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LAUNCHXL-F28027: Noise in the ADC reading.

Damith Pavithra
Expert 1125 points
Part Number: LAUNCHXL-F28027
Other Parts Discussed in Thread: AMC1200

I configured the ADC using c2000 embedded coder support package  and connected the A0 pin to the ground and the 3.3V pins in the launchpad. For both readings get a noise. Following image shows the reading when the A0 connected to the 3.3V pin. Why is that happening. 
configuration parameters of ADC shown below. ePWM1 frequency is 20kHz. I changed the board, changed the power supply and nothing works. 


  • 0
    TI__Guru* 77460 points
    Damith,
    For the 3.3V on the launchpad, this is generated by a LDO from the 5V USB. While this is more than adequate for the FTDI and F28027 power source, this will not be a "clean" voltage signal for any ADC and you will observe noise on this supply.

    Do you see reasonable behaivor from the ground connection?

    Best,
    Matthew
  • 0 in reply to MatthewPate
    Expert 2055 points

    Damith

    Hi, I was dealing with this thing by hardware, I could recommend you to put a ferrite bead in series with the ADC input pin, also resolving by software substracting that kind of "offset" . Does the signal you have to measure is, at least, 100 times bigger than that noise, if yes then I could tell you not to worrie about, but if not, then you will have to make some sort of digital/filter in order to keep your measurement in a good shape

    Mike

  • 0 in reply to MatthewPate
    Expert 2055 points
    Matthew

    How can achieve a good "clean" source voltage for the adc input pin?
  • 0 in reply to MatthewPate
    Expert 1125 points

    Yes I got the same noise for the ground pin as well. 

  • 0 in reply to Mike_3007
    Expert 1125 points
    I have to consider the noise even though it is within 0-100 range because i measure the mains sine wave and i need to measure frequency, voltage etc. because of this noise, near the zero crossing, there will be a problem.
    Thank you
  • 0 in reply to Damith Pavithra
    Expert 2055 points

    Damith

    How are you measuring the mains? I could recommend you the AMC1200 or any of that family of The AMC or you can use  the ISO 224 , in the AMCXXXX you have The output mounted with DC offset and you can put a comparator with hysteresis to avoid multiples Zero crossings

    Regards

    Mike

  • 0 in reply to Mike_3007
    TI__Guru* 77460 points

    Damith,

    I was re-reviewing the intial embedded coder GUI picture; can you comment/show the channel listing tab?  There is an errata regarding the first sample of this ADC from any idle state; that is it will be corrupt.  

    There are two workarounds listed in the errata http://www.ti.com/lit/er/sprz292p/sprz292p.pdf

    1)Double sample the first channel in a conversion sequence and discard the first sample result

    2)Enabled the ADCNONOVERLAP bit as well as the ADCCLKDIV bit.  This will slow down the ADC clock, but the first sample is usuable.

    Let me know if this addresses the issue.

    Matt

  • 0 in reply to MatthewPate
    TI__Guru* 77460 points
    Damith,
    Wanted to check to see if the above response has helped resolve the issue.

    Matt