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TMS320F280049: ADC sampling result has a different value with the average

Johnny Chen
Expert 4450 points
Part Number: TMS320F280049

Hi Champ,

I am asking for the customer.

 They are using the same trigger source to trigger 6 ADC channel for conversion.

The picture could see there is a value has an obviously different value than the average in ADCC.

The ADC pin is using C3, and the sampling time is 10, and ADCC is set high priority mode all, the condition happens in this pin and the priority mode every time. (ADCCLK = 50MHz)

Is it a normal condition for this C3 reading value deviation to occur under this priority mode setting ? It shouldn't be affected by the way how they set the priority, am I correct?

Thanks a lot !

BR, Johnny

  • +1
    TI__Guru 60865 points

    Hi Johnny,

    Is the trigger source that is triggering the 6 conversions the only trigger source active in the system?  Is C3 the only channel of the 6 that is seeing this noise?

    In general, this looks like some noise is coupling into your signal.  Is there anything else running in the system that runs slower than the ADC trigger source, runs asynchronously to the ADC, or runs intermittently?  Maybe serial comms like SPI, UART, or I2C?  

    You also mentioned that the S+H time = 10 (100ns?).  What is driving channel C3?  Specifically, what is the impedance of the source and bandwidth of the op-amp (if any) in the driving circuit?

  • 0 in reply to Devin Cottier
    TI__Expert 4450 points

    Hi Devin,

    Thanks for your suggestion. I have solved the issue with the customer, it is because of the bandwidth of the op-amp that causes the noise couples into the signal. Thanks a lot for your help.

    BR, Johnny