Signal Distortion caused by # channels sampled and sampling rate

[TitanLabview]

I am using a USB-6211 DAQ (250kS/s) and looking to sample 3 channels at 80kS/s. (Labview 2012)

When I sample just one channel, it appears fine (1 Channel_Sampled First_250kS), but when I add another channel to be sampled, the signal is dragged down and depends on which channel is sampled (2 Channels(Different)_Sampled First_40kS and 2 Channels_Sampled First_30kS).  Adding the 3rd channel drags it down even more.  I noticed as well that the sampling rate also distorts the signal i.e. the higher the sampling rate, the more the signal is dragged down.

The DAQ IS sampling the signals 'correctly' in that when I run my Labview VI my external hardware starts reading in correctly based on the signal distortions.

What is causing this and is there a way to fix this probelm?

I have attached the waveform captures mentioned above, and can post others if needed.

Thanks in advance,

WBrenneman

[TitanLabview]

Another note is that this happens whether I am using AI sense (NRSE) or GND (RSE)

[JÞB]

a fast search for the term "Ghosting" yields this thread. where Albet offers a workaround.

a detailed explaination can be found in this KB

[TitanLabview]

This problem is not caused by ghosting, as it was altering the signals being measured on the board level.  I have instead placed a buffer in between the device output and the DAQ, which seems to have solved the problem.  More of a problem with device design and not the Labview/DAQ setup.

[RavensFan · Accepted solution]

That is exactly what ghosting means.  The measurement of later signals is being affected by the other signals.  It usually happens if you have a high impedance input signal.  Adding buffers such as you did can help solve that problem by making the input signal to be a lower impedance.

Ghosting probably would look worse at higher frequency sampling rates, just as you said you were having problems, because it provides less time between samples for the amplifier to settle to the new voltage level when the multiplexer switches between input signals.