Channel Ghosting issue

[Hansolo080973]

I have a setup and I am getting channel Ghosting. Input on channel 1 24V, Channels 2,3,4,5 all showing 12-13Volts. Is there a way to fix this in Flexlogger I have seen a fix for LabView. 

[crossrulz]

What hardware are you using? How is it wired? How do you have the channels configured?

[Hansolo080973]

Hi the setup is as follows. 

Using a cDAQ-9174(4-slot USB)

I have the following cards in. 

NI 9217 2 channel of RTD. No issue with this. 

NI 9209(DSUB) wired in to measure current over a 1 Ohm resistor. No issue with this. 

NI 9221 (DSUB) Have 5 channels all measuring 24V. Channel 0 to 4 allocated. If there is 24V on Channel 0 all the others are reading 12/13 Volts. 

I have however put a resistor across the inputs and this has removed the ghosting. That would indicate that the NI module is not a fan of floating channels when the measuring value is high, which was said in another thread. The setup will not normally have nothing in them when the test is running. 

I have read up that this can be sorted in LabVIEW by adjusting the scanning timing, I was wondering if that was the same for the FlexLogger. 

I am very new to NI and I have tried LabVIEW but its over complicated for doing simple Logging data jobs. I liked FlexLogger for its easy to use and off you go working. 

[santo_13]

I am less hopeful that there is a simple software way to eliminate ghosting in FlexLogger. Instead, I would call it a best practice on hardware wiring to short unused inputs to GND with a moderate-sized resistor.

NI needs to add a FlexLogger feature that allows users to remove ghosting by adopting the repeated channel method or adjusting the ADC conversion time and settling delay.

https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z000000P84RSAS&l=en-US

[Hansolo080973]

Hi Cheers for the info. Its working now so will see how I get one with the test. 

[crossrulz · Accepted solution]

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@Hansolo080973 wrote:

I have however put a resistor across the inputs and this has removed the ghosting. That would indicate that the NI module is not a fan of floating channels when the measuring value is high, which was said in another thread. The setup will not normally have nothing in them when the test is running. 

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Nope. ADCs do not like floating inputs. You need your voltages to have some reference to ground. I highly recommend giving this article a good read: Field Wiring and Noise Considerations for Analog Signals