voltage ni9206 labview signal express does not work for multiple channels

[Help Please]

Hi,

I am using LabView signal express to capture voltage measurements using NI 9206 analog input.  It works great to capture a single voltage.  But when I connect two channels, so it can record two voltages at the same time, it does not give correct readings at all.

I have a known 0.80 V voltage source and a known 2.75 V voltage source. If I connect one at a time to my NI 9206 it shows the correct reading.  If I connect them both at the same time to two different channels it shows an output of 2.08 V and 3.64 V.  I checked with a multimeter and sure enough, the voltage is 2.08 and 3.64 when they are both connected to the NI card but if one is disconnected the correct voltage reading returns.

What is wrong and hope can I fix it?

Thanks

Jay

[Jesse_O]

Hello Jay,

That is strange behavior.  The 3.6V reading you are getting is almost equal to the 0.8V and the 2.75V added together.  Can you please confirm your external connections to the 9206?  You can find this information inside the user manual found here on page 12 to 19.

If this does not help I have a few other questions.  If you only connect the 0.8V signal, can you read it correctly?  If not, do you have the same problems if you switch channels or input configuration type (i.e. if you are using differential, does NRSE or RSE fix your problem)?  Note:  Changing the input confiruation with your external connections will require you change your software setup.

If none of this helps, can you please post more information about your external connections (what pins are you connecting to the channels and com)?  Also a description of your voltage sources might help (i.e. they are floating or grounded).

Regards,

Jesse O.
Applications Engineering
National Instruments