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TMDSCNCD280049C: Capacitors at anlougue inputs

User1164857
Intellectual 380 points
Part Number: TMDSCNCD280049C

Hello everyone,

I am designing a board for an F280049. Doing so I also take a look at the ControlCard Schematics that TI provides. In case of the mentioned Piccolo there are several 330pF capcitors directly connected to the analougue inputs at the control card. I usually work with the Delfino F28335 or F28379D and I never saw something like this there. What is the purpose of those capacitors?

Additionally the recommended VREF capacitance is 2x2.2uF on the control card with F280049. The delfino cards show a much higher vref capacitance and even small resistor values in series. I assume to damp possible oscillations. Where does this need for such a higher capacitance come from. The analogue modules seem similar. Is it just the additional adc channels and higher adc clock?

Kind regards,
Martin

  • 0
    TI__Mastermind 41385 points

    Martin,

    These capacitors are used with the series resistor to form a low-pass filter on the ADC's input pin. This will help with noise filtering. Because the noise that you will be filtering is system dependent you will need to tune this filter to your system. We have suggested 330pF in the schematic as a good starting point, however these marked as "do not populate" so you will need to populate the capacitors.

    The recommended VREF capacitance will change from ADC to ADC.

    22uF is required on the F28379D controlCARD to support the ADC's 16-bit mode. If 12-bit mode is used the capacitance can be reduced.

    Please note that F280049C only has a 12-bit ADC, and therefore requires less capacitance on its VREF pin.

    Regards,
    Cody