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TMS320F280036C-Q1: ADC conversion time doesn't match the theoretical value

Kita Li
Expert 3675 points

Part Number: TMS320F280036C-Q1
Other Parts Discussed in Thread: SYSCONFIG

Tool/software:

Hi experts,

Good day! I am asking for customer.

Now the customer is using EPWM to trigger ADC, ADC will converse a single channel. After that EOC will trigger interrupt.

The configuration like that: The ADC clock is 60MHz; sample window in Sysconfig is 9 so ACQPS=9; Use single sampling mode;

Here is the test result:

1. with early interrupt: INTPULSEPOS=0, from EPWM trigger to ADC interrupt(use a GPIO toggle in interrupt service) will take 400ns.

2. with late interrupt: INTPULSEPOS=0, from EPWM trigger to ADC interrupt(use a GPIO toggle in interrupt service) will take 500ns.

These test result is far from the theoretical values. Theoretically, the time of late interrupt should be equal to tSH+tEOC = (9+21)/60MHz=250ns. The time of both interrupt are much longer than theoretical value. Could you give some advices of this issue? Or should there be certain times that need to be considered?

Thanks!

Best Regards

Kita

  • 0
    TI__Mastermind 44580 points

    Hi Kita,

    From timing perspective, the total conversion time from table below would be:

    SH = ACQPS+1

    tCONV  =  (SH +  tEOC)*(1/SYSCLK) = (9+1+23)*(1/120MHz) = 275ns.  Regardless whether the ISR duration is 400ns or 500ns, it should be plenty enough time for the conversion to complete.

    Couple of things to check:

    - Ensure that VREFHI is set up properly

    - Ensure that the ACQPS selected matches the actual input circuit.  ACQPS of 9 basically mean that the input impedances of the signal source is minimal.

    - You can actually verify the correct ACQPS setting by using the Sample Time Calculator in Sysconfig under SOC setup.  Click on Use Sampe Time Calcultor option and there will be entry for Cs (Input Capacitance) and Rs (Input Resistance).  Enter the values based on actual customer circuit and the sampling time and ACQPS will be automatically calculated.  Click on Use Calculated Sample Time for This SOC Option so code will pick up the correct ACQPS value.

    Regards,

    Joseph