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TMDSCNCD280039C: The problem of ADC pin interference.

Green Deng
Expert 4490 points
Part Number: TMDSCNCD280039C
Other Parts Discussed in Thread: TMDSHSECDOCK, C2000WARE

Dear team:

HW: TMDSCNCD280039C + TMDSHSECDOCK

SW: C:\ti\c2000\C2000Ware_4_01_00_00\driverlib\f28003x\examples\adc\adc_ex1_soc_software.c

My customer measured the ADC pin of the chip with an oscilloscope based on the above configuration, and found that the pin would have a clutter with an amplitude of about 200mV /4mhz when it was suspended.

And when the customer modifies the conversion time of ADC module, the frequency of clutter will also change.

Is this waveform caused by external interference or the internal circuit characteristics of ADC module?

Best regards,

Green

  • +1
    TI__Mastermind 27185 points

    Hi Green,

    The SAR ADC in F280039 has a sampling switch that toggles at a rate specified by ACQPS to charge the internal sampling capacitor through the ADC input pin.  The maxmum switching rate for the internal sampling capacitor is 4MHz on F280039 so what customer is measuring is the switching of the internal sampling switch as it tries to capture a floating high impedance signal (as ADC channels are left floating).  If there is a low impedance signal source connected to the ADC, the amplitude would settle to the voltage of the source signal and will not show the ringing behavior.

    Regards,

    Joseph