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Multiple ADS1256EVM Strange Noise Issue

Daniel Vamos
Genius 3950 points
Other Parts Discussed in Thread: ADS1256

Dear Community,

I have been working with ADS1256EVM board connected to TivaC Launchpad without MMB0. I am using Labview to monitor the ADS conversion result and precision of a strain gauge. I am using 4wire Strain gauge (Load Cell) measurement where the excitation voltage is the ADS1256EVM AVDD. AGND, DGND and the excitation GND are common.

This system work very well and quite accurate. I sense noise when I touch the load cell with my hands, however connecting the load cell metal housing to the GND this noise is no more.

I would like to increase the channel count of my system by adding another ADS1256EVM TivaC. I made the proper wiring and software update but I faced a very strange problem that I can not solve it on my own. 

There are two load cells, with common excitation voltage source. This voltage source is used for the second ADS1256EVM too. One load cell V+/V- signal wires connected to one ADS1256, the second load cell connected to other ADS1256EVM respectively.

When I connect the 2nd load cell wires to 2nd ADS the first ADS signal become very noisy. 

  • I connected the 1st ADS RefP and RefN to 2nd ADS Ext.RefP / Ext.RefN  - No success.
  • If both cells are connected to the same ADS there is no noise.
  • If both load cells are connected and ADS input and I remove any load cell excitation the noise is gone.

Do you have any idea why? And how to solve this noise issue? 

Thank you very much!

  • 0
    Genius 3950 points
    Hello,

    I found something very important. I could totally eliminate the noise by setting the PGA value to 1. Increasing the PGA value will increase the noise peak on signal.

    Could you give me some explanation? Thanks
  • 0 in reply to Daniel Vamos
    TI__Mastermind 44680 points

    Hi Daniel,

    I hope you're well!

    May I ask for clarification by what you mean by...

    Daniel Vamos said:
    I could totally eliminate the noise by setting the PGA value to 1.

    Any noise in the input signal will be gained up by the PGA; therefore, it makes sense that increasing the gain would increase the signal's noise peaks. You're not implying that the noise completely went away, are you? If so, then perhaps there is a software issue where only one ADC conversion result is being reported.

     

    Regarding the increased noise with the 2nd load cell; where do you connect the load cells to the analog supply, and how are you powering the ADS1256EVMs? You could be creating a large loop that is picking up additional noise.

    Is your supply capable of supplying the required amount of current to both load cells?

    Are the load cell cables shielded, twisted pair wires and does the shield connect to AGND? Unfortunately, may load cell cables are not twisted pair and not always well shielded. This can generate additional noise in the ADC results. Grounding the shield on both sides of the cable is recommended.

    The noise could also be picked up and enter in through the ADC's reference voltage. If you apply a clean input signal and still see the noise, then likely it is getting in through the reference.

    Best Regards,
    Chris