Offset voltage when channel isn't set as first in the Daq Assistant...... any idea....?

[marc france]

Hello everyone,

I am using Labview 8.0 and a PCI 6221 card.

I have got an offset pb with my AI channels. In order to avoid a software pb I am using the Assistant DAQ to check the voltages. All my AI work fine when I measure 0.150 V generated by a power supply. However when I measure the voltage of the signal coming out of my circuit (what a realy want to measure in the future), which is roughly 0.150 V, the mesaurement is correct only when the signal is set as first in the list in the daq assistant. If my signal is connected to channel 1 for instance, the measure  will only be correct if I choose the order the order 1, 0,2 or 1,2,0. If Channel 1 isn't put in first place I get a 30 to 60 mV offset on this channel. This doesn't happen at all  if I use a power supply with the same voltage, and in any order channel.

Would anybody know how I could solve this pb?

Thanks a lot,

regards,

Marc

[mikeporter]

This sounds like a scanning/settling time problem (pb?).

Are the other input channels terminated? What is the range of the signal that you are measuring on them?

You should always terminate (tie to ground) unused input channels, and avoid large range swings between channels. For example, if you have a +-10V signal on channel 0, you are going to have trouble reading +- 1mV on channel 1 in a single scan.

Mike...