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Problem with ADC input on the F28335

Thanh Bui
Prodigy 60 points


Dear all,

Actually, i am working with eZdsp F28335 but i have a problem with reading ADC. I have already connected ADCLO to GND, i used internal reference but i can not find ADCREFIN on the card (for connecting to GND). Otherwise, when i change the input voltage of ADC from 0 to 3 volt, it work with the range 0 - 700 mV,. (I used a multimetter to see the change of voltage), so when the voltage is 1 Volt for exemple, the voltage of the ADC input is around 800mV (there are consomable), but from 0 to 700mV, it is same.

Please help me to explain why the ADC work when the input is from 0 to 700mV (in the data sheet say from 0 to 3V), maybe cause of do not connect ADCREFIN to GND?

Many thanks

Dang-Thanh

  • 0
    TI__Guru* 81440 points

    If you are measuring this at the pin(not just conversion showing problem) it sounds like there is contention on the channel from another source on the board.  Do you see this on every channel?  Are you able to try a different board?  The pin voltage should track to your input, if not I would suspect board issue rather than ADC.

    Best,

    Matthew

  • 0 in reply to MatthewPate
    Prodigy 60 points

    Hello Matthew and other,

     

    I want to explain more detail my problems:

     

    I am working with Matlab Simulink (toolbox: Target Support Package), I used the three components with the configuration following:

     

    F28335 eZdsp:

    -> Peripherals: ADCLKPS =1; CPS =1; ACQ_PS=1; Offset=0;

     

    ADC C280x/C28x3x:

    ADC Control: Module: A; Conversion mode: Sequential; Start of conversion: software; Sample time: 0.001; Data type: uint16

    Input Channels: Number of conversion:1; Conversion no. 1: ADCINA4

     

    Actually, I connect the ADC output to “Data” of the block  SCI Transmit

     

    I change the input voltage of ADCINA4 (P9-Pin10) from 0 to 700mV, I see in the multimetter as same result, and the result displays on the PC like (it's number of ADC in uint16):

     

    Vin= 100mV --------> ~250

    Vin= 200mV --------> ~500

    Vin= 300mV --------> ~750

     

    Here is the first problem, it means if Vin=300mV (using function generator, DC mode), the output value of ADC  is around 409 (because Vin =0 to 3V corresponding with the output value is 0 to 4095) in stead of displaying ~750.

     

    When I continue increasing the input voltage of ADCINA4 more than 700mV, the value displays in the multimetter is difference,  for example:

     

    V(function generator) = 1 V;         V(mutilmetter) =Vin = 834mV

    V(function generator) = 1.2 V;      V(mutilmetter) =Vin = 885mV

    V(function generator) = 1.4 V;      V(mutilmetter) =Vin = 930mV

     

    Here is the second problem, why there is difference when the input voltage is greater 700mV although, in data sheet say the ADC input can work in the range from 0 to 3V.

     

    Thanks before,

     

    Dang-Thanh