USB 6008 using all analogue input channels simultaneously

[miikaelf]

Hi all,

I'm using NI USB 6008 to record some data, from its analog input channels. I need to use all 8 input channels at once (A0-A7). When using A0-A3, everything works well, but when using A4-A7, their signals also cause a signal change in the A0-A3 channels (but not vice versa).

All channels have grounds connected. I know A4-A7 are meant also to be used for differential measurements, but according to NI manuals, it should also be possible to use them all as individual input channels. So any thoughts how to use them all as independent input channels, without interference?

[crossrulz]

Can you share your code?  My first thought is that you have the channels set up to be Differential instead of Reference Single Ended.

[Hooovahh]

The 6008 is not a simultaneous sampling device.  It has one analog to digital convert on the hardware and it will mux that hardware to each channel one at a time as fast as it is told to.  If you have two sine waves, one on AI0 one on AI1 taking the measurments in the same DAQ task will not have them being taken at the same exact time, because the device only has one of those ATD devices.  Other more expensive hardware does have multiple ATD, one for each channel and then you are sure not to get any cross talk between channels you mux between.

I don't know your setup, and I don't know how much time can be between sampling the different channels.

[johnsold]

Do some of the signals have high source impedance? Although the 6008 has a relatively low input impedance, signal sources with high output impedance can still result in ghosting.

Lynn