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28335 ADC Calibration Problem

wu chung-yi
Prodigy 40 points


Hi, I have a develop board with 28335.

And when i am testing the adc module, i found though it can read the data i input from the power supply.

But it's not accurate. Any value above 1.8V DC input will read as 4095. I know i have to calibrate and i have followed datasheet of ADC chapter, section 1.8.1 and activated the instruction of ADC_Cal().

But it still has gain and offset error. Next i am going to calibrate from external. But i have a question now, is the input range already limited to 1.8V(since my board will read as 4095 any voltage above 1.8V), no matter i have calibrated it?

Or is there any way to solve the adc problem?

  • 0
    TI__Genius 16281 points

    Hi,

    The full scale input voltage for F28335 is 0-3.0V whether using internal or external reference and the ADC_Cal() routine should not matter whether internal or external reference is selected since it is not actually calibrating the ADC but rather copying factory determined calibration values into the appropriate registers.

    It is true, however running in external reference mode can help determine whether you are dealing with a silicon issue or setup issue.  If you encounter the same issue using external reference then I would suspect something wrong with the hardware, particularly ADCREFP, ADCREFM, or ADCRESEXT or the analog power supplies (note that some are 1.8V supplies and some are 3.3V supplies)

    Can you clarify whether your statement that you are using a "devel board with 28335" means you are using a TI designed board such as a control card or development kit or your own custom hardware?  If the latter did you follow all the guidelines in the following document?

    http://www.ti.com/lit/pdf/spraas1

    Regards, Joe

  • 0 in reply to Joe Prushing
    Prodigy 40 points

    Thank you so much for replying

    I am using develop board ezDSP28335 produced from Spectrum. I have two boards which have problem on each one.

    One has the usb connectivity problem with PC, and the other's main 28335 chip has been burned. So i change the good chip from one device to the other which's usb is in good situation.

    And now the "new" one can compiled, pwm is also good, but the ADC's value has much different from the actual one i send from power supply.

    I don't know is the hardware has been broken cause the offset and gain error. If did, is that means even i calibrate from either internal or external ways, i still lose the AD resolution? Which means my analog input should not exceed 1.8V?