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CCS/TMS320F28379D: ADC Results Interference

Hasan
Intellectual 405 points

Part Number: TMS320F28379D

Tool/software: Code Composer Studio

Hi everyone,

I discussed this issue before. Didn't receive any response. I am trying to run the example "adc_soc_software_cpu01.c". The results i am getting in the ADC Result Regs (A2,A3,B2,B3). The issue is whenever i change the analog value on ADC Channel A2, it gives ADC Result A2 Value accordingly but at the same time changing the ADC Result A3 Value at some extent. The same thing happens vice versa and when i am using ADC B Channels. I have added some screen shots that may help you. In some cases, when ADC Channel A2 is disconnected and A3 is connected, there will still be some value at A2 which varies when i change the analog value at channel A3. Help needed.

In this screen shot, there is nothing connected at AdcaResult0 and AdcbResult1, but still there are some values which changes as i vary AdcaResult1 AdcbResult0.

  • 0
    TI__Guru 60865 points
    Hi Hasan,

    If A2 is disconnected, it is expected that other conversion results on the same ADC may have some impact; the S+H capacitor on the ADC will have residual charge from the previous conversion which will show up in the result absent some external driver.

    A similar phenomenon can occur even if the input is driven if the input driver has high impedance and/or if the input driver has low bandwidth and/or if the selected S+H duration is too short.
  • 0 in reply to Devin Cottier
    Intellectual 405 points
    Hi Devin,

    Thanks for your reply. If we assume that the S+H capacitor on the ADC will have residual charge from the previous conversion which will show up in the result so how do i counter it??

    Currently, I am using 2 variable resistors on ADC Channel A3 and B2. Both varying the voltage from 0 to 3.3 Volt. But what happen in Debug window is, even though nothing is connected at channel A2, if the voltage at channel A3 is 1v, the voltage at channel A2 shows 0.5v (which can't be neglected)and as I increase the voltage at channel A3, the voltage at channel A2 also increases.

    Should I increase the acquisition window size?
  • 0 in reply to Hasan
    TI__Guru 60865 points

    Hi Hasan,

    See the following equations.  You can find the values of Ron, Ch, and Cp in the datasheet for your device.  Instead of using the equations, you can also simulate the settling in SPICE.  

  • 0 in reply to Devin Cottier
    Intellectual 405 points
    Hi Devin,

    Thanks. I will get back to you.
  • 0 in reply to Hasan
    Intellectual 405 points
    Thanks Devin, the acquisition window was too small for 5 ADC Channels that i was using.