USB 6211 DAQ - When one analog input changes, the results of all other inputs shift.

[Scott_T]

I have a USB 6211 DAQ and am controlling it with a modified version of the NI supplied analogIn example program. As a test I have ground on one channel, a steady 1.0 volt signal on another, and a force sensor on a third. At steady state all looks normal. When pressure is applied to the force sensor the DAQ results move positive or negative as appropriate. However, the other two channels shift in the opposite direction. The ground and 1.0 volt signals are normal at the input pins as seen with an oscilloscope, so something is going on in the DAQ. Various ground schemes don't make any difference. Any insights would be appreciated. Thanks.

[Jim_S]

Hello Scott,

Could you explain the shifting in a little more detail and maybe some screen shots of this behavior? Does shifting in opposite directions mean that gnd becomes a higher voltage and 1V becomes a lower voltage? When this happens do you read the force sensor correctly? Also what speed were you sampling at?

If you could answer these questions, I will be able to help narrow down what the problem might be. Also could you try to reproduce this by creating a task in MAX? Thanks.

[Scott_T]

Hi Jim,

In DAQmxCreateAIVoltageChan() I changed the terminal configuration from DAQmx_Val_Cfg_Default to DAQmx_Val_RSE and everything is now working properly.

Out of curiosity, in the default mode what criteria does the DAQ use to select the terminal configuration?

Thanks,

Scott T

[Jim_S]

Hello Scott,

After looking at the DAQmx help, the default input terminal configuration depending upon for channels are being used. In order to simply matters, here the paragraph from the help explaining this.

For devices with eight channels: differential for the first four channels, referenced single-ended for the next four channels. For devices with 16 channels or more: differential for eight channels followed by referenced single-ended for eight channels. For instance, channels 0-7, 16-23, and 32-39 are differential. Channels 8-15, 24-31, and 40-47 are referenced-single ended.