6211 Channel Interference Problem daf

[zidaneharold]

Hi,

I am using NI-USB 6211 ADC module with Labview 8.6 on Windows 7. I have this problem where I connect two inputs to any two channel using RSE model, but the signal on input 1 will interfer with the signal on input 2.

My inputs are +3 to -3V to channel 13. When I connect one input and enable the channel in Labview using DAQ assistance, the output waveform is perfect.

However, as soon as I add another input channel in the same DAQ assistance, the input in channel 13 will mirror on the other channel, even when the other input is floating. When I connect the second input, both waveform get distroted.

It seems like there is a setup issue with my Labview code, such that two channels are talking to each other. I tried different pins, all behave the same.

Thanks,

Harold

[MikhailK]

Hi Harold,

What is you are experiencing is potentially a ghosting effect on your channels. To eliminate this problem follow the steps below as described in the following article: How Do I Eliminate Ghosting From My Measurements?

To overcome ghosting you have several options:

1. Select a transducer with lower source impedance.
2. Reduce sampling rate and increase interchannel delay to allow enough settling time for the amplifier. See KnowledgeBase 292A3QB5: How do I Increase Interchannel Delay Using NI-DAQmx or Traditional NI-DAQ (L...
3. Implement a voltage follower or buffer circuit to decrease source impedance to less than 1 kOhm. See KnowledgeBase 208AD10T: Decreasing the Source Impedance of an Analog Input Signal
4. Arrange signals to minimize voltage swings between channels.
5. Choose the high-impedance channel as the first channel in the scan list.
   Note: This will only help if you are not sampling in round-robin mode.
6. Avoid multiplexing – sample one channel at a time or switch to a simultaneous sampling DAQ device.

[zidaneharold]

Hi MikhailK,

Thanks for the reply, it was really helpful. I am now able to sample two channels simultaneously, with some acceptable of distrotion. I scaled one of the input voltage down to the same range as the other one and lower the output impedance.

However, I could not follow the second point you metioned, adding delay between channels. I used the DAQ assisstance to communicate with USB 6211. I have no idea how to use the DAQmx. Could you provide me a brief tutorial on that?

Thanks

Harold

[MikhailK]

Hi Harold,

You can increase the interchannel delay, by decreasing AI Convert clock rate through DAQmx property node. Here are some helpful links that will get you started with using DAQmx functions instead of DAQ Assistant.

Getting Started with NI-DAQmx: Basic Programming with NI-DAQmx

Learn 10 Functions in NI-DAQmx and Handle 80 Percent of Your Data Acquisition Applications

Basics of NI-DAQmx Property Nodes