Single High-Impedance voltage source on a DAQ scan (PCI-6355, DAQmx)

[Jed_Davidow]

I have a DAQ task with 80 channels.  A few of these channels are high-impedance voltage divider sources (~1MOhm or so).  When sampling, I see significant ghosting to the previous channel in the scan.  If I hook up a scope to the test point, I can see a blip and decay that coincides with the overall scan clock.

We have tried to add a 0.1uF cap across the lower leg of the voltage divider:

Source------1M--------<Test Point>--------DAQ
             |

           ------

           |    |

          1M    0.1uF

           |    |

           ------

              |

             GND

(Note:  I cannot change the 1M resistors in this voltage divider)

This helps, but doesn't alleviate the issue.

I have also tried slowing down the rate (just to see what I can do, I still need faster sampling):

  Originally:  1KHz Scan Clock, ~90909 Conversion Clock

  Test:   5Hz Scan Clock, 400Hz Conversion

This also helps, but it's not entirely stable yet.

I am not 100% familiar with the timing; does the MUX switch immediately after the previous measurement?  What is my dwell time for the selected channel?  Can I set that on a per-channel basis, or is it determined solely by the Conversion Clock?

I am considering adding dummy samples on these channels in a group, to increase the dwell.  But if it isn't stable at 2500us of dwell (400Hz conversion clock), then would this help?

[santo_13]

You definitely need to add a unity gain amp or buffer to ensure DAQ is able to sample the signal properly.

[crossrulz]

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@Jed_Davidow wrote:

When sampling, I see significant ghosting to the previous channel in the scan.

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A very large percentage of the time when there is ghosting, you need an op-amp set up in a volage follower configuration before going to your DAQ. This will give the DAQ a low impedance source, allowing the sampling capacitor a place to discharge.