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[F28335] ADC PROBLEM

Ahmed Shakeel
Intellectual 410 points

Hi

I am using f28335 control card (R1.0 with 30 MHz crystal) with my custom gpio board. My board with control card was working well, now I am experiencing a problem with my system while using ADC. My adc results are not 0 when I give 0v at analog input, I understand there can be an offset but the value that I get is 900, earlier it was around 30 or 50 and I subtracted this from every digital value. Now If I sample other voltages at analog input (0-3 volts) this offset is not seemed to added to the theoretical values (4096*analog volts/3v). If every reading that I got had this offset I could minus the offset and use the value. This control card works fine with same software on my other board, and docking station from TI so there is some issue with my board. 5 volts board supply and control card 3.3 and 1.8 are fine in both board. ADCREFP(pin p5 ) gives 1.3 volts when measured on capacitor c 30, while ADCREFIN is ground on control card while formula for conversion is (4096*analog volts/3v) Page 10 of Spru 812. This was confusing for me as pin p5 on control card shows 1.3volts. Internal reference is selected from software. I hope someone can help me debug this problem.

  • 0
    TI__Guru* 84340 points

    Ahmed,

    Since we are suspecting a board issue, I will address that aspect.  You need to make sure that the ADCREFP/M pins have 2.2uF capacitor and have voltages in range that is in the DS.  Pin ADCRESEXT should have a 22kOHm resistor.  Additionally pin ADCLO should be connected to VSSA.

    Please confirm these first and we can proceed from there.

    Best,
    Matthew

  • 0 in reply to MatthewPate
    Intellectual 410 points

    Hi

    Thanks for the reply, i will check what you have suggested and get back to you. Also my findings are that all channels in seq1  are working ok with fixed offset for given analog input volts, while seq2 channels i.e b0-b7 are having trouble. i am using cascaded sequencer mode.

    Ahmed Shakeel

  • 0 in reply to MatthewPate
    Intellectual 410 points

    Hi Mathew

    As i mentioned in my first post that  I am using f28335 control card (R1.0 with 30 MHz crystal) it has 2.2 uf caps  c30 and c31 on ADCREFP/M . ADCLO pin is short with ADCREFIN both are tied to ground. ADCREFEXT is pulled down with 22kohm. ADCREFPand ADCREFM have 1.3 and .55v volts respectively across cap c30 and c31,when my code runs.

    Ahmed Shakeel

  • 0 in reply to Ahmed Shakeel
    TI__Guru 55700 points

    Ahmed,

    Your ADCREFP, ADCREFM, ADCRESEXT connections are good and it is expected to see 1.3v on ADCREFP and .55v on ADCREFM while using internal reference.Did you double check whether ADCINB0-ADCINB7 are infact connected to analog ground when you taking these readings. Also, are you using start stop mode (or) continuous run mode?

    Regards,

    Manoj

     

  • 0 in reply to Manoj Santha Mohan
    Intellectual 410 points

    Hi Manoj

    I am using ADCINB0 to ADCINB7 for conversions so i have given them some analog inputs so  they are ofcourse not connected to analog ground. Problem is that none of Seq2 is giving right result, but seq1  channel  are  behaving ok. Also since i am using TI control Card there is nothing like different analog or digital ground, all analog and digital grounds are tied to coomon ground. i am using start stop mode as COUNT_RUN=0; in ADCTRL1.

    Ahmed Shakeel