When I try to take measurements from bridges with offsets that are outside the range of the 9237 (± 12.5 mV @ 10 V Excitation ?) I just get a flat line at the rail.

I noticed there was an offeset that can be assigned in the Create_Virtual_Channel.vi for the bridge. Is this value used before the D to A conversion or after such that it won't help me?

If I can't solve this through software, would using a Current module (NI 9265) allow me to inject current into the bridge on one leg and reduce this offset? If so, Is the EX- on the 9237 in any way connected to the return line of a 9265 or would there be any other complications?

Thanks for your help.  

[Frank-L]

Have you tried doing a reset / self test / self calibrate in Measurement and Automation Explorer to see if the device is able to be reset to see if this alleviates the offset seen in the 9237? Are you using a homemade or purchased Full Bridge device, as most purchased Full Bridge devices should be calibrated fairly well and would not cause the railing you are experiencing. Also isn't the range of the 9237 250 mV total (25mv/Vex * 10 Vex), hopefully resetting / recalibrating this would fix the railing issues as 12.5 mV should not cause the device to rail..