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F28335 ADC conversion problem

Akhil Francis
Intellectual 420 points

HI,

 I am using F28335 with XDS100V3 and CCSv5 

I am giving 20Mhz Crystal oscillator into  XCLKIN pin.CLKINDIV is 2 and PLLCR is 10. I am using internal refernce.

I have some problem with my adc conversion code.

I checked with the demo code in Controlsuit.

I configured the adc registers many ways. I am converting  ADCINA0 with applying 2.048 voltage. I grounded ADCREFIN pin and Other circuits are following same as data sheet.

But i getting almost +/- 100 difference in each conversion. 

How can i avoid this?which is the best ADC configuartion for 100Mhz Devices?

I am expecting a fast help from you experts.

Thanks in advance

Akhil Francis

  • 0
    Guru 192875 points

    Akhil Francis said:
    But i getting almost +/- 100 difference in each conversion. 

    You mean to say 100 digital counts? Do you've a RC filter on the hardware?

  • 0 in reply to Gautam Iyer
    Intellectual 420 points
    Gautam Iyer,

    Thanks for your reply.
    Actually i am directly connecting the 2.048 volt into ADCINA0 pin. i checked the voltage in oscilloscope, there is no noise or something like that.

    -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
    And i am using 100Mhz crystal.

    ADCCLKPS is 2

    ACQ_PS is 15

    CPS is 1

    HSPCP is 2

    -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

    i am applying 2.048 voltage but still variations are there.
    I getting values are in range of 2700 - 2980

    How can i avoid this?

    Akhil
  • 0 in reply to Akhil Francis
    Guru 14640 points
    You may want to check out SPRAAP6A - "An Overview of Designing Analog Interface With TMS320F28xx/28xxx DSCs"
    very good stuff...
  • 0 in reply to Todd Anderson78572
    TI__Guru 60865 points

    Hi Akhil,

    Be careful of concluding that there isn't any noise due to a scope capture; the vertical resolution is usually only 8-bits and the vertical range is usually set to a higher value than the ADC range (e.g. 5V scope range to see a 3V ADC range).  This make very significant noise (from the ADC perspective) look like a small ripple in the scope plot.  

    You can get away with driving a voltage onto the ADC pin directly, but you may want to add some LP filtering.  To see if the ADC is working, i'd recommend you add a capacitor to the input in the ballpark of 200nF.  You may need to limit the sample rate based on the LP filter created by the capacitor and the output impedance of your DC signal source (otherwise the ADC input may droop over time).  

  • 0 in reply to Devin Cottier
    Intellectual 420 points
    Hai experts,

    thanks for your reply. I solved that issue

    Thanks in Advance
    Akhil Francis