Ordering Analog Inputs

[DeOdderKeith]

I've set up a number of Analog Inputs and, depending on the order of the inputs, I get different readings for some of the signals.

ADC SLOT6 ai 7   N
ADC SLOT6 ai 22 N
ADC SLOT6 ai 19 D
ADC SLOT6 ai 21 D
ADC SLOT6 ai 23 N
ADC SLOT6 ai 31 N
ADC SLOT6 ai 30 N
ADC SLOT6 ai 17 D
ADC SLOT6 ai 32 N
ADC SLOT6 ai 16 D
ADC SLOT6 ai 9   N
ADC SLOT6 ai 10 N
ADC SLOT6 ai 11 N
ADC SLOT6 ai 5   N
ADC SLOT6 ai 18 D
ADC SLOT6 ai 6   N
ADC SLOT6 ai 20 D

Here is the current order, but when I change the order, especially on ai 7 and ai 22, they seem to fail the test.

However, I go to MAX, and monitor the values, and they are exactly where I expect them to be.

Of course, N indicates NRSE, and D indicates Differential

Can anyone shed any light on this?

DOK

[John_P1]

Hi DOK,

I'm not sure what hardware you're using, but with 32 input channels I think it's a safe bet that it's a multiplexed board.  If the source of your input signals has a high impedance, you might be seeing what we refer to as Ghosting.

If you're using one of our DAQ devices with DAQmx, the driver configures the device (by default) to the fastest scan rate possible +10 us to allow for settling (at high rates the 10 us is reduced to fit all the channels within a single sample clock period).  To allow more time for settling in between channels, you can increase the time between conversions using the following property node (assuming LabVIEW):

If you are acquiring at too high of a rate to do this, or if the impedance of your signal source is too high for this to help, you might consider a voltage follower on any high-impedance sources.

If this isn't the issue, we'll need a bit more information (screenshots and details about how the task is configured) to diagnose what you're seeing.  From what you've described however, it sounds like ghosting would explain the behavior.

Best Regards,

John